Direct tyrosine oxidation using the MLCT excited states of rhenium polypyridyl complexes

Article abstract of DOI:10.1021/ja0510360

Rhenium(I) polypyridyl complexes have been designed for the intramolecular photogeneration of tyrosyl radical. Tyrosine (Y) and phenylalanine (F) have each been separately appended to a conventional Re^I(bpy)(CO) ₃CN framework via an

Full text of DOI:10.1021/ja0510360

Published on Web 06/02/2005
Direct Tyrosine Oxidation Using the MLCT Excited States of
Rhenium Polypyridyl Complexes
Steven Y. Reece and Daniel G. Nocera*
Contribution from the Department of Chemistry, 6-335, Massachusetts Institute of Technology,
77 Massachusetts AVenue, Cambridge, Massachusetts 02139-4307
Received February 17, 2005; E-mail: nocera@mit.edu
Abstract: Rhenium(I) polypyridyl complexes have been designed for the intramolecular photogeneration
of tyrosyl radical. Tyrosine (Y) and phenylalanine (F) have each been separately appended to a conventional
ReI(bpy)(CO)3CN framework via an amide linkage to the bipyridine (bpy) ligand. Comparative time-resolved
emission quenching and transient absorption spectra of Re(bpy-Y)(CO)3CN and Re(bpy-F)(CO)3CN show
that Y is oxidized only upon its deprotonation at pH 12. In an effort to redirect electron transport so that it
is more compatible with intramolecular Y oxidation, polypyridyl ReI complexes have been prepared with
the amide bond functionality located on a pendant phosphine ligand. A [Re(phen)(PP-Bn)(CO)2](PF6) (PP
) bis(diphenylphosphino)ethylene) complex has been synthesized and crystallographically characterized.
Electrochemistry and phosphorescence measurements of this complex indicate a modest excited-state
potential for tyrosine oxidation, similar to that for the (bpy)ReI(CO)3CN framework. The excited-state oxidation
potential can be increased by introducing a monodentate phosphine to the ReI(NN)(CO)3+ framework (NN
) polypyridyl). In this case, Y is oxidized at all pHs when appended to the triphenylphosphine (P) of
[Re(phen)(P-Y)(CO3)](PF6). Analysis of the pH dependence of the rate constant for tyrosyl radical generation
is consistent with a proton-coupled electron transfer (PCET) quenching mechanism.
Introduction
the reaction on a time scale fast enough to allow for direct
observation of the actual ET tunneling event. The same temporal
Amino acid radicals are increasingly identified as reactive
intermediates along the primary metabolic pathways of many
enzymes.1 Owing to their high reactivity, the radicals are often
present as fleeting intermediates along a directed pathway so
as to minimize their time for participation in deleterious one-
electron/radical side reactions. Accordingly, investigations of
reaction mechanisms involving amino acid radicals require
methods to generate the radical on fast time scales. Of the
various time-resolved methods available, laser flash photolysis
techniques provide the largest temporal window for mechanistic
studies because reactions may be turned on “instantaneously”
with a pulse of light. Nowhere are the benefits of this approach
better exemplified than in the study of electron transfer (ET) in
biology. First developed for the ET reactions of modified
cytochromes2,3 and subsequently generalized to other proteins
and enzymes,4-13 the laser flash photolysis approach can initiate
benefits may be retained for mechanistic investigations of amino
acid radicals with the caveat that both electron and proton
inventories need to be managed14,15 in the generation of the
amino acid radical. The photon energy must, therefore, be
sufficiently energetic to overcome barriers for both electron and
proton transfer.16-21 We have been particularly interested in
photogenerating tyrosyl radical (Y•) and modified derivatives
for the study of the 35 Å proton-coupled electron transfer
(PCET) pathway of class I ribonucleotide reductases (RNRs).22-25
Methods have been developed to photogenerate Y• by laser
(13) Nocek, J. M.; Zhou, J. S.; Forest, S. D.; Priyadarshy, S.; Beratan, D. N.;
Onuchic, J. N.; Hoffman, B. M. Chem. ReV. 1996, 96, 2459.
(14) Cukier, R. I.; Nocera, D. G. Annu. ReV. Phys. Chem. 1998, 49, 337.
(15) Chang, C. J.; Chang, M. C. Y.; Damrauer, N. H.; Nocera, D. G. Biochim.
Biophys. Acta 2004, 1655, 13.
(16) Damrauer, N. H.; Hodgkiss, J. M.; Rosenthal, J.; Nocera, D. G. J. Phys.
Chem. B 2004, 108, 6315.
(17) Kirby, J. P.; Roberts, J. A.; Nocera, D. G. J. Am. Chem. Soc. 1997, 119,
9230.
(1) Stubbe, J.; van der Donk, W. A. Chem. ReV. 1998, 98, 705.
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J. Am. Chem. Soc. 1982, 104, 5798.
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J. Am. Chem. Soc. 1984, 106, 5145.
(4) Electron Transfer in Chemistry; Balzani, V., Ed.; Wiley-VCH: Weinheim,
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3783.
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1997, 263, 395.
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Chem. Soc. 2003, 125, 10506.
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9
9448
J. AM. CHEM. SOC. 2005, 127, 9448-9458
10.1021/ja0510360 CCC: $30.25 © 2005 American Chemical Society

Tyrosine Oxidation Using MLCT Excited States of Re Complexes
A R T I C L E S
excitation of tyrosine oxalate esters26 and through photoioniza-
tion of tryptophan (W), producing W• which in turn oxidizes Y
below pH 10.27 In both cases, a UV exciting photon is needed
to overcome the barriers for radical generation. Whereas the
intrinsic efficiency for Y• photogeneration is high, radical yields
are mitigated in the biological milieu owing to high absorption
cross-sections of UV photons by proteins.28 We, therefore, have
sought new photochemical methods for Y• generation in higher
yields using an excitation profile to the red of protein absorption
envelopes.
complex in the protein with the native Y108 residue appears to
produce •Y108; however, the reaction was not kinetically
characterized.38
The implementation of the flash-quench method becomes
more cumbersome when the electron and proton transfer events
are coupled to the chemistry of a functioning enzyme. This is
the case for the radical chemistry in RNR, where typical
quenchers of the flash-quench method can directly interfere with
allosteric regulation of the enzyme, radical intermediates of the
PCET pathway, and the chemistry attendant to substrate
activation and turnover. For these reasons, we are interested in
developing more reactive photo-oxidants that can efficiently
produce tyrosine radicals directly from an MLCT excited state
without the need for external quenchers. We now describe the
synthesis of ReI polypyridyl complexes with primary coordina-
tion spheres that are covalently modified with tyrosine along
with the kinetics and mechanism for the photogeneration of
tyrosyl radical directly from the MLCT excited state.
Metal-to-ligand charge transfer (MLCT) excited states of
metal complexes can be used to initiate biological redox
processes using visible light. The MLCT excited state of RuII
bipyridyl (bpy) and its derivatives has been especially useful
for redox processes involving metallo-cofactors.4 Owing to the
modest redox potential of RuII polypyridyl MLCT excited states,
their application is best-suited for the study of ET reactions
involving metal-containing cofactors with mild reduction po-
tentials, such as hemes,3 iron-sulfur clusters,29 and blue copper
proteins.30 The utility of the RuII polypyridyl MLCT excited
state may be extended when implemented as part of the flash-
quench method.31,32 With this technique, the more powerfully
reducing RuI or oxidizing RuIII centers are photogenerated by
bimolecularly quenching the MLCT excited state with an
external reductant or oxidant, respectively. The RuI or RuIII
complex persists with sufficient lifetime, owing to a slow
bimolecular back-reaction or irreversible quenching step, to
enable it to initiate a subsequent redox process on time scales
longer than that for the decay of the corresponding MLCT
excited state. Owing to the greater oxidizing and reducing power
of the flash-quenched products, as compared to that of the RuII
MLCT excited state, redox processes of nonmetal-based cofac-
tors may be initiated. The approach has been extended to amino
acid radicals by Hammarstro¨m and co-workers,33 who using the
flash-quench-generated RuIII(bpy)33+, observed oxidation of a
tyrosine residue covalently bound to one of the bpy ligands.
The rate of charge transport is 5 × 104 s-1, and tyrosyl radical
formation occurs by a PCET mechanism with proton loss to
bulk solvent.34,35 The rate for radical generation can be
accelerated by increasing the oxidizing strength of the flash-
quenched product. For instance, a Re(NN)(CO)3(Q107H)+-
modified azurin Y108W mutant can be flash-quenched to
generate the ReII oxidation state, which in turn has been
observed to oxidize the adjacent W108 residue to •W108 with
Experimental Section
Materials and Methods. L-Tyrosine tert-butyl ester (Y-OtBu),
L-alanine tert-butyl ester hydrochloride (A-OtBu‚HCl), 1-hydroxyben-
zotriazole (HOBt) (NovaBiochem), L-phenylalanine tert-butyl ester
hydrochloride (F-OtBu‚HCl) (Advanced ChemTech), magnesium
sulfate (MgSO4) (EMD), thallium hexafluorophosphate (TlPF6) (Strem),
potassium thiocyanate (KSCN), sodium cyanide (NaCN), 1-(3-(dim-
ethylamino)propyl)-3-ethylcarbodiimide hydrochloride (WSC‚HCl),
4-(dimethylamino)pyridine (DMAP), N-methylmorpholine (NMM),
pentacarbonylchlororhenium (Re(CO)5Cl), 1,10-phenanthroline (phen),
4-diphenylphosphinobenzoic acid (Ph2PPh-COOH) (Aldrich), and
trifluoroacetic acid (TFA, 99.9%) (J. T. Baker) were used as received.
4′-Methyl-2,2′-bipyridine-4-carboxylic acid (bpy-COOH),39 cis-1,2-
(bisdiphenylphospino)ethylene-(1,10-phenanthroline)(dicarbonyl)rhenium-
(I) hexafluorophosphate ([Re(phen)(dppe)(CO)2](PF6)),40 and tri-
carbonylchloro(1,10-phenanthroline)rhenium(I) (Re(phen)(CO)3Cl)41
were prepared as previously described. cis-1-Carboxy-2-(N-benzyl-
carbamoyl)-1,2-bis(diphenylphosphino)ethene (HOOC-PP-Bn)42 was
prepared from 2,3-bis(diphenylphosphino)maleic anhydride (bma)43 and
benzylamine (Aldrich).
Tricarbonylchloro(4′-methyl-2,2′-bipyridine-4-carboxylic acid)-
rhenium(I) (Re(bpy-COOH)(CO)3Cl). Re(CO)5Cl (347 mg, 0.962
mmol, 1.0 equiv) and bpy-COOH (206 mg, 0.962 mmol, 1.0 equiv)
were added to toluene (25 mL) contained in a round-bottom flask, and
the mixture was heated at reflux overnight. The bright orange solution
was cooled to room temperature, and the yellow/orange solid was
isolated by filtration, washed with ether, and dried under vacuum (470
1
mg, 94%). H NMR (300 MHz, d-acetone, 25 °C): δ ) 2.67 (s, 3H,
a rate constant of 2.8 × 106 s-1 36,37
.
Flash quench of the same
bpy-CH3), 7.66 (m, 1H, bpy-H), 8.21 (m, 1H, bpy-H), 8.81 (s, 1H,
bpy-H), 8.95 (d, 1H, bpy-H), 9.00 (s, 1H, bpy-H), 9.29 (d, 1H, bpy-
H). 1H NMR (300 MHz, CD3OD, 25 °C): δ ) 2.62 (s, 3H, bpy-CH3),
7.55 (m, 1H, bpy-H), 8.13 (m, 1H, bpy-H), 8.57 (s, 1H, bpy-H), 8.87
(d, 1H, bpy-H), 8.98 (s, 1H, bpy-H), 9.19 (d, 1H, bpy-H).
(26) Chang, M. C. Y.; Miller, S. E.; Carpenter, S. D.; Stubbe, J.; Nocera, D. G.
J. Org. Chem. 2002, 67, 6820.
(27) Reece, S. Y.; Stubbe, J.; Nocera, D. G. Biophys. Biochim. Acta 2005, 1706,
232.
(28) Chang, M. C. Y.; Yee, C. S.; Stubbe, J.; Nocera, D. G. Proc. Natl. Acad.
Sci. U.S.A. 2004, 101, 6882.
(29) Babini, E.; Bertini, I.; Borsari, M.; Capozzi, F.; Luchinat, C.; Zhang, X.;
Moura, G. L. C., IV; Kurnikov, D. N.; Beratan, A.; Ponce, A.; Di Bilio, A.
J.; Winkler, J. R.; Gray, H. B. J. Am. Chem. Soc. 2000, 122, 4532.
(30) Pascher, T.; Karlsson, B. G.; Nordling, M.; Malmstro¨m, B. G.; Vanngård,
T. Eur. J. Biochem. 1993, 212, 289.
(36) Miller, J. E.; Gra˜dinaru, C.; Crane, B. R.; Di Bilio, A. J.; Wehbi, W. A.;
Un, S.; Winkler, J. R.; Gray, H. B. J. Am. Chem. Soc. 2003, 125, 14220.
(37) Miller, J. E.; DiBilio, A. J.; Wehbi, W. A.; Green, M. T.; Museth, A. K.;
Richards, J. R.; Winkler, J. R.; Gray, H. B. Biochim. Biophys. Acta 2004,
1655, 59.
(38) Di Bilio, A. J.; Crane, B. R.; Wehbi, W. A.; Kiser, C. N.; Abu-Omar, M.
M.; Carlos, R. M.; Richards, J. H.; Winkler, J. R.; Gray, H. B. J. Am.
Chem. Soc. 2001, 123, 3181.
(39) McCafferty, D. G.; Bishop, B. M.; Wall, C. G.; Hughes, S. G.; Mecklenberg,
S. L.; Meyer, T. J.; Erickson, B. W. Tetrahedron 1995, 51, 1093.
(40) Schutte, E.; Helms, J. B.; Woessner, S. M.; Bowen, J.; Sullivan, B. P. Inorg.
Chem. 1998, 37, 2618.
(31) Chang, I.-J.; Gray, H. B.; Winkler, J. R. J. Am. Chem. Soc. 1991, 113,
7056.
(32) Bjerrum, M. J.; Casimiro, D. R.; Chang, I. J.; Di Bilio, A. J.; Gray, H. B.;
Hill, M. G.; Langen, R.; Mines, G. A.; Skov, L. K.; Winkler, J. R. J.
Bioenerg. Biomembr. 1995, 27, 295.
(33) Magnuson, A.; Berglund, P.; Korral, P.; Hammarstro¨m, L.; Åkermark, B.;
Styring, S.; Sun, L. C. J. Am. Chem. Soc. 1997, 119, 10720.
(34) Sjo¨din, M.; Styring, S.; Åkermark, B.; Sun, L.; Hammarstro¨m, L. J. Am.
Chem. Soc. 2000, 122, 3932.
(35) Sjo¨din, M.; Styring, S.; Wolpher, H.; Xu, Y.; Sun, L.; Hammarstro¨m, L. J.
Am. Chem. Soc. 2005, 127, 3855.
(41) Wrighton, M.; Morse, D. L. J. Am. Chem. Soc. 1974, 96, 998.
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9
J. AM. CHEM. SOC. VOL. 127, NO. 26, 2005 9449

A R T I C L E S
Reece and Nocera
Tricarbonylthiocyanato(4′-methyl-2,2′-bipyridine-4-carboxylic
acid)rhenium(I) (Re(bpy-COOH)(CO)3SCN). Re(bpy-COOH)(CO)3Cl
(200 mg, 0.385 mmol, 1.0 equiv) and KSCN (3.73 g, 38.5 mmol, 100
equiv) were combined in a flask with 20 mL of 1:1 95% ethanol/water
and refluxed overnight under N2. The ethanol was removed in vacuo,
and the mixture was extracted with ethyl acetate. The various extracts
were combined, dried over MgSO4, and the solvent was removed in
(300 MHz, d-acetone, 25 °C): δ ) 1.47 (m, 12H, tBu), 2.59 (s, 3H,
bpy-CH3), 4.41 (m, 1H, Y-CH), 7.61 (d, 1H, bpy-H), 8.02 (m, 1H,
bpy-H), 8.70 (s, 1H, bpy-H), 8.85 (d, 1H, bpy-H), 8.95 (s, 1H, bpy-
H), 9.13 (d, 1H, bpy-H), 9.31 (m, 1H, N-H) Anal. Calcd for C22H23-
ClN3O6Re: C, 40.83; H, 3.58; N, 6.49. Found: C, 40.96; H, 3.72; N,
6.52.
Tricarbonylchloro(4′-methyl-2,2′-bipyridine-4-tyrosine tert-butyl
ester)rhenium(I) (Re(bpy-Y-OtBu)(CO)3Cl). Re(CO)5Cl (467 mg,
1.29 mmol, 1.0 equiv) and bpy-Y-OtBu (560 mg, 1.29 mmol, 1.0 equiv)
were combined in a round-bottom flask with 50 mL of toluene, and
the mixture was heated at reflux overnight. The yellow suspension was
cooled in an ice bath, and the yellow solid was isolated by filtration,
washed with ether, and dried under vacuum (668 mg, 70%). 1H NMR
(300 MHz, d-acetone, 25 °C): δ ) 1.36 (s, 9H, tBu), 2.58 (s, 3H,
bpy-CH3), 3.01 (m, 2H, Y-CH2), 4.57 (m, 1H, Y-CH), 6.66 (d, 2H,
phenol-H), 7.08 (d, 2H, phenol-H), 7.61 (d, 1H, bpy-H), 7.97 (m, 1H,
bpy-H), 8.65 (s, 1H, bpy-H), 8.86 (m, 2H, bpy-H), 9.17 (m, 1H, bpy-
H), 9.33 (m, 1H, N-H) Anal. Calcd for C28H27ClN3O7Re: C, 45.50; H,
3.68; N, 5.68. Found: C, 45.36; H, 3.75; N, 5.74.
1
vacuo to furnish a red powder (136 mg, 65%). H NMR (300 MHz,
d-acetone, 25 °C): δ ) 2.70 (s, 3H, bpy-CH3), 7.73 (d, 1H, bpy-H),
8.28 (m, 1H, bpy-H), 8.87 (s, 1H, bpy-H), 8.99 (d, 1H, bpy-H), 9.07
(s, 1H, bpy-H), 9.33 (d, 1H, bpy-H). Anal. Calcd for C16H10N3O5ReS:
C, 35.42; H, 1.86; N, 7.75; S, 5.91. Found: C, 35.35; H, 1.81; N, 7.71;
S, 5.97.
Tricarbonylcyano(4′-methyl-2,2′-bipyridine-4-carboxylic acid)-
rhenium(I) (Re(bpy-COOH)(CO)3CN). Re(bpy-COOH)(CO)3Cl (200
mg, 0.385 mmol, 1.0 equiv) and NaCN (1.89 g, 38.5 mmol, 100 equiv)
were combined in a round-bottom flask with 40 mL of 1:1 95% ethanol/
water, and the mixture was heated at reflux for 6 h. The ethanol was
removed in vacuo, yielding a bright yellow precipitate, which was
isolated by filtration, washed with copious amounts of water and ether,
Tricarbonylthiocyanato(4′-methyl-2,2′-bipyridine-4-tyrosine tert-
butyl ester)rhenium (I) (Re(bpy-Y-OtBu)(CO)3SCN). Re(bpy-Y-
OtBu)(CO)3Cl (111 mg, 0.150 mmol, 1.0 equiv) and KSCN (1.46 g,
15 mmol, 100 equiv) were combined in a round-bottom flask with 40
mL of 1:1 95% ethanol/water and heated at reflux overnight. The
ethanol was removed in vacuo, and the red precipitate was isolated by
filtration, washed with copious amounts of water, and dried under high
1
and dried under vacuum (168 mg, 86%). H NMR (300 MHz, CD3-
OD, 25 °C): δ ) 2.63 (s, 3H, bpy-CH3), 7.54 (d, 1H, bpy-H), 8.03
(m, 1H, bpy-H), 8.49 (s, 1H, bpy-H), 8.89 (m, 2H, bpy-H), 9.08 (d,
1H, bpy-H). Anal. Calcd for C16H10N3O5Re: C, 37.65; H, 1.97; N,
8.23. Found: C, 37.53; H, 2.06; N, 8.20.
4′-Methyl-2,2′-bipyridine-4-alanine tert-butyl ester (bpy-A-OtBu).
Bpy-COOH (0.502 g, 2.34 mmol, 1.0 equiv), A-OtBu‚HCl (0.425 g,
2.34 mmol, 1.0 equiv), WSC‚HCl (0.493 g, 2.57 mmol, 1.1 equiv),
and HOBt (0.374 g, 2.57 mmol, 1.1 equiv) were combined in a round-
bottom flask. CH2Cl2 (100 mL) and NMM (1 mL, 9.4 mmol, 4 equiv)
were added, and the reaction mixture was stirred overnight. The reacted
solution was washed in a separatory funnel with 2 × 50 mL of water,
10% citric acid, and 1.0 M NaHCO3, and the organic layer was collected
and dried over MgSO4. The solvent was removed in vacuo to yield an
oil, which solidified under high vacuum as a white mass (0.5947 g,
1
vacuum (91 mg, 80%). H NMR (300 MHz, d-acetone, 25 °C): δ )
1.45 (s, 9H, tBu), 2.70 (s, 3H, bpy-CH3), 4.87 (m, 1H, Y-CH), 6.79
(m, 2H, phenol-CH2), 7.19 (m, 2H, phenol-CH2), 7.73 (m, 1H,
bpy-H), 8.12 (m, 1H, bpy-H), 8.46 (d, 1H, N-H), 8.69 (m, 1H,
bpy-H), 8.90 (m, 1H, bpy-H), 8.98 (m, 1H, bpy-H), 9.25 (m, 1H,
bpy-H).
Tricarbonylcyano(4′-methyl-2,2′-bipyridine-4-phenylalanine)rhe-
nium(I) (Re(bpy-F)(CO)3CN). Re(bpy-COOH)(CO)3(CN) (130 mg,
0.321 mmol, 1.0 equiv), F-OtBu‚HCl (72 mg, 0.321 mmol, 1.0 equiv),
WSC‚HCl (53 mg, 0.353 mmol, 1.1 equiv), and HOBt (37 mg, 0.353
mmol, 1.1 equiv) were combined in a round-bottom flask with 25 mL
of DMF. NMM (0.12 mL, 4.0 equiv) was added, and the mixture was
stirred overnight. The solvent was removed in vacuo, and the resulting
yellow oil was dissolved in 100 mL of CH2Cl2 and washed with 2 ×
25 mL water, 10% citric acid, 1.0 M NaHCO3, and water in that order.
The organic phase, isolated with a separatory funnel, was dried over
MgSO4, and the solvent was removed in vacuo. The resulting yellow
solid was dissolved in 10 mL of 1:1 TFA/CH2Cl2 and stirred for 1 h.
The solution was concentrated to <1 mL under a stream of N2 and
triturated with ether. The bright yellow solid that precipitated was
isolated by filtration, washed with copious amounts of ether, and dried
under vacuum (82 mg, 39%). 1H NMR (300 MHz, CD3OD, 25 °C): δ
) 2.64 (s, 3H, bpy-CH3), 3.13 (m, 2H, F-CH2), 4.96 (m, 1H, F-CH),
7.30 (m, 5H, F-ph), 7.57 (m, 1H, bpy-H), 7.86 (m, 1H, bpy-H), 8.49
(m, 1H, bpy-H), 8.79 (m, 1H, bpy-H), 8.89 (m, 1H, bpy-H), 9.14 (m,
1H, bpy-H). Anal. Calcd for C25H19N4O6Re: C, 45.66; H, 2.91; N,
8.52. Found: C, 45.48; H, 2.85; N, 8.42.
1
57%). H NMR (300 MHz, CDCl3, 25 °C): δ )1.47 (m, 12H, tBu
and A-CH3), 2.45 (s, 3H, bpy-CH3), 4.72 (m, 1H, A-CH), 7.07 (d, 1H,
N-H), 7.15 (d, 1H, bpy-H), 7.70 (d, 1H, bpy-H), 8.25 (s, 1H, bpy-H),
8.55 (d, 1H, bpy-H), 8.70 (s, 1H, bpy-H), 8.78 (d, 1H, bpy-H).
4′-Methyl-2,2′-bipyridine-4-tyrosine tert-butyl ester (bpy-Y-
OtBu). Bpy-COOH (0.750 g, 3.50 mmol, 1.0 equiv), Y-OtBu (0.830
g, 3.50 mmol, 1.0 equiv), WSC‚HCl (0.738 g, 3.85 mmol, 1.1 equiv),
HOBt (0.520 g, 3.85 mmol, 1.1 equiv), and DMAP (43 mg, 0.35 mmol,
0.1 equiv) were combined in a round-bottom flask. Methylene chloride
(CH2Cl2, 200 mL) and NMM (1.7 mL, 15.4 mmol, 4.4 equiv) were
added, and the reaction mixture was stirred overnight. The reacted
solution was washed in a separatory funnel with 4 × 100 mL of water,
and the organic layer was collected and dried over MgSO4. The solvent
was removed in vacuo to yield a yellow oil, which under high vacuum,
produced an off-white solid. The solid was dissolved in a few milliliters
of ethyl acetate, loaded onto a Chromatotron plate (alumina, 2 mm),
and eluted with ethyl acetate. The first band to elute was collected and
the solvent removed in vacuo to yield an oil, which solidified under
1
high vacuum as an off-white solid (0.5921 g, 39%). H NMR (300
MHz, CDCl3, 25 °C): δ )1.47 (s, 9H, tBu), 2.45 (s, 3H, bpy-CH3),
3.16 (m, 2H, Y-CH2), 4.96 (m, 1H, Y-CH), 6.68 (d, 2H, phenol-H),
7.02 (m, 2H, phenol-H and bpy-H), 7.17 (d, 1H, N-H), 7.70 (m, 1H,
bpy-H), 8.23 (s, 1H, bpy-H), 8.53 (d, 1H, bpy-H), 8.63 (s, 1H, bpy-
H), 8.78 (d, 1H, bpy-H).
Tricarbonylcyano(4′-methyl-2,2′-bipyridine-4-tyrosine)rhenium-
(I) (Re(bpy-Y)(CO)3(CN)). The synthesis of the tyrosine derivative
was the same as that for the phenylalanine derivative, except that Re-
(CO)3(bpy-COOH)(CN) (193 mg, 0.377 mmol, 1.0 equiv), WSC‚HCl
(80 mg, 0.415 mmol, 1.1 equiv), and HOBt (43 mg, 0.415 mmol, 1.1
equiv) were combined with Y-OtBu (90 mg, 0.377 mmol, 1.0 equiv),
Tricarbonylchloro(4′-methyl-2,2′-bipyridine-4-alanine tert-butyl
ester)rhenium(I) (Re(bpy-A-OtBu)(CO)3Cl). Re(CO)5Cl (500 mg,
1.38 mmol, 1.0 equiv) and bpy-A-OtBu (471 mg, 138 mmol, 1.0 equiv)
were combined in a round-bottom flask with 50 mL of toluene, and
the mixture was heated at reflux overnight. The yellow suspension was
cooled in an ice bath, and the yellow solid was isolated by filtration,
washed with ether, and dried under vacuum (722 mg, 81%). 1H NMR
1
instead of F-OtBu‚HCl (49%). H NMR (300 MHz, CD3OD, 25 °C):
δ ) 2.64 (s, 3H, bpy-CH3), 3.03 (m, 2H, Y-CH2), 4.87 (m, 1H, F-CH),
6.71 (d, 2H, phenol-H), 7.11 (d, 2H, phenol-H), 7.57 (m, 1H, bpy-H),
7.87 (m, 1H, bpy-H,), 8.48 (m, 1H, bpy-H), 8.76 (m, 1H, bpy-H), 8.89
(m, 1H, bpy-H), 9.15 (m, 1H, bpy-H). Anal. Calcd for C25H19N4O7Re:
C, 44.57; H, 2.84; N, 8.23. Found: C, 44.42; H, 2.76; N, 8.17.
9
9450 J. AM. CHEM. SOC. VOL. 127, NO. 26, 2005

Tyrosine Oxidation Using MLCT Excited States of Re Complexes
A R T I C L E S
Tricarbonyl(acetonitrile)(1,10-phenanthroline)rhenium(I) Hexa-
fluorophosphate ([Re(phen)(CO)3(CH3CN)](PF6)). Re(phen)(CO)3Cl
(400 mg, 0.823 mmol, 1.0 equiv) and TlPF6 (374 mg, 1.07 mmol, 1.3
equiv) were combined in a round-bottom flask with 40 mL of CH3CN
and heated at reflux in the dark overnight. The solvent was removed
in vacuo, leaving a yellow oil, which was dissolved in CH2Cl2. Any
remaining solids were filtered off, and ether was slowly dripped into
the solution to produce long yellow/green luminescent crystals, which
were isolated by filtration, washed with ether, and dried under vacuum
(450 mg, 86%). 1H NMR (300 MHz, d-acetone, 25 °C): δ ) 2.17 (s,
3H, CH3CN-Re), 8.27 (m, 2H, phen-H), 8.42 (m, 2H, phen-H), 9.11
(d, 2H, phen-H), 9.63 (d, 2H, phen-H). Anal. Calcd C17H11F6N3O3-
PRe: C, 32.08; H, 1.74; N, 6.60. Found: C, 32.15; H, 1.66; N, 6.47.
Tricarbonyl(diphenylphosphinobenzoic acid)(1,10-phenanthro-
line)rhenium(I) Hexafluorophosphate ([Re(phen)(Ph2PPh-COOH)-
(CO)3](PF6)). [Re(phen)(CO)3(CH3CN)](PF)6 (877 mg, 1.37 mmol, 1.0
equiv) and Ph2PPh-COOH (633 mg, 2.07 mmol, 1.5 equiv) were
combined in a round-bottom flask with 40 mL of N2-purged acetone.
The solution was heated at reflux overnight under a N2 atmosphere in
the dark. The solvent was removed in vacuo without heating, and the
yellow solid was taken up in CH2Cl2. The resulting light yellow
precipitate was isolated by filtration and washed with CH2Cl2 (1.118
g, 90%). 1H NMR (300 MHz, d-acetone, 25 °C): δ ) 7.05-7.80 (m,
14H, Ph2PPh-R), 8.02 (m, 2H, phen-H), 8.27 (s, 2H, phen-H), 8.90 (d,
2H, phen-H), 9.44 (d, 2H, phen-H). MALDI-TOF calcd (found): [M
- PF6]+ 757.09 (757.03); [M - CO,PF6]+ 729.10 (729.05). Anal. Calcd
C34H23F6N2O5P2Re: C, 45.29; H, 2.57; N, 3.11. Found: C, 45.21; H,
2.63; N, 3.06.
(0.258 g). The crude product, which contained some Re(HOOC-PP-
Bn)(CO)3Cl, was used in the next step without further purification.
MALDI-TOF calcd (found): [M - Cl]+ 800.11 (800.22).
[Re(phen)(PP-Bn)(CO)2](PF6). Re(PP-Bn)(CO)3Cl (150 mg, 0.171
mmol, 1.0 equiv), phen (34 mg, 0.188 mmol, 1.1 equiv), and TlPF6
(64 mg, 0.188 mmol, 1.1 equiv) were combined in a round-bottom
flask with 25 mL of anhydrous o-dichlorobenzene. The mixture was
bubbled with N2 for 30 min and heated at reflux for 5 h under N2. The
dark yellow solution was filtered over Celite, which was rinsed with
CH2Cl2. The solvent was then removed from the filtrate in vacuo; the
residue was dissolved in a minimal volume of CH2Cl2 and loaded onto
a Chromatotron plate (alumina, 1 mm). The product eluted with 10%
CH3CN/CH2Cl2 as a golden yellow band. The solvent was removed in
vacuo, yielding a yellow solid, which was characterized as a mixture
of two isomers. 31P NMR (300 MHz, CDCl3, 25 °C): δ ) 35.3 (d),
44.0 (d), 49.9 (d), 60.9 (d). MALDI-TOF calcd (found): [M - PF6]+
952.19 (952.25); [M - CO,PF6]+ 924.19 (924.25).
X-ray Crystal Structure of [Re(phen)(dppe)(CO)2](PF6) and [Re-
(phen)(PP-Bn)(CO)2](PF6)‚CDCl3. Single crystals of the complexes
were grown by slow evaporation from a concentrated solution in CDCl3.
A crystal was removed from the supernatant, coated with Paratone N
oil, and mounted on a glass fiber. X-ray diffraction data were collected
at -123 °C on a Siemens diffractometer equipped with a CCD detector,
using the Mo KR radiation. The data were integrated to hkl intensity,
and the unit cell was calculated using the SAINT version 4.050 program
from Siemens. Solution and refinement were performed by direct
methods (SHELXTL version 6.10, Sheldrick, G. M., and Siemens
Industrial Automation, Inc., 2000). The structures were solved by the
Patterson method; non-hydrogen atoms were refined anisotropically,
and hydrogen atoms were placed at calculated positions. Figure S1 and
Tables S1-S6 contain details regarding the refined data, cell parameters,
and results of the solution of the X-ray diffraction data of [Re(phen)-
(PP-Bn)(CO)2](PF6)‚CDCl3; similar data accompanying the X-ray
crystal structure of [Re(phen)(dppe)(CO)2](PF6) are presented in Figure
S2 and Tables S7-S12 of the Supporting Information.
[Re(phen)(P-Y)(CO)3](PF6). Re(phen)(CO)3(Ph2PPh-COOH)](PF6)
(200 mg, 0.222 mmol, 1.0 equiv), Y-OtBu (53 mg, 0.222 mmol, 1.0
equiv), WSC‚HCl (47 mg, 0.244 mmol, 1.1 equiv), HOBt (33 mg, 0.244
mmol, 1.1 equiv), and NMM (100 µL, 0.888 mmol, 4.0 equiv) were
combined in a round-bottom flask with 20 mL of CH2Cl2 and stirred
overnight. The solution was diluted with 50 mL of CH2Cl2, washed
with 2 × 25 mL of water, and dried over MgSO4. The solution was
concentrated to a few milliliters in vacuo and loaded onto a silica gel
Chromatotron plate (2 mm). The highly emissive band that eluted with
5% MeOH/CH2Cl2 was collected and the solvent removed in vacuo to
yield a yellow solid, which was dissolved in 10 mL of 1:1 TFA/CH2-
Cl2 and stirred for 1 h. The solution was concentrated under a stream
of N2. Addition of ether caused a yellow solid to precipitate; the solid
1
Physical Measurements. H and 31P NMR spectra were recorded
on a Varian Mercury 300 MHz NMR at the MIT Department of
Chemistry Instrumentation Facility (DCIF) and externally referenced
to tetramethylsilane (1H) and phosphoric acid (31P). MALDI-TOF mass
spectrometry was performed on a Bruker Omniflex instrument in the
DCIF using dithranol as the matrix. The instrument was calibrated with
a quadratic polynomial using a mixture of bradykinin fragment 1-7
(757.3997), angiotensin II (1046.5423), and P14R synthetic peptide
(1533.8582) (Sigma) with R-cyano-4-hydroxycinnamic acid as the
matrix. Elemental analysis was performed by H. Kolbe Mikroanalytis-
ches Laboratorium in Mu¨lheim an der Ruhr, Germany.
UV-vis absorption spectra were recorded on a Cary 17D modified
by On-Line Instrument Systems (OLIS) to include computer control
or a Spectral Instruments 440 spectrophotometer. Steady-state emission
spectra were recorded on an automated Photon Technology International
(PTI) QM 4 fluorimeter equipped with a 150 W Xe arc lamp and a
Hamamatsu R928 photomultiplier tube. Time-resolved emission and
transient absorption (TA) measurements on the >20 ns time scale were
made with pump light provided by the third harmonic (355 nm) of an
Infinity Nd:YAG laser (Coherent) running at 20 Hz as previously
described.44 Time-resolved emission measurements on the <20 ns time
scale were made with the frequency doubled (400 nm) pump light
provided by a Ti:sapphire laser system (100 fs pulse width) and
collected on a Hamamatsu C4334 Streak Scope streak camera as
previously described.16
1
was isolated by filtration and dried under vacuum (140 mg, 59%). H
NMR (300 MHz, d-acetone, 25 °C): δ ) 3.18 (m, 2H, Y-CH2), 4.89
(s, 1H, Y-CH), 6.85 (d, 2H, phenol-H), 7.20 (d, 2H, phenol-H), 7.27-
7.85 (m, 14H, Ph2-P-Ph-R), 7.98 (m, 2H, phen-H), 8.14 (s, 2H, phen-
H), 8.80 (m, 2H, phen-H), 9.42 (m, 2H, phen-H). 31P NMR (300 MHz,
d-acetone, 25 °C): δ ) 19.5 (s, Re-P). MALDI-TOF calcd (found):
[M - PF6]+ 920.15 (920.05); [M - CO,PF6]+ 892.16 (892.07). Anal.
Calcd C43H32F6N3O7P2Re: C, 48.50; H, 3.03; N, 3.95. Found: C, 48.42;
H, 2.95; N, 4.06.
[Re(phen)(P-F)(CO)3](PF6). The synthesis of the phenylalanine
derivative was the same as that for the tyrosine derivative, except that
F-OtBu‚HCl was used instead of Y-OtBu (74%). 1H NMR (300 MHz,
d-acetone, 25 °C): δ ) 3.29 (m, 2H, F-CH2), 4.93 (m, 1H, F-CH),
6.72-7.54 (m, 19H, F-phenyl and Ph2PPh-R), 7.97 (m, 2H, phen-H),
8.14 (s, 2H, phen-H), 8.81 (m, 2H, phen-H), 9.43 (m, 2H, phen-H).
31P NMR (300 MHz, d-acetone, 25 °C): δ ) 19.3 (s, Re-P). MALDI-
TOF calcd (found): [M - PF6]+ 904.16 (903.94); [M - CO,PF6]+
876.16 (875.98). Anal. Calcd C43H32F6N3O6P2Re: C, 49.24; H, 3.08;
N, 4.01. Found: C, 49.18; H, 3.04; N, 3.96.
The corrected emission spectra were analyzed by a standard Franck-
Condon analysis:45-47
Re(PP-Bn)(CO)3Cl. Re(CO)5Cl (126 mg, 0.14 mmol, 1.0 equiv)
and HOOC-PP-Bn (200 mg, 0.174 mmol, 1.0 equiv) were combined
in a flask with 25 mL of N2-purged toluene, and the mixture was heated
at reflux under N2 overnight. The solution was concentrated to a few
milliliters in vacuo and triturated with hexanes. The white precipitate
was isolated by filtration, washed with hexanes, and dried under vacuum
3
2
Sυ
5
E0 - υpω
Vj - E0 + υpω
∆Vj1/2
I(Vj) )
exp - 4ln2
(1)
∑
( )
{(
)
(
) }
]
[
E0
υ!
υ)0
9
J. AM. CHEM. SOC. VOL. 127, NO. 26, 2005 9451

A R T I C L E S
Scheme 1 a
Reece and Nocera
a Conditions: (a) WSC‚HCl, HOBt, NMM, CH2Cl2; (b) Re(CO)5Cl, 4, toluene; (c) KSCN, EtOH; H2O (1:1); (d) NaCN, EtOH:H2O(1:1); (e)
[Re(phen)(CO)3(MeCN)]PF6, acetone; (f) TFA:CH2Cl2(1:1).
in which I(Vj) is the corrected emitted intensity in quanta (I(λ) × λ2).
Synthesis. Scheme 1 highlights the two approaches under-
E0, S, frequency (pω), and bandwidth (∆Vj1/2) values for the emission
Results
The other parameters of eq 1 have been described elsewhere.46,47 The
taken in the syntheses of the amino acid containing ReI
spectra were obtained by fitting the emission bands to eq 1 using
complexes. The Re(bpy-AA-OtBu)(CO)3(X) complexes (AA )
amino acid, A, and Y; X ) Cl-, SCN-) were prepared by
precoupling the tert-butyl ester protected amino acid to the
carboxylic acid of the bpy ligand using standard peptide coupling
conditions. This modified bpy ligand was then added to the
appropriate ReI synthon, followed by deprotection of the amino
acid with trifluoroacetic acid. We note that these deprotection
conditions led to scrambling of the 1H NMR peaks in the bpy-
aryl region, and biexponential emission decay lifetimes were
observed for the acid-treated material. These results are
consistent with an exchange of the X- ligand by the deprotected
carboxylate of the coupled amino acid. Similar complexes,
Re(phen)(CO)3(L)+ (L ) pyridine, imidazole, His), have
previously been shown to exchange L for carboxylate in
reactions with acetic acid and/or synthetically appended car-
boxylate groups.51 For the preparation of Re(bpy-AA-OtBu)-
(CO)3(CN) (AA ) F and Y), exchange of X ) Cl- by CN-
did not proceed cleanly in the precoupling synthesis scheme,
and thus this method was abandoned. Instead, we found that a
post-coupling strategy proved to be more synthetically feasible,
in which bpy-COOH was first complexed to a ReI synthon
employing methods used to obtain other Re(NN)(CO)3Cl (NN
) polypyridyl) complexes.52 The Cl- anion was then exchanged
for CN- and the amino acid coupled to the Re(bpy-COOH)-
(CO)3(CN) complex. Unlike the X ) Cl- and SCN- derivatives,
the X ) CN- derivative proved to be inert to carboxylate
exchange under the conditions of amino acid deprotection, thus
enabling the isolation of pure Re(bpy-AA)(CO)3(CN) com-
pounds for spectroscopic study.
Microcal Origin. Without a direct and independent measure of these
parameters, the fit is overparametrized, and the values obtained are, at
best, an approximation. Relative quantum yields of samples, Φsam, were
calculated using [Ru(bpy)3](PF6)2 in CH3CN and water as the reference
actinometer according to
2
Aref Isam ηsam
Φsam ) Φref
(2)
(
)(I )(η
)
Asam
ref
ref
2+
Φref is the emission quantum yield of the Ru(bpy)3 reference taken
to be 0.062 in CH3CN48 and 0.053 in water.49 A is the measured
absorbance; η is the refractive index of the solvent, and I is the
integrated emission intensity, which was obtained by plotting and
integrating the entire emission band fitted using eq 1. Samples for
phosphorescence quantum yield and time-resolved spectroscopic mea-
surements were freeze-pump-thaw degassed for five cycles to 10-6
Torr. Unless otherwise noted, all spectroscopy was performed in 10
mM aqueous phosphate buffer with the pH adjusted using 1 M NaOH
or HCl. In cases of limited solubility, the samples were sonicated in a
Branson 3200 sonicator and filtered through a 0.2 micron Gelman
Acrodisc polypropylene syringe filter prior to measurement.
Electrochemical measurements were performed using a Bioanalytical
Systems (BAS) Model CV-50W potentiostat/galvanostat. Differential
pulse voltammetry (DPV) was performed using a platinum disk working
electrode, a Ag/AgCl reference electrode, and a platinum wire auxiliary
electrode. DPV experiments were performed in dry CH3CN with 0.1
M tetrabutylammonium hexafluorophosphate (Bu4N)PF6 as the sup-
porting electrolyte, and solutions were bubbled with Ar to remove
dissolved O2. All potentials are reported versus E°(Fc+/Fc) ) 0.65 V
versus NHE in CH3CN.50 Spectroelectrochemistry was performed in
dry DMF with 0.1 M (Bu4N)PF6 as the supporting electrolyte and Pt
gauze as the working electrode in a 2 mm path length quartz optical
cell. A blank spectrum was recorded on the solution prior to electrolysis
at -1.65 V versus Fc+/Fc. Spectra were recorded every 10 s for 5
min.
The amide linkage required for amino acid modification is
furnished from carboxylic acid-containing phosphine ligands of
+ 53-56
the analogous monodentate (P) Re(NN)(P)(CO)3
and
(51) Chang, M. C. Y. Ph.D. Thesis, Massachusetts Institute of Technology, 2003.
(52) Caspar, J. V.; Meyer, T. J. J. Phys. Chem. 1983, 87, 952.
(53) Caspar, J. V.; Sullivan, B. P.; Meyer, T. J. Inorg. Chem. 1984, 23, 2104.
(54) Hori, H.; Koike, K.; Ishizuka, M.; Takeuchi, K.; Ibusuki, T.; Ishitani, O.
J. Organomet. Chem. 1997, 530, 169.
(55) Koike, K.; Tanabe, J.; Toyama, S.; Tsubaki, H.; Sakamoto, K.; Westwell,
J. R.; Johnson, F. P. A.; Hori, H.; Saitoh, H.; Ishitani, O. Inorg. Chem.
2000, 39, 2777.
(44) Loh, Z.-H.; Miller, S. E.; Chang, C. J.; Carpenter, S. D.; Nocera, D. G. J.
Phys. Chem. A 2002, 106, 11700.
(45) Caspar, J. V.; Meyer, T. J. J. Am. Chem. Soc. 1983, 105, 5583.
(46) Chen, P.; Meyer, T. J. Chem. ReV. 1998, 98, 1439.
(47) Striplin, D. R.; Reece, S. Y.; McCafferty, D. G.; Wall, C. G.; Friesen, D.
A.; Erickson, B. W.; Meyer, T. J. J. Am. Chem. Soc. 2004, 126, 5282.
(48) Calvert, J. M.; Caspar, J. V.; Binstead, R. A.; Westmoreland, T. D.; Meyer,
T. J. J. Am. Chem. Soc. 1982, 104, 6620.
(56) Koike, K.; Okoshi, N.; Hori, H.; Takeuchi, K.; Ishitani, O.; Tsubaki, H.;
Clark, I. P.; George, M. W.; Johnson, F. P. A.; Turner, J. J. J. Am. Chem.
Soc. 2002, 124, 11448.
(49) Henderson, L. J., Jr.; Cherry, W. R. J. Photochem. 1985, 28, 143.
(50) Connelly, N. G.; Geiger, W. E. Chem. ReV. 1996, 96, 877.
9
9452 J. AM. CHEM. SOC. VOL. 127, NO. 26, 2005

Tyrosine Oxidation Using MLCT Excited States of Re Complexes
A R T I C L E S
Table 1. Spectroscopic and Photophysical Properties of the
MLCT Excited State of ReI Polypyridyl Carbonyl Complexes in
Aqueous Solution at Room Temperature
a
Eabs
Eem
Φem
τem
(ns)a
1
1 a
complex
(cm-
)
(cm-
)
Re(bpy-COOH)(CO)3Cl
Re(bpy-COOH)(CO)3SCN
Re(bpy-COOH)(CO)3CN
Re(bpy-F)(CO)3CN
[Re(phen)(P-F)(CO)3](PF6)
[Re(phen)(dppe)(CO)2](PF6)b
[Re(phen)(PP-Bn)(CO)2](PF6)b,c
∼28500 15700 1.1 × 10-3
27400 15100 1.6 × 10-3
28600 16400 1.4 × 10-2
27900 15800
5.5
12.0
86.5
59.4
27100 19100 9.4 × 10-2 3900
∼27000 16300 7.8 × 10-2 3200
940
a λexc ) 355 nm. b CH3CN solvent. c Isomer shown in Figure 1. Isolated
solids from synthesis exhibit biexponential behavior arising from the
presence of the two isomers (see text).
Figure 1. Thermal ellipsoid plot of a single isomer of [Re(phen)(PP-Bn)-
-
(CO)2]PF6‚CDCl3 shown at 50% probability. The PF6 anion, the solvent
molecule, and the phosphine phenyl groups have been removed for clarity.
+ 40
bidentate (PP) Re(NN)(PP)(CO)2
complexes. The parent
complex, [Re(phen)(dppe)(CO)2](PF6), originally synthesized by
Schutte et al.,40 has been obtained in single crystal form and
structurally characterized by X-ray diffraction (Figure S2, Tables
S7-S12). The dppe analogue can be derivatized by introducing
carboxylic acid functionality on the ethylene backbone in the
protected form as the 2,3-bis(diphenylphosphino)maleic anhy-
dride (bma). An amide bond linkage has been established by
nucleophilically attacking bma with benzylamine.42 This amide
functionalized ligand was adapted to ReI coordination chemistry
by reacting it with Re(CO)5Cl and subsequently chelating with
phen to yield the model complex, [Re(phen)(PP-Bn)(CO)2](PF6).
The extra carboxylic acid group, which remained on the Bn-
PP-COOH ligand upon ring opening the anhydride, was
observed to decarboxylate with the application of the heat
required for subsequent PP and phen chelation. The crystal
structure of one geometric isomer of [Re(phen)(PP-Bn)(CO)2]-
(PF6), shown in Figure 1, confirms the formation of the crucial
amide bond to one carboxylic acid and the decarboxylation of
the other. The two CO ligands adopt a cis stereochemistry, and
the phosphorus atom on the derivatized side of the chelating
ligand is trans to CO. The other isomer, which did not
crystallize, has this phosphorus trans to the nitrogen of phen.
Tables S3 and S4 list the bond lengths and angles, respectively,
obtained from the solution of the X-ray structure. The bite angles
of the PP-Bn and phen ligands ( P(1)-Re(1)-P(2) ) 80.72°
and N(1)-Re(1)-N(2) ) 75.37°) create a distorted octahedral
coordination geometry. The Re-C bond distances are slightly
longer (d(Re-C) ) 1.902 and 1.928 Å) and the C-O bond
distances slightly shorter (d(C-O) ) 1.169 and 1.164 Å) than
a crystallographically characterized relative, Re(CO)2(P(OEt)3)-
(PPh3)(4,4′-Me2-2,2′-bpy)+, which features two monodentate
phosphine ligands trans to one another (d(Re-C) ) 1.84(3)
and 1.88(3) Å; d(C-O) ) 1.18(4) and 1.17(3) Å).55
Figure 2. Normalized, corrected emission spectra for Re(bpy-COOH)-
(CO)3(X) [X ) CN- (green line), Cl- (orange line), SCN- (red line)] and
[Re(phen)(P-F)(CO)3](PF6) (blue line) in 10 mM phosphate buffer at pH 7
and [Re(phen)(dppe)(CO)2](PF6) (black line) in CH3CN.
upon amino acid deprotection, as exhibited by the single,
downfield shifted peak in the 31P NMR spectrum (versus free
phosphine), satisfactory MALDI-TOF and elemental analysis,
and the single-exponential behavior of the emission decay (vide
infra).
Photophysics. The spectroscopic properties for the ReI
polypyridyl carbonyl complexes are summarized in Table 1.
The MLCT transition for the ReI polypyridyl carbonyl com-
plexes appears as a shoulder on the strong π f π* absorption
profile of the polypyridyl ligand. As has been observed
previously,56 the MLCT absorption band of the biscarbonyl
complexes is red shifted from that of their tricarbonyl counter-
parts. Excitation into the MLCT absorbance manifold produces
the broad, structureless emission bands shown in Figure 2, where
the intensity has been scaled in quanta (I × λ2) and normalized.
The emission maximum (Eem) of Re(bpy-COOH)(CO)3(X)
decreases across the series X ) CN- > Cl- > SCN-.
Derivatization of the bpy ligand with an amino acid causes a
red shift of the MLCT emission, as evidenced from the
The carboxylic acid derivatives of the monodentate and
bidentate phosphines can be modified with amino acids. We
present the chemistry of only the former owing to their greater
water solubility and higher MLCT energy (vide infra). The [Re-
(phen)(P-AA)(CO)3](PF6) (AA ) F and Y) complexes were
synthesized using the AA-ligand post-coupling strategy of
Scheme 1. These compounds were inert to carboxylate exchange
9
J. AM. CHEM. SOC. VOL. 127, NO. 26, 2005 9453

A R T I C L E S
Reece and Nocera
Table 2. Emission Spectral Parameters Obtained from Fitting
comparison of the Re(bpy-COOH)(CO)3(CN) and Re(bpy-F)-
(CO)3(CN) emission maxima in Table 1. Numerous studies of
the room temperature emission properties of ReI(NN)(CO)3L+
complexes establish the emission to be of 3MLCT parentage.57
A more detailed analysis of ReI(NN)(CO)3Cl photophysics
reveals that the MLCT emission arises from three states of triplet
orbital parentage in thermal equilibrium at room temperature.58
The situation may be further complicated by the presence of
an intraligand 3ππ* excited state that is energetically proximate
to the 3MLCT excited state, as observed for ReI(s-phen)(CO)3-
Cl complexes (s-phen ) 4,7-Me2-1,10-phen, 5,6-Me2-1,10-phen,
3,4,7,8-Me2-1,10-phen).59 The case of energetically proximate
intraligand and charge-transfer excited states, however, does not
appear to be pertinent to ReI(phen)(CO)3(phosphine)+ complexes
because the room temperature emission does not exhibit vibronic
structure, which is a signature of 3ππ* emission from transition-
metal polypyridyl complexes.60 Accordingly, we ascribe the
emission band of ReI(phen)(CO)3(P-F)+ shown in Figure 2 to
a predominantly 3MLCT parentage, as assigned for other
Re(phen)(CO)3(L)+ (L * phosphine) complexes with long
lifetimes and similar emission envelopes.61 As noted previously,
removal of a CO ligand to form ReI(NN)(dicarbonyl)+ com-
Corrected Emission Band to eq 1, and Energy of the MLCT
Excited State Calculated from eq 3
E0
(cm-
S
pω
(cm-
∆ν1/
¯
∆
GoMLCT
(eV)
2
1
1
1
compound
)
)
(cm-
)
Re(bpy-COOH)(CO)3Cl
Re(bpy-COOH)(CO)3SCN
Re(bpy-COOHOH)(CO)3CN
Re(bpy-F)(CO)3CN
16400 1.0 1900
16300 1.5 1700
16800 0.8 2000
16500 0.9 1800
20300 1.8 1200
2800
3000
3100
3200
1800
2600
2.47
2.52
2.60
2.59
2.70
2.42
[Re(phen)(P-F)(CO)3](PF6)
[Re(phen)(dppe)(CO)2](PF6)a 16700 0.7 1700
a Solvent is CH3CN.
paralleled by a decrease in the emission quantum yields (Φem
)
and lifetimes (τem) listed in Table 1. This behavior is in
accordance with the predictions of the energy gap law.57
There are several other notable features of the Φem and τem
listed in Table 1. (1) [Re(phen)(dppe)(CO)2](PF6) in CH3CN
exhibits a slightly faster τem and lower Φem than that previously
reported in the less polar CH2Cl2 solvent.40 (2) Solutions of the
benzyl amide conjugate, [Re(phen)(PP-Bn)(CO)2](PF6), exhibit
biexponential emission decay behavior (τ1 ) 1.1 µs and τ2 )
4.3 µs) as a result of a mixture of geometric isomers. A crystal
of the isomer characterized by X-ray diffraction (shown in
Figure 1) dissolved in CH3CN exhibits a monoexponential decay
(τ ) 0.96 µs) equivalent to τ1 of the isomer mixture. We,
therefore, ascribe the short lifetime decay to the excited state
of the isomer shown in Figure 1 and the longer lifetime decay
to its isomeric congener. (3) The [Re(phen)(CO)3(P-F)](PF6)
3
plexes lowers the energy of the MLCT excited state relative
to their tricarbonyl counterparts. In accordance with these
observations, the emission band maximum of [Re(phen)(dppe)-
(CO)2](PF6) is 2800 cm-1 to the red of [Re(phen)(P-F)(CO)3]-
(PF6).
The precise determination of the 3MLCT excited-state energy
of the ReI polypyridyl carbonyl complexes is problematic for
several reasons. Location of the E0,0 for the emitting state cannot
be deduced from the overlap of the absorption and emission
bands, as predicted by the Franck-Condon principle, because
complex exhibits the longest τem and highest Φem and ∆Go
MLCT
of all the complexes in this study. The more energetic excited
state and/or disposition of a phosphine trans to a CO may be
responsible for a photolysis reaction that we observe for this
complex upon extensive laser irradiation. A similar photo-
chemical behavior observed for ReI(NN)(CO)3(PR3)+ complexes
upon irradiation with light (λ > 320 nm) has been attributed to
loss of a CO ligand.56
1
absorption occurs into the MLCT state and emission occurs
from the corresponding 3MLCT state. Moreover, the room
temperature emission bands of d6 polypyridyl complexes are
usually broad and featureless, and thus the υ′ ) 0 f υ ) 0
transition cannot be directly observed within the emission
envelope. Evaluation of the emission energy from low-temper-
ature glasses of ReI complexes is further complicated by the
rigidochromic effect.41,62
The high values of ∆GoMLCT for the amino acid conjugates,
[Re(phen)(CO)3(P-F)](PF6) and Re(bpy-F)(CO)3CN, make these
platforms excellent candidates for promoting the oxidation of
tyrosine directly from the excited state. The excited-state
reduction potentials for the ReI polypyridyl tricarbonyl com-
plexes are listed in Table 3 along with the modified Latimer
diagram from which they were determined. The portion of the
diagram relevant to tyrosine oxidation is highlighted in black.
The ground state E°(ReI/0) and E°(ReII/I) reduction potentials
were determined from differential pulse voltammetry (DPV)
measurements. Reversible electrochemical behavior is observed
for the E°(ReI/0) redox process, whereas the E°(ReII/I) redox
couple is quasi-irreversible and is, therefore, reported as a peak
potential, Ep, in Table 3.
In the absence of a direct measure of the excited-state energy,
a Franck-Condon analysis of the emission band shapes was
employed. The values of E0, S, frequency (pω), and bandwidth
(∆Vj1/2) listed in Table 2 were obtained by fitting the emission
spectra shown in Figure 2 to eq 1. The E0 and ∆Vj1/2 values
may be used calculate the free energy of the MLCT excited
state, ∆GoMLCT, according to the following:45,46
2
(∆Vj1/2
)
∆GoMLCT ) E0 +
(3)
16 kBT ln(2)
With the excited-state reduction potentials in hand, we next
investigated the facility of tyrosine to quench the emission of
these powerful excited-state oxidants. A comparison of the
luminescence properties of Re(bpy-AA)(CO)3CN (AA ) Y and
F) complexes in 10 mM aqueous phosphate buffer at pH 7
established no phosphorescence quenching of the tyrosine
complex. However, deprotonation of the tyrosine phenol (pKa
) 10.1 for L-tyrosine)63 at pH 12 induces significant emission
The values of ∆GoMLCT are listed in Table 2. Within the
homologous polypyridyl series, Re(bpy)(CO)3X, the decrease
in ∆GoMLCT across the series X ) CN- > SCN- > Cl- is
(57) Stufkens, D. J.; Vle`ek, A., Jr. Coord. Chem. ReV. 1998, 177, 127.
(58) Striplin, D. R.; Crosby, G. A. Chem. Phys. Lett. 1994, 426, 426.
(59) Striplin, D. R.; Crosby, G. A. Coord. Chem. ReV. 2001, 211, 163.
(60) Ayala, N. P.; Flynn, C. M., Jr.; Sacksteder, L.; Demas, J. N.; DeGraff, B.
A. J. Am. Chem. Soc. 1990, 112, 3837.
(61) Sacksteder, L.; Zipp, A. P.; Brown, E. A.; Streich, J.; Demas, J. N.; DeGraff,
B. A. Inorg. Chem. 1990, 29, 4335.
(62) Lumpkin, R. S.; Meyer, T. J. J. Phys. Chem. 1986, 90, 5307.
(63) Tommos, C.; Skalicky, J. J.; Pilloud, D. L.; Wand, S. J.; Dutton, P. L.
Biochemistry 1999, 38, 9495.
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9454 J. AM. CHEM. SOC. VOL. 127, NO. 26, 2005

Tyrosine Oxidation Using MLCT Excited States of Re Complexes
A R T I C L E S
Table 3. Differential Pulse Voltammetrya and Excited-State
Reductionb and Oxidation Potentialsc of Amino Acid-Derivatized
ReI Polypyridyl Tricarbonyl Complexes
Ep
E
°
E
°
E
°
/
I
a
0
a
/
0
b
/
II c
complexes
(ReII
)
(ReI/
)
(ReI*
)
(ReI*
)
Re(bpy-A-OtBu)(CO)3Cl
Re(bpy-F)(CO)3CN
[Re(phen)(P-F)(CO)3](PF6)
0.94
1.09
1.52
-1.58
-1.65
-1.57
1.59
1.78
0.85
0.53
a Reported in volts vs Fc+/Fc with 0.1 M (Bu4N)PF6 as supporting
electrolyte in CH3CN. b E°(ReI*/0 ∆GMo LCT E°(ReI/0
vs NHE.
c E°(ReI*/II) ) ∆GMo LCT- E°(ReII/I) vs NHE.
)
)
+
)
quenching. A rate constant for this process may be determined
from the excited-state lifetimes in the presence and absence of
quenching
Figure 3. Plot of the rate constant for emission quenching, kq, versus pH
for [Re(P-Y)(phen)(CO)3]PF6. Three pH regions are highlighted corre-
sponding to the protonated (black circles) and deprotonated (red circles)
forms of the tyrosine carboxylic acid and deprotonated (green circles)
tyrosine phenol.
1
1
kq )
-
(4)
nm excitation of the Re(bpy-Y)(CO)3(CN) complex in pH 7
and 12 solutions are presented in Figure 4. The ∆OD spectrum
at pH 7 is the same as that obtained for Re(bpy-F)(CO)3(CN)
at all pHs (not shown) and consists of maxima at 380 and ∼475
nm and a minimum at 400 nm. These features are in accord
τY τF
where τY and τF are the lifetimes of emission decay for the Y
and F derivatives, respectively. The measured lifetime of τY )
1.2 ns for Re(bpy-Y)(CO)3CN yields a kq ) 8.2 × 108 s-1
.
3
with that observed for the MLCT state of ReI(NN)(CO)3L
In contrast to the pH behavior of Re(bpy-Y)(CO)3CN
quenching, the emission from [Re(phen)(P-Y)(CO)3](PF6) was
quenched at all pHs. At pH 7, τY ) 2.2 µs and Φem,Y ) 0.059
for the tyrosine derivative whereas τF ) 3.9 µs and Φem,F )
0.094 for the phenylalanine derivative (τF and Φem,F are
independent of pH), yielding a quenching rate constant of kq )
2.0 × 105 s-1. As noted above, the [Re(phen)(P-AA)(CO)3](PF6)
complexes are subject to CO dissociation under prolonged
irradiation. We contend that the kinetics for CO dissociation
upon complex excitation are the same for the F and Y derivatives
and do not vary with pH. Under this assumption, eq 4 isolates
the quenching process due to Y. kq for [Re(phen)(P-Y)(CO)3]-
(PF6) exhibits the pH dependence shown in Figure 3 as a result
of the pH dependent τY. The points are separated into three
color-coded regions to represent the different protonation states
of the tyrosine residue; black and red points correspond to the
protonated and deprotonated forms of the tyrosine carboxylic
acid, respectively, while the green points are for the deprotonated
tyrosine phenol. Above pH 10, the quenching of the excited
state increases significantly; the vestige of this increase is
indicated by the displaced rate constant at pH 10. The quenching
mechanism appears to change at basic pHs due to the afore-
mentioned lability of the sixth coordination site of Re(CO)3-
(NN)+. We believe that phosphine is substituted upon intra-
molecular attack of the phenolate oxygen of the deprotonated
tyrosine. We rule out an intermolecular attack by hydroxide as
the excited-state lifetime of [Re(phen)(P-F)(CO)3](PF6) does not
change at very basic pHs.
complexes.57,64 In support of a similar assignment here, the
single wavelength transient signals obtained at 380 and 475 nm
shown in the top insets of Figure 4 decay to baseline with time
constants (61 and 60 ns, respectively) identical to the emission
3
lifetime of the MLCT excited state (60 ns). In contrast, time-
resolved absorption spectra of Re(bpy-Y)(CO)3(CN) at pH 12
are not that of the 3MLCT excited state. The transient profile is
dominated by absorptions at 410 and 520 nm. The 410 nm
feature coincides with the absorption maximum of Y•,65 and
the 520 nm feature resembles the absorption maxima of
bpy•-.66,67 With regard to the latter, the TA feature is similar to
the spectroelectrochemical UV-vis absorption spectrum of the
reduced Re(bpy•--Y-OtBu)(CO)3CN in DMF (shown in Figure
S3 in the Supporting Information). These results provide direct
evidence for the charge separated state, Re(bpy•--Y•)(CO)3(CN).
The time-evolved gray traces establish that the overall profile
does not change with time. The single wavelength transient
signals obtained at 410 and 500 nm and shown in the bottom
insets decay concomitantly with a rate constant of kCR ) 2.2 ×
107 s-1, which we attribute to charge recombination. The low
solubility of [Re(phen)(P-Y)(CO)3](PF6) in water prevented its
investigation by transient absorption methods.
Discussion
Rhenium(I) polypyridyl complexes provide suitable platforms
for the intramolecular photogeneration of tyrosyl radicals, owing
(64) Kalyanasundaram, K.; Graetzel, M.; Nazeeruddin, M. K. Inorg. Chem. 1992,
31, 5243.
(65) Bent, D. V.; Hayon, E. J. Am. Chem. Soc. 1975, 97, 2599.
(66) Damrauer, N. H.; McCusker, J. K. J. Phys. Chem. A 1999, 103, 8440.
(67) Kreje`´ık, M.; Vle`ek, A. A. J. Electroanal. Chem. 1991, 313, 243.
Nanosecond transient absorption spectroscopy establishes that
the quenching process of the tyrosine derivatives occurs by
charge transfer. The transient absorption profiles following 355
9
J. AM. CHEM. SOC. VOL. 127, NO. 26, 2005 9455

A R T I C L E S
Reece and Nocera
The energetics of the PCET contribution to Y• generation is
accounted for by the pH dependence of E°(Y•/Y)63
2.303RT
10-pH
Eo(Y•/Y) ) Eo(Y•/Y-) +
log 1 +
(6)
10-pK (Y)
(
)
a
nF
where E°(Y•/Y-) is the reduction potential for forming the
deprotonated tyrosine phenolate (0.65 V vs NHE), and pKa(Y)
is that for the tyrosine phenol (9.9 for N-acetyl-L-tyrosinamide).63
The overall form of the pH-dependent quenching, shown in
Figure 3, is in accordance with the predictions of eq 6. At low
pH, tyrosine is more difficult to oxidize, and rates for Y•
generation should be slowest. A smoothly varying monotonic
increase in the rate constant is expected as the pH is increased
to the pKa of the tyrosine. This overall behavior is generally
observed excepting a discontinuity at pH ∼ 5, as indicated by
the different color coding in Figure 3. At this pH, the carboxylic
acid group of the amino acid backbone is deprotonated,
augmenting the dipole moment of the ground-state complex in
a direction favorable for ET.27,68,69 The slight displacement of
the quenching rate constants and increase in slope in the pH )
5-9 range are consistent with this behavior. After pH )
pKa(Y), the rate constant should be invariant to pH changes as
is E°(Y•/Y-) for the deprotonated tyrosine phenolate. The green
points represent the ET rate for oxidation of the deprotonated
tyrosine phenolate without interference of the proton. A similar
jump in the rate constant of tyrosine oxidation upon its
deprotonation has been observed by Sjo¨din et al.,34 who used a
flash-quench-generated RuIII oxidant to photogenerate tyrosyl
radical appended to a bpy ligand.
The inability to oxidize Y with the shorter-lived MLCT
excited state of the Re(bpy-Y)(CO)3(CN) complex is also
consistent with a PCET mechanism. The PCET rate constant
for intramolecular quenching of [Re(phen)(CO)3(P-Y)]PF6 (at
pH < 9) is an order of magnitude slower than the ET rate
constant for [Re(phen)(CO)3(P-Y-)]PF6 oxidation (at pH > 9).
A similar order of magnitude decrease in the rate of Y• formation
Figure 4. Top: Transient absorption spectra following 355 nm laser
excitation (fwhm ) 3 ns) of pH 7 solution of Re(bpy-Y)(CO)3(CN) at 15
(black line) and 65, 115, and 215 ns (gray line, top to bottom). Insets: Single
wavelength kinetic traces and monoexponential fits for the disappearance
of the 380 and 475 nm (3MLCT) signals. Bottom: Same experiment at pH
12 over the same time course of the top panel. Insets: Single wavelength
kinetic traces and monoexponential fits for the disappearance of the 410
(Y•) and 500 nm (bpy•-) signals.
for Re(bpy-Y)(CO)3(CN) would furnish a PCET rate of 106 s-1
,
which barely competes with the normal excited-state decay (107
s-1). Thus, our inability to observe excited-state quenching for
this complex at pHs where Y is protonated is reasonable.
Conversely, the higher driving force for Y oxidation in
to the significant oxidizing power of their MLCT excited states.
The driving force for the intramolecular MLCT quenching
process, ∆GoPCET, is
3
[Re(phen)(CO)3(P-Y)]PF6, coupled to its longer-lived MLCT
excited state, allows for the relatively sluggish PCET reaction
(kPCET ) 1.6 × 105 s-1 at pH 7) to be observed.
∆GoPCET ) E°(ReI*/0) - E°(Y•/Y)
(5)
Intramolecular shuttling of electrons between tyrosine and
the MLCT excited state may occur along two fundamentally
different pathways, designated in Scheme 2 as pop-pop and
pop-fizz. The latter describes the electron-transfer events
induced by excitation of typical Re(NN)(CO)3X complexes, in
which the donor is attached to the polypyridyl ligand, as is the
case for Re(bpy-Y)(CO)3CN. Photon absorption (“pop”) forms
the 3[ReII(bpy•--Y)(CO)3CN] MLCT state. Subsequent tyrosine
oxidation occurs by a contraposed electron flow, whether
through the bpy or through space. The pathway is anticipated
to incur several barriers to Y• generation. Oxidation of tyrosine
brings the electron through the reduced bpy ligand. Though the
effect of superexchange through a reduced polypyridyl ligand
where E°(ReI*/0) is given in Table 3, and E°(Y•/Y) is the
previously reported pH-dependent reduction potential for Y•.63
In the above expression, ∆GoPCET is not corrected for Debye-
Huckel electrostatic work terms since the Y• radical is neutral.
The ReI complexes have considerable overpotential for the
oxidation of tyrosine, but two observed results suggest that Y•
generation does not proceed by a simple ET. (1) Despite
significant oxidizing strength at all pHs, the MLCT excited state
of Re(bpy-Y)(CO)3CN is only a competent oxidant of tyrosine
when it is deprotonated (pH > 10). (2) Although [Re(P-Y)-
(phen)(CO)3]PF6 oxidizes Y at all pHs, Figure 3 shows that the
rate constant for this process is pH-dependent. Taken together,
these results clearly indicate a pronounced role for the proton
in Y• generation and implicate proton-coupled electron transfer
(PCET) as the mechanism for tyrosine oxidation.
(68) Batchelder, T. L.; Fox, R. J., III; Meier, M. S.; Fox, M. A. J. Org. Chem.
1996, 61, 4206.
(69) Galoppini, E.; Fox, M. A. J. Am. Chem. Soc. 1996, 118, 2299.
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Tyrosine Oxidation Using MLCT Excited States of Re Complexes
A R T I C L E S
Scheme 2
has not been investigated critically, general theory suggests that
the increased energy gap between reduced donor and oxidized
acceptor levels would retard the tunneling rate for Y• genera-
tion.70 More problematic is the favored back reaction resulting
from the disposition of a neighboring hole and electron on Y•
and bpy•-, respectively. The correlation between the back
electron-transfer rate and proximity of hole/electron pairs has
been examined in a comparative study of ReI(bpy)(CO)3(py)+
complexes containing a phenothiazine (PTZ) electron donor
coupled to the pyridine ligand in Re(4,4′-Me2-bpy)(CO)3-
(py-PTZ)+ and bipyridine ligand in Re(PTZ-CH2-bpy)(CO)3-
(4-Etpy)+.71 An accelerated back electron-transfer rate was
established for the former, which exhibits a Re(PTZ•+-CH2-
bpy•-)(CO)3(4-Etpy)+ charge distribution upon intramolecular
quenching of the MLCT excited state. A similar charge
distribution is retained in the charge-separated state of the
tyrosine-modified ReI polypyridyl complex, ReI(bpy•--Y•)(CO)3-
CN, thus accounting for its short lifetime of 45 ns (“fizz”).
The hole/electron pair is optimally separated by appending
the tyrosine ligand to the sixth ligand, away from the polypyridyl
ligand. We chose phosphine ligands as the scaffold for the amino
acid owing to the high energies and long lifetimes of Re(NN)-
(PP)(CO)2+ and Re(NN)(P)(CO)3+ model compounds. Of these
two platforms, the latter was more ideally suited to Y• generation
owing to a greater water solubility and intrinsically higher
MLCT excited-state energy. The architecture of the ReI(phen)-
(CO)3(P-Y)+ complex permits the charge distribution in the
pop-pop pathway of Scheme 2 to be established. Excitation
pops the electron to the phen ligand in the MLCT state to
produce ReII(phen•-)(CO)3(P-Y)+, which then allows for the
electron to flow from the phosphine ligand, quenching the ReII
hole, and furnishing the ReI(phen•-)(CO)3(P-Y•)+ radical in-
termediate. The electron and hole are disposed at maximal
distance within the primary coordination sphere of the complex,
presumably retarding the rate of charge recombination.
overpotential for tyrosine oxidation. The additional energy of
the MLCT excited state is crucial for amino acid oxidation
because, unlike a simple ET quenching process, it is needed to
overcome the barrier for proton transfer, as well.34,72 Second,
the phosphine complexes exhibit a dramatically extended
lifetime of the MLCT excited state, allowing for a higher Stern-
Volmer constant (SV ) kqτo) for quenching.73 Finally, by
appending the tyrosine to an ancillary ligand, remote from the
electron-accepting polypyridyl ligand in MLCT excitation, the
intramolecular shuttling occurs via a unidirectional electron
cascade for the efficient generation and preservation of the
photogenerated Y•.
The ReI phosphine scaffolds presented here provide a
convenient method for investigating radical-based mechanisms
in biology because, unlike previous approaches, the amino acid
radical can be generated directly and on microsecond time scales
without the need for external oxidants or reductants. Moreover,
unlike previous ReI complexes used to photogenerate tyrosyl
radicals, such as ReI(phen)(CO)3(His)+, the ReI phosphine
complexes are resistant to carboxylate exchange. Thus, these
MLCT states may be used to initiate and study radical reactions
on peptides or within proteins containing multiple aspartate or
glutamate residues. Finally, we note that this method is not
necessarily specific to tyrosine but can be extended to other
amino acids by simple derivatization of the monophosphine.
The complexes reported here are particularly convenient for
phototriggering amino acid radicals in biological PCET path-
ways because the kinetics are not convoluted with the bi-
molecular events of the flash-quench process. The simplicity
of this approach is especially beneficial for the study of the
PCET pathway in class I RNR, which catalyzes the conversion
of nucleotides to deoxynucleotides, providing all of the mon-
omeric precursors required for DNA replication and repair in
the host organism.74 Enzyme function is derived by connecting
a radical initiation site (•Y122, E. coli numbering) to a cysteine
(C439) at the active site of substrate turnover through a 35 Å
radical transport pathway that spans two subunits (R2 and R1).75
This intersubunit pathway is postulated to be gated by a tyrosine,
Y356,22-25 which is located near the C-terminus of R2 and is
the bellwether of a YYY pathway, Y356 f Y731 f Y730 f
Conclusions
Phosphines of ReI polypyridyl complexes are ideal platforms
for the direct intramolecular photogeneration of amino acid
radicals, and especially tyrosine for three reasons. First, the
MLCT excited state is highly energetic and presents a significant
(72) Sjo¨din, M.; Ghanem, R.; Polivka, T.; Pan, J.; Styring, S.; Sun, L.;
Sundstro¨m, V.; Hammarstro¨m, L. Phys. Chem. Chem. Phys. 2004, 6, 4851.
(73) Balzani, V.; Moggi, L.; Manfrin, M. F.; Bolletta, F. Coord. Chem. ReV.
1975, 15, 321.
(74) Jordan, A.; Reichard, P. Annu. Rev. Biochem. 1998, 67, 71.
(75) Uhlin, U.; Eklund, H. Nature 1994, 370, 533.
(70) Lewis, F. D.; Liu, J.; Weigel, W.; Rettig, W.; Kurnikov, I. V.; Beratan, D.
N. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 12536.
(71) Chen, P.; Duesing, R.; Graff, D. K.; Meyer, T. J. J. Phys. Chem. 1991, 95,
5850.
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J. AM. CHEM. SOC. VOL. 127, NO. 26, 2005 9457

A R T I C L E S
Reece and Nocera
C439. Radical transport along this pathway may be examined
by synthesizing modified variants of the 20-mer C-terminal
peptide tail of the R2 subunit. This peptide segment preserves
the binding determinant of R2 to R1, and it contains the critical
Y356, as well. The inclusion of phototriggers proximate to Y356
in the heavily carboxylated C-terminus tail provides a method
for generating Y356• without the need for R2.15 In this way,
radical transport along the Y356 f Y731 f Y730 f C439
pathway can be “turned on” promptly by laser excitation of the
20-mer peptide terminus of R2 bound to R1. Moreover, Y356•
may be photogenerated with excitation wavelengths that lie well
outside the protein absorption envelope (ꢀ280 ) 190 000 M-1
cm-1), and the relatively fast time scale (>106 s-1) for Y
generation provides a large temporal window through which to
observe Y356 f Y731 f Y730 f C439 radical transport.
Experiments to couple ReI(phen)(CO)3(P-Y)+ to the C-terminal
R2 19-mer peptide are currently underway.
C. Y. Chang and Jeffrey Hirsch for preliminary contributions
to the synthesis of Re(bpy-AA)(CO)3 complexes, David R.
Manke for assistance with crystallography, and Durwin R.
Striplin for contributions to our understanding of ReI photo-
physics. We thank the National Institutes of Health for support
of this work, GM47274.
+
Supporting Information Available: Crystal structure of [Re-
(phen)(dppe)(CO)2](PF6); atom numbering schemes, tables of
crystal data, atomic coordinates, bond lengths and angles,
anisotropic thermal parameters, and hydrogen coordinates for
[Re(phen)(PP-Bn)(CO)2](PF6)‚CDCl3 and [Re(phen)(dppe)-
(CO)2](PF6); X-ray crystallographic files, in CIF format; spec-
troelectrochemical spectrum for reduction of Re(bpy-Y-OtBu)-
(CO)3CN. This material is available free of charge via the
Internet at http://pubs.acs.org.
Acknowledgment. We thank Linda M. Rodriguez for insights
into electron shuttling mechanisms and nomenclature, Michelle
JA0510360
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9458 J. AM. CHEM. SOC. VOL. 127, NO. 26, 2005