Intramolecular electron transfer across amino acid spacers in the picosecond time regime. Charge-transfer interaction through peptide bonds

Article abstract of DOI:10.1021/jp9832802

For a series of alanine-based peptides having 1-3 amino acid residues as spacers, the chromophore, pyrenesulfonyl (Pyr), has been attached at the N-terminus and an electron donor, dimethyl-1,4-benzenediamine (DMPD), covalently bound at the C-terminus. Evidence for an intramolecular charge-transfer interaction involving the electron donor and acceptor groups has been obtained from absorption spectra. Intramolecular electron transfer involving the end groups, Pyr (electron acceptor) and DMPD (electron donor) has been confirmed by ultrafast pump-probe methods. The radical-ion pair states that are generated on Ti/sapphire laser excitation at 400 nm decay in the picosecond to nanosecond time domain and generally show multiexponential decay kinetics. These rates of charge recombination are among the fastest yet observed involving electron transfer between terminal groups for peptide oligomers. The falloff of rate constants for ion pair recombination is irregular in terms of the through-bond distance that separates Pyr and DMPD groups for the various peptide links; i.e., back electron transfer remains fast for the tripeptide, Pyr-Ala-Ala-Ala-DMPD, despite an average through-bond distance between photoactive groups that reaches 18 Aì?. Molecular modeling studies show that the peptides are free to adopt conformations in essentially random fashion, without showing evidence for long range ordering of the peptide chain. ? 1999 American Chemical Society.

Full text of DOI:10.1021/jp9832802

572
J. Phys. Chem. B 1999, 103, 572-581
Intramolecular Electron Transfer across Amino Acid Spacers in the Picosecond Time
Regime. Charge-Transfer Interaction through Peptide Bonds1
Guilford Jones II,* Lily N. Lu, Hongning Fu, Catie W. Farahat, and Churl Oh
Department of Chemistry and the Center for Photonics, Boston UniVersity, Boston Massachusetts 02215
Scott R. Greenfield,† David J. Gosztola,† and Michael R. Wasielewski*,†,‡
Chemistry DiVision, Argonne National Laboratory, Argonne, Illinois 60439, and Department of Chemistry,
Northwestern UniVersity, EVanston Illinois 60208
ReceiVed: August 5, 1998; In Final Form: NoVember 23, 1998
For a series of alanine-based peptides having 1-3 amino acid residues as spacers, the chromophore,
pyrenesulfonyl (Pyr), has been attached at the N-terminus and an electron donor, dimethyl-1,4-benzenediamine
(DMPD), covalently bound at the C-terminus. Evidence for an intramolecular charge-transfer interaction
involving the electron donor and acceptor groups has been obtained from absorption spectra. Intramolecular
electron transfer involving the end groups, Pyr (electron acceptor) and DMPD (electron donor) has been
confirmed by ultrafast pump-probe methods. The radical-ion pair states that are generated on Ti/sapphire
laser excitation at 400 nm decay in the picosecond to nanosecond time domain and generally show
multiexponential decay kinetics. These rates of charge recombination are among the fastest yet observed
involving electron transfer between terminal groups for peptide oligomers. The falloff of rate constants for
ion pair recombination is irregular in terms of the through-bond distance that separates Pyr and DMPD groups
for the various peptide links; i.e., back electron transfer remains fast for the tripeptide, Pyr-Ala-Ala-Ala-
DMPD, despite an average through-bond distance between photoactive groups that reaches 18 Å. Molecular
modeling studies show that the peptides are free to adopt conformations in essentially random fashion, without
showing evidence for long range ordering of the peptide chain.
Introduction
for A-spacer-D peptides that supports a mechanism of electronic
coupling involving the peptide chain (an approximate expo-
nential dependence on through-bond distance in accord with
theoretical models8). In a more general context, the results
provide some insight with regard to the role of amino acid
linkages in large de noVo peptide assemblies (e.g., “bundles”)
and in proteins for which electron transport is important.9,10
Over the past decade a number of structures have been
prepared having electron donor and acceptor groups that are
covalently bound to peptide oligomers.2-6 Principal objectives
of this work have encompassed the elucidation of the role of
amino acid linkages in providing either a relatively rigid
framework for the systematic placement of donor and acceptor
groups at a varying distance of separation or the assembly of
potential electron-transfer reactants in larger peptide assemblies
(e.g., the scaffolding of an R-helix). For several series involving
synthetic peptides that are substituted with non-native organic
or transition metal groups or with native reactive side chains
(e.g., tyrosine or tryptophan),2-7 time constants for electron
transfer in the nanosecond to millisecond domain have been
observed (generally, a range of rate constants, k ) 103-108
s-1). Peptide links have featured prominently the amino acid
proline,2-4 a residue that provides a relatively rigid link having
limited conformational possibilities for pendant groups.
The assumption made in most electron-transfer studies
involving peptides is that the connecting amino acid residues
are not electroactive (i.e., not involved in discrete electron-
transfer steps). The peptide links play a more subtle role in
which electron-transfer rates vary with the number of amino
acid residues in an intervening peptide linkage. Although there
are interesting variations (vide infra), a general finding is a
falloff in the rate of photoinduced electron transfer with distance
In this paper a new series of synthetic peptides (2-4) is
reported that employs one of the simplest amino acids, L-alanine
(Ala), as a spacer. The amine terminal group of alanine or a
short peptide sequence is modified by attachment of a chro-
mophore, the pyrenesulfonyl group (Pyr), a moiety that also
serves as an electron acceptor. The C-terminal group of the
spacer is further modified by placement of an electron donor
group, N,N-dimethyl-1,4-benzenediamine (DMPD), as the car-
boxamide derivative. These peptides provide another assessment
of the roles of structure and electron-transfer dynamics having
three distinct features. Compared to the oligoproline links (6),
the alanine oligomers are free to adopt a larger variety of
conformations and represent, to a greater degree, the confor-
mational dispersion found more generally for peptide chains.
In addition, we have focused in this study on the observation
of phototransients and rates of charge recombination; the result
is an assessment of peptide electron-transfer behavior for the
Marcus “inverted region” (for a thermodynamic driving force,
∆Get < -2.0 eV). The values determined using ultrafast laser
flash methods are among the fastest yet measured for electron
transfer along a peptide chain (for the series, 2-4, dominant
decay constants fall in the 10-100 ps domain). Also of value
† Argonne National Laboratory.
‡ Northwestern University.
10.1021/jp9832802 CCC: $18.00 © 1999 American Chemical Society
Published on Web 12/31/1998

Electron Transfer across Amino Acid Spacers
J. Phys. Chem. B, Vol. 103, No. 3, 1999 573
Peptide Coupling Methodology and General Procedures.
Traditional methods for solution-phase coupling of amino acids
were employed by following general steps11 that include
protection at the N-terminus with the benzyloxycarbonyl group
(Cbz), activation of the carboxylic acid group with nitrophenol,
and coupling of the amino and carboxylic acid groups using
the diimide reagent, DCC. Silica gel 60 (230-400 mesh, E.
Merck Inc.) was employed as the stationary phase for flash
column chromatography (typically 2.5 cm i.d. × 32 cm for a
0.5-1.0 g sample of crude product) with a gradient of petroleum
ether-ethyl acetate as typical elution solvent.
N-Acetyl-N′,N′-dimethylbenzene-1,4-diamine (Ac-DMPD).
Freshly distilled N′,N′-dimethylbenzene-1,4-diamine (DMPD)
(5.0 g, 0.038 mol) was dissolved in 30 mL of Ar-purged pyridine
in the dark. On addition of acetic anhydride (3.7 mL, 3.9 g,
0.038 mol), the reaction mixture was refluxed at 80 °C for 30
min. The pH of the cooled solution was adjusted to 7.0 by
adding 2.0 N NaOH. The resulting solution was extracted with
ethyl ether (3 × 50 mL), the organic extracts were washed with
5% HCl, 5% Na2CO3, and water successively and then dried
over Na2SO4. Removal of solvent in vacuo gave a solid that
was recrystallized from benzene/petroleum ether to yield 5.5 g
1
of off-white crystals (81%) (mp 134-135 °C). H NMR (400
MHz, CD3CN): δ 8.22 (br, D-NH), 7.32 (d, J ) 10.0 Hz,
D-H-2, D-H-6), 6.67 (d, J ) 9.0 Hz, D-H-3, D-H-5), 2.84
(s, D-N(CH3)2), 2.00 (s, CH3-CdO). HRMS (EI 70 eV): m/z
178.1114 (M+, calcd for C10H14N2O 178.1106).
N-(1-Pyrenesulfonyl)-N′,N′-dimethylbenzene-1,4-di-
amine (Pyr-DMPD). 1-Pyrenesulfonyl chloride12 (0.50 g, 1.7
mmol) was dissolved in 15 mL of THF and 2 mL of
triethylamine. N,N-Dimethylbenzene-1, 4-diamine (0.24 g, 1.7
mmol, freshly distilled) in 3 mL of THF was added dropwise
with stirring. The reaction was stirred at room temperature for
12 h. The precipitate was filtered off and the solvent removed
by rotary evaporation, yielding a brownish yellow solid. The
solid was purified by flash column chromatography on silica
gel and recrystallized from ethyl acetate/petroleum ether to yield
0.21 g (30%) of orange needles (mp 175-177 °C): 1H NMR
(400 MHz, CD3COCD3): δ 9.12 (d, J ) 9.4 Hz, H-10), 8.92
(br, D-NH), 8.53 (d, J ) 8.3 Hz, H-2), 8.45 (d, J ) 7.7 Hz,
H-8), 8.43 (d, J ) 7.7 Hz, H-6), 8.38 (d, J ) 9.4 Hz, H-9),
8.33 (d, J ) 9.0 Hz, H-5), 8.27 (d, J ) 8.3 Hz, H-3), 8.21 (d,
J ) 9.0 Hz, H-4), 8.18 (t, J ) 7.7 Hz, H-7), 6.83 (d, J ) 9.1
Hz, D-H-2, D-H-6), 6.39 (d, J ) 9.1 Hz, D-H-3, D-H-5),
2.83 (s, D-N(CH3)2). HRMS (EI 70 eV): m/z 400.1194 (M+,
calcd for C24H20N2O2S 400.1246).
for this series is the finding that the strength of electronic
coupling involving interaction of the remotely placed Pyr and
DMPD groups can be evaluated qualitatively through the
observation of weak charge-transfer bands that contribute to
absorption spectra.
The systems reported are analogous to others in which a
simple aromatic chromophore (usually the hydrocarbon moiety,
pyrene, serving as electron acceptor) has been used in conjunc-
tion with peptides that have been conjugated with aromatic
amines as electron donors.5 In these complementary studies, the
novel rate effects that have been reported have been restricted
to the forward electron-transfer step (as in eq 1) in which a
remotely placed amine donor donates an electron to a photo-
excited pyrene, an event that is normally monitored through
observation of pyrene fluorescence quenching. The objective
of the present work was to highlight the charge recombination
step and to attempt to sort out some of the complexities that
attend examination of systems that display conformational
freedom.
N-[N-(1-Pyrenesulfonyl)-L-alanyl]-N′,N′-dimethylbenzene-
1,4-diamine (Pyr-Ala-DMPD). Benzyloxycarbonyl-L-alanine-
p-nitrophenyl ester (2.5 g, 7.3 mmol) was dissolved in 25 mL
of pyridine, and the solution was cooled to 0 °C. N,N-
Dimethylbenzene-1,4-diamine (0.99 g, 7.3 mmol, freshly dis-
tilled) in 10 mL pyridine was added dropwise, and the reaction
was stirred at 0 °C for 2 h and at room temperature overnight.
The solvent was removed by rotary evaporation and the resulting
solid washed with ethyl ether to yield 1.9 g (77%) of a light
yellow precipitate (mp 184-186 °C). A mixture of the product,
N-benzyloxycarbonyl-L-alanyl-N′,N′-dimethylbenzene-1,4-di-
amine (Cbz-Ala-DMPD, 0.60 g, 1.8 mmol), in 10 mL of 80%
acetic acid and 0.06 g of Pd/C (10%) hydrogenation catalyst
was stirred under hydrogen provided by a balloon for 24 h. The
Pd/C was filtered and the solvent removed by rotary evaporation
to yield a solid that was washed with ethyl ether to give 0.44 g
(92%) of a white precipitate (mp 123-125 °C). The resulting
acetate salt of N-(L-alanyl)-N′,N′-dimethylbenzene-1,4-diamine
Experimental Section
Materials. The preparation of Pyr-AlaOEt (5) has been
previously described.7 The benzyloxycarbonyl-protected amino
acids were obtained from Bachem Bioscience Inc., and the
amino acid ethyl esters from Sigma. N,N-Dimethylbenzene-1,
4-diamine (DMPD) (Aldrich) was freshly distilled at 0.1 mmHg
pressure (bp ca. 100 °C) under an argon atmosphere before use.
Acetonitrile, dioxane (HPLC grade, Fisher), and dimethylfor-
mamide (DMF) (<0.005% water, Aldrich) were used as
received.

574 J. Phys. Chem. B, Vol. 103, No. 3, 1999
Jones et al.
(0.36 g, 1.3 mmol) was dissolved in a pyridine/methanol mixture
(1:1 v/v) along with 2 mL of triethylamine. 1-Pyrenesulfonyl
chloride (0.50 g, 1.8 mmol) in 5 mL of pyridine was added
dropwise, and the reaction was stirred at room temperature for
12 h. The solvent was removed by rotary evaporation, and the
product was purified by flash column chromatography on silica
gel (ethyl acetate and petroleum ether gradient). The solid was
recrystallized from ethyl acetate/petroleum ether to yield 0.14
g (23%) of orange crystals (mp 198-199 °C): 1H NMR (400
MHz, CD3COCD3): δ 9.23 (d, J ) 9.4 Hz, H-10), 8.81 (d, J )
8.2 Hz, H-2), 8.69 (br, D-NH), 8.56 (d, J ) 7.6 Hz, H-8),
8.55 (d, J ) 7.6 Hz, H-6), 8.54 (d, J ) 9.4 Hz, H-9), 8.43 (d,
J ) 8.2 Hz, H-3), 8.41 (d, J ) 8.9 Hz, H-5), 8.30 (t, J ) 7.6
Hz, H-7), 8.30 (d, J ) 8.9 Hz, H-4), 7.35 (d, J ) 8.3 Hz,
A-NH), 6.97 (d, J ) 9.1 Hz, D-H-2, D-H-6), 6.42 (d, J )
9.1Hz,D-H-3,D-H-5),4.15(m,A-CH),2.95(s,D-N(CH3)2),
1.31 (d, J ) 7.0 Hz, A-CH3). HRMS (EI, 70 eV): m/z
471.1602 (M+, calcd for C27H25N3O3S 471.1617).
DMSO-d6) δ 9.55 (s, D-NH), 8.99 (d, J ) 9.4 Hz, H-10), 8.58
(d, J ) 8.0 Hz, H-2), 8.52 (d, J ) 8.4 Hz, A1-NH), 8.48 (d, J
) 7.4 Hz, H-8), 8.46 (d, J ) 7.4 Hz, H-6), 8.42 (d, J ) 9.4 Hz,
H-9), 8.38 (d, J ) 8.0 Hz, H-3), 8.38 (d, J ) 8.9 Hz, H-5),
8.28 (d, J ) 8.9 Hz, H-4), 8.21 (t, J ) 7.4 Hz, H-7), 7.39 (d,
J ) 7.9 Hz, A2-NH), 7.82 (d, J ) 7.8 Hz, A3-NH), 7.33 (d,
J ) 8.6 Hz, D-H-2, D-H-6), 6.65 (d, J ) 8.6 Hz, D-H-3,
D-H-5), 4.24 (m, A3-CH). 3.92 (m, A1-CH), 3.79 (m, A2-
CH), 3.33 (s, D-N(CH3)2), 1.17 (d, J ) 6.9 Hz, A3-CH3),
0.98 (d, J ) 6.9 Hz, A1-CH3), 0.81 (d, J ) 6.9 Hz, A2-CH3).
HRMS (EI, 70 eV): m/z 613.2360 (M+, calcd for C33H35N5O5S
613.2359).
General Methods and Procedures. Steady-state emission
spectra were recorded using a PTI QuantaMaster Luminescence
spectrometer model SE-900M. For fluorescence quantum yield
measurements,13 argon-purged solutions in 1 × 1 cm quartz cells
were used (OD < 0.2 at the excitation wavelength); the reference
was coumarin 1 (Φf ) 1.0, CH3CN). The oxidation potentials
for CH3CONHC6H4N(CH3)2 (Ac-DMPD) and for 1 (first
oxidation wave) (E1/2 ) 0.53 and 0.56 V vs SCE, respectively)
and the reduction potential for Pyr-AlaOEt (5) (E1/2 ) -1.69
V vs SCE) were determined by cyclic voltammetry using an
EG&G Princeton Applied Research model 273 potentiostat/
galvanostat, a platinum bead working electrode, and a platinum
wire working electrode. Reversible waves were observed for
nitrogen-purged acetonitrile solutions (22 °C) with 0.1 M LiClO4
and scan rates of 0.2-10 V/s.
Ultrafast Transient Detection. The femtosecond transient
absorption apparatus consists of a self-mode-locked Ti/sapphire
oscillator that produces 25 fs pulses at 97 MHz. The oscillator
ouput was temporally stretched in a single grating pulse
stretcher, then amplified in a 23 kHz Ti:sapphire regenerative
amplifier that is pumped by an intracavity frequency-doubled,
Q-switched, Nd:YAG laser. Seeding and subsequent ejection
of the amplified pulse was accomplished with a 2 mm quartz
acousto-optic modulator (NEOS) with a carrier wave that is both
phase and frequency locked to 4 times that of the oscillator
(388 Hz). Both the pulse stretcher and compressor utilize
quadruply passed holographic transmission gratings (Kaiser
Optical). The pulse stretcher utilizes a one-to-one inverting
telescope, whereas the compressor’s telescope is noninverting.14
The compressed, amplified output was split and 80% was
frequency-doubled to serve as the pump beam.
N-[N-(1-Pyrenesulfonyl)-L-alanyl-L-alanyl]-N′,N′-dimeth-
ylbenzene-1,4-diamine (Pyr-Ala-Ala-DMPD). Following a
procedure similar to that for preparation of Pyr-Ala-DMPD, the
N-benzyloxycarbonyl-L-alanyl-L-alanine-p-nitrophenyl ester (white
solid, mp 163-164 °C, 60%) was first prepared from N-
benzyloxycarbonyl-L-alanyl-L-alanine (2.4 g, 8 mmol) and
p-nitrophenol (1.1 g, 8 mmol) using DCC coupling. Subsequent
condensation with N,N-dimethylbenzene-1,4-diamine provided
Cbz-Ala-Ala-DMPD (mp 245-247 °C), which was subjected
to catalytic hydrogenration (10% Pd/C), as described. The
resulting acetate salt of N-(L-alanyl-L-alanyl)-N′,N′-dimethyl-
benzene-1,4-diamine (0.45 g, 1.3 mmol) was allowed to react
with 1-pyrenesulfonyl chloride (0.54 g, 1.8 mmol). The product
was purified by recrystallization three times from ethyl acetate
to yield 0.12 g (17%) of light yellow needles (mp 251.0-251.5
°C): 1H NMR (400 MHz, CD3COCD3): δ 9.11 (d, J ) 10 Hz,
H-10), 8.76 (br, D-NH), 8.72 (d, J ) 7.6 Hz, H-2), 8.47 (d, J
) 8.0 Hz, H-8), 8.46 (d, J ) 8.0 Hz, H-6), 8.44 (d, J ) 10 Hz,
H-9), 8.42 (d, J ) 7.6 Hz, H-3), 8.38 (d, J ) 8.4 Hz, H-5),
8.28 (d, J ) 8.4 Hz, H-4), 8.21 (t, J ) 8.0 Hz, H-7), 7.56 (d,
J ) 6.8 Hz, A2-NH), 7.41 (d, J ) 9.2 Hz, D-H-2, D-H-6),
7.33 (br, A1-NH), 6.67 (d, J ) 9.2 Hz, D-H-3, D-H-5), 4.16
(m, A2-CH), 3.92 (m, A1-CH), 2.85 (s, D-N(CH3)2), 1.14
(d, J ) 6.8 Hz, A2-CH3), 1.03 (d, J ) 7.6 Hz, A1-CH3).
HRMS (EI, 70 eV): m/z 542.1959 (M+, calcd for C30H30N4O4S
542.1977).
Samples were typically 10 mM, held in a stirred, 1 cm cuvette,
and excited with 0.5 µJ, 70 fs, 400 nm pulses. The remaining
20% of the amplified 800 nm light was used to generate a white-
light continuum probe by focusing into a 2 mm sapphire
window. The probe beam was polarized at the magic angle
(54.7°) with respect to the pump beam. Amplified photodiodes
were used to detect single wavelengths of the probe light, after
it passed through a computer-controlled monochromator (SPEX
model 270M). The photodiode outputs were digitized and
recorded using a personal computer. Multiexponential fits of
the kinetic data were determined using nonlinear least-squares
analysis based on the Leven-Marquardt algorithm.
Molecular Modeling. The software employed was based on
QUANTA from Molecular Simulations, Inc.15 It included the
conformational search subroutine that interfaces with the
CHARMm molecular mechanics program. Additionally, a
semiempirical quantum mechanical program, MOPAC (version
6.0; QCPE 455), was used in determining charge distributions.
To arrive at an array of low-energy (most probable) conforma-
tions for a particular peptide, the sequence of steps began with
a random selection of all the flexible dihedral angles. The
N-[N-(1-Pyrenesulfonyl)-L-alanyl-L-alanyl-L-alanyl]-N′,N′-
dimethylbenzene-1,4-diamine (Pyr-Ala-Ala-Ala-DMPD). Ac-
cording to the manual solid-phase procedure of Stewart and
Young11 a support resin, 1% cross-linked polystyrene, was
functionalized by chloromethylation for reaction with N-
protected amino acid. For the pyrene-labeled tripeptide, the
1-pyrenesulfonyl group is coupled with the aminoacyl resin in
THF in the presence of triethylamine in the last cycle. The
product was cleaved from the resin using anhydrous HF, and
the solution of free peptide was washed with anhydrous ether.
The free C-terminal peptide was directly coupled with DMPD,
and the final product purified by reversed-phase HPLC. Materi-
als used in solid-phase peptide coupling included Boc-amino
acid resin esters and Boc-amino acids obtained from Peninsula
Laboratories. Preparative HPLC separation of the tripeptide
utilized a Dynamax-60 C-18 reversed-phase column and a
gradient mobile phase of acetonitrile and water (12 mL/min flow
rate); Pyr-Ala-Ala-Ala-DMPD was obtained as a light yellow
crystal (yield 30% - 40%, based on available amino acid
residues on the resin) (mp > 300 °C): 1H NMR (400 MHz,

Electron Transfer across Amino Acid Spacers
J. Phys. Chem. B, Vol. 103, No. 3, 1999 575
Figure 1. Absorption spectra for 2 and 5, 30 µM in DMF.
resulting high-energy conformation was energy-minimized by
the adopted-basis Newton-Raphson algorithm. For these com-
putations, the solvent dielectric constant was assumed to be that
of DMF (36.7) and to be radial-distance-dependent; the amide
carbonyl and amide proton positions were assumed to be
antiparallel and coplanar.
Figure 2. Long wavelength absorption assigned to contribution of a
charge-transfer band for 1 (solid lines, concentration increasing, 0.013,
0.063, 0.12, 0.33, and 0.53 mM) and for model compound 5 (dashed
line, 0.53 mM) (top) and for 0.53 M conjugates 2 (thick line), 3 (thin
line), and 5 (dashed line) (bottom).
Results
for the CT band at 410 nm could be obtained for the two shortest
links: 302 (1) and 110 (2) M-1 cm-1. For this approximation,
the CT bands were constructed as Gaussian functions (Figure
3), using long wavelength portions of the absorption bands for
the DMPD conjugates after subtraction of absorption by Pyr-
AlaOEt (5) to provide the long wavelength portion of the
Gaussian fit. Using the relationship derived by Mulliken and
Hush (eq 2),18,19 it was possible to evaluate the electronic
Absorption and Fluorescence Properties of Peptide Con-
jugates. For the series 1-4, features in common with the
pyrenesulfonamide chromophore attached to the amino acid,
alanine, were observed; the conjugate, Pyr-AlaOEt (5), absent
in the DMPD donor group, was available for comparison.7
Principal maxima appeared at 278 and 350 nm (for 5, ꢀ )
44 100, and 34 900 M-1 cm-1, respectively) with enhancements
at 280-310 nm for conjugates bearing the DMPD group (Figure
1). Also, for these DMPD conjugates, a very weak tail intruded
into the visible region just past 400 nm. The extent of this long
wavelength absorption depended on chain length, suggesting
that a perceptible (and variable) interaction was important for
Pyr and DMPD acceptor-donor moieties and operates at
relatively long range. Relevant also was the observation that
the DMPD conjugates were lightly colored as crystalline solidss
orange for 1 and 2 and light yellow for 3 and the tripeptide,
Pyr-Ala-Ala-Ala-DMPD (4).
The anticipated charge-transfer interaction16,17 between elec-
tron acceptor and donor groups in 1-4 was examined in more
detail with expansion of the absorption data in the 400-500
nm region. The spectra that result for 1 and for oligomeric
peptides are shown in Figure 2; these absorption tails were not
the result of an aggregation phenomenon since the absorption
of solutions having varied concentrations of the conjugates
obeyed Beer’s law. In no case did the weak charge-transfer (CT)
absorptions appear as well resolved bands, but approximate
extinction coefficients corresponding to an estimated maximum
2.06 × 10-2 (νmaxꢀmax∆ν1/2)1/2
Hab (cm-1) )
(2)
rab
coupling matrix elements (Hab) associated with the interaction
of Pyr and DMPD groups. The interaction energies obtained
using this approximate procedure were 609 (1) and 225 (2)
cm-1. Although the CT bands were not sufficiently defined for
3 and 4, one could estimate that for these peptides, ꢀ410 ) < 30
M-1 cm-1 (Hab < 60 cm-1). For the Mulliken-Hush relation,
νmax and ∆ν1/2 correspond to the frequency of the reconstructed
CT band maxima and bandwidths at half-height, respectively,
ꢀ is the extinction coefficient for peak CT absorption, rab is the
distance of separation (through bonds, from sulfur on the Pyr
acceptor to NH for the DMPD group), and Hab is the electronic
coupling matrix element. We emphasize that the values for Hab
obtained by this procedure are highly approximate, given the
poor resolution of the charge-transfer absorption features.
It was important to determine for comparison purposes
whether CT interaction for Pyr and DMPD groups was indicated

576 J. Phys. Chem. B, Vol. 103, No. 3, 1999
Jones et al.
Figure 4. Fluorescence spectra for 2-4 in acetonitrile (25 µM samples;
excitation at 337 nm).
TABLE 1: Fluorescence Quantum Yield and Transient
Absorption Parameters for Pyrenesulfonamide Conjugatesa
Figure 3. Absorption spectra in the long wavelength (charge transfer)
region for 1 and 2, obtained by subtraction of the spectrum for the
reference pyrenesulfonyl derivative 5 from the absorption of the
conjugates for solvents indicated.
Φfb
τ1 (a1)
τ2 (a2)
τ3 (a3)
τ
av (ps)
1
2
3
4
5
<0.0005
0.0045
0.024
0.021
0.83
1.2 (1.0)
9.7 (0.43) 53. (0.57)
13.5 (0.44) 60.9 (0.36) 1470 (0.20) 318
11.1 (0.37) 53.5 (0.39) 1530 (0.24) 379
0.83
43.1
in spectra for an intermolecular system. Complex formation
could be detected for solutions of Pyr-AlaOEt (5) in the presence
of the model electron donor, CH3CONHC6H4N(CH3)2 (Ac-
DMPD), again as a weak end absorption in the 400-430 nm
range for acetonitrile solutions ranging in concentration up to
10 mM in 5 and Ac-DMPD. With the determination of an
equilibrium constant (K ) 3.4 × 103 M-1) from data associated
with the quenching of Pyr-AlaOEt fluorescence with Ac-
DMPD,20 it was possible to obtain an extinction coefficient for
absorption by the complex (ꢀ410 ) 12 M-1 cm-1, corresponding
to Hab ) 18 cm-1). For this comparison, it was assumed that
intermolecular complexation of Pyr-AlaOEt and Ac-DMPD
results in formation of a conventional CT complex having a
plane-parallel arrangement of the Pyr-AlaOEt and Ac-DMPD
π systems.16 The π-stack arrangement that is likely for 5 and
Ac-DMPD is permissible (although not ideal) for Pyr-Ala-
DMPD, but more probable for energetically allowed folded
conformations of the longer peptides 3 and 4 (note discussion
of models below). In contrast, the strongest evidence for CT
interaction for the linear conjugates appears in the data for Pyr-
DMPD (Figure 2) for which (plane parallel) π interaction is
not possible. In addition, the CT contribution (Figure 3) falls
off with the size of the linkage (1 > 2 > 3, 4), signaling further
that the dominant mechanism of electronic coupling between
terminal moieties in the ground electronic state is most likely
to be through-σ-bonds. This finding has been commonly made
for linked donor-acceptor systems, including, for example,
those featuring proline oligomers2-4 and rigid hydrocarbon
spacers.21,22
a Transient absorption for Ar-purged DMF solutions with lifetimes
associated with 495 nm transient decays, τ1-3 (ps) values that result
from single, double, and triple exponential fits with relative contribu-
tions, a1-a3, and a computed weighted average, τav. b Emission yield
data for 25 µM Ar-purged CH3CN solutions.
SCHEME 1
donor and acceptor groups in linked pyrene systems23 has not
been observed. Notably, the direct connection of Pyr and DMPD
groups in 1 led to virtually complete quenching of Pyr
fluorescence. Consistent with an electron transfer mechanism
of quenching that is sensitive to the presence of spacer amino
acid residues, fluorescence was substantially restored for the
series of oligomeric peptides. However, values of emission
quantum yield for peptides 3-5 fell in the same range and did
The fluorescence spectra for 2-5 are shown in Figure 4, and
the quantum yields of emission for the conjugates are collected
in Table 1. The emission spectral features for the DMPD
derivatives were identical to that for 5, which displays the
fluorescence of the locally excited pyrenesulfonyl chromophore,
having a vibronic progression of bands (ca. 900 cm-1) and an
origin at 390 nm. For the peptides in several solvents, longer
wavelength emission that has been previously assigned to a
mechanism of exciplex formation involving remotely attached
not reveal a clear dependence on peptide chain length.24,25
A
model for the relative placement in energy of low-lying excited
and radical-ion pair states that are accessible to the peptides is
shown in Scheme 1. Used in this model are the energies derived
from redox potential data obtained using cyclic voltammetry
(Experimental Section) for the entries that serve as models
lacking the peptide links, 5 and Ac-DMPD, and for 1.

Electron Transfer across Amino Acid Spacers
J. Phys. Chem. B, Vol. 103, No. 3, 1999 577
Figure 5. Transient spectrum (at 2.0 ps) and 495 nm transient decay
curve obtained on pulsed laser photolysis of 3 in DMF (λexc ) 400
nm).
Figure 6. Transient spectrum (at 2.0 ps) and 495 nm transient decay
curve obtained on pulsed laser photolysis of 4 in DMF (λexc ) 400
nm).
the di- and tripeptides 3 and 4 were most satisfactorily fit to
triple exponential functions that included a long τ component
that reached the nanosecond regime. The decay data are
organized in the table in terms of single and multiexponential
lifetimes (τ1-τ3), along with a set of weighted averages, τav.
Molecular Modeling. The peptides selected for computer
simulation were the alanine derivative 2 and the tripeptide, Pyr-
Ala-Ala-Ala-DMPD (4), for which >500 local minima were
inspected. The simulations provided comparisons of geometry
and distance relationships for ensembles of the Pyr chromophore
and the donor group, DMPD, and their energies relative to an
arbitrary reference low-energy conformer. The distribution of
conformations obtained for 4 is shown in Figure 7, which
displays distributions of conformations in histogram form
according to energy and critical distances. The average distances
between putative electron donor and acceptor groups, defined
in terms of through-space spans between points “center-to-
center” for aromatic rings are 7.6 and 10. 2 Å, respectively for
2 and 4; the respective through-bond distances “edge-to-edge”
are 9.2 and 18.0 Å. The distribution of conformations for 4 is
displayed in another way in terms of the distribution of dihedral
angles made by main chain segments; these results are shown
in the form of circle graphs (Figure 8).
Transient Absorption Properties of Peptide Conjugates.
The events associated with formation and decay of electron
transfer intermediates could be directly examined using fem-
tosecond/picosecond transient absorption methods that employed
laser excitation with a Ti/sapphire system capable of 70 fs pulses
at the frequency-doubled wavelength (400 nm). For the reference
conjugate, Pyr-AlaOEt, transient absorption was observed (λmax
) ca. 520, a band that was convoluted with stimulated emission)
with an instrument-limited delay following laser pulse. The
transient associated with 5 was long-lived (>10 ns), consistent
with its identification with the fluorescent transient (the local
Pyr S1 state).6,24
On irradiation of the DMPD conjugates at 400 nm, a
wavelength associated with the CT end absorption (Figure 2),
a transient that grew in with the laser pulse (a rise time of ca.
300-600 fs) appeared with λmax ) 495 nm (Figures 5 and 6).
This transient was assigned to the radical-anion of the pyrene-
sulfonyl moiety (Pyr•-), since it is similar in appearance to the
absorption reported for the radical-anion of the parent hydro-
carbon, pyrene (λmax ) 490 nm, ꢀ ) 49 200) and that of pyrene
derivatives.26 Transients also appeared to include weak absorp-
tion at longer wavelengths (a shoulder in the 520-560 nm
range), consistent with the simultaneous appearance of the less
strongly absorbing DMPD radical-cation transient, which has
been observed in independent nanosecond flash photolysis
experiments involving the model donor, Ac-DMPD.27
Decay times, assumed to represent the rates of charge
recombination to the ground state (eq 3 and Scheme 1)28 were
obtained by monitoring the Pyr•- transient at 495 nm for the
series of Pyr conjugates (Table 1). The general pattern consisted
of very rapid (1 ps) depletion for the directly linked 1 and a
less rapid return electron transfer associated with longer links,
2-4. The kinetics were complex, consistent with decay via back
electron transfer for a multiplicity of species. For 2 a double
exponential function was most suitable, whereas the data for
Discussion
In much of the important work to date regarding long-range
electron transfer for synthetic peptide oligomers, the amino acid,
L-proline, provides a relatively rigid link due to closure of the
five-membered lactam ring (6). In the most extensive studies,
Isied and co-workers3 have deployed ruthenium, rhodium, and
osmium transition metal complexes for terminal modification
of proline oligomers. In these studies a charge shift process has

578 J. Phys. Chem. B, Vol. 103, No. 3, 1999
Jones et al.
at longer spacings of reactive residues and through-space
interaction implicated for shorter links (n ) 1-3), proposed
by Bobrowski et al.4
Other investigations also address rates of end-to-end electron
transfer across oligomer prolines, including the linked an-
thracene/dimethylaniline pairs of Maruyama et al.29 and the
connections of transition metal complexes with organic electron-
transfer agents reported by Schanze and co-workers.30 In another
investigation by Fox and Miller,31 radical-anion charge-shift
reactions have been observed between organic groups that are
linked, as in the present study, via alanine (Ala) residues. In
these prior investigations, rates of electron transfer involving
excited-state quenching or charge shift reach values of about
109 as an upper limit for short spans of amino acids (n ) 1-3).
Taken together these studies show that moderately large energy
barriers for electron transfer remain for short links due to the
influence of modest thermodynamic driving force or large
reorganization energies.32
By any of these comparative measures, the rates of electron
transfer (charge recombination, eq 3) for conjugates 1-4 are
large. The trends in rate are expected to fall in the Marcus
inverted regime (i.e., for the relationship of log k-et and ∆G-et32),
since the thermodynamic driving force for charge recombination
for these peptides is sizable (for the step, eq 3, ∆G-et ) -2.1
eV). The inverted behavior is illustrated further by comparison
with the series of similar tryptophan (Trp) conjugates that
includes Pyr-Trp and Pyr-Ala-Trp,6 derivatives that exhibit
similar distance relationships vis-a`-vis donors and acceptors as
do 2 and 3, respectively. Rates of charge recombination for the
Trp series are about 1 order of magnitude slower than those
associated with DMPD derivatives, in accord with a higher
driVing force for the Trp series (by about 0.5 eV).6
The interpretation of rate data for 1-4 must take into account
the extensive conformational averaging that is important for
these systems.31 The modeling studies were carried out in order
to confirm that peptide structures did not favor particular
conformers based on some principle of long range ordering of
the chains and pendant groups. Folded conformations, if favored,
would introduce a mechanism of electron transfer that could
be dominated by through-space interaction, if distances between
electroactive pendants were as close as 5 Å.8 For the peptide
oligomers, a very large number of favorable conformations,
representing, for 4, distances of center-to-center separation of
Pyr and DMPD groups of 8-15 Å, lie within a range of 5-7
kcal/mol in relative energy (>200 conformations in the simula-
tion, Figure 7). The structural analysis for these conjugates that
is in accord with 2D-NMR (NOE) findings33 is one of peptide
geometries in which the terminal groups (and chain segments)
are oriented in largely random fashion, but with a large
population of extended conformations (note the near random
distributions of bond angles for d3-d6, d8 for 4, Figure 8). The
simulations do not address the dynamics of such arrays, but a
reasonable assumption is that conformational interconversions
of significant dimensions are bounded in the microsecond to
nanosecond time scale, as revealed in NMR and other studies
of peptides.34 Importantly, the dominant decay times for return
electron transfer are very short, by comparison with this time
scale for conformational averaging.
Figure 7. Molecular modeling results for 4 shown in histogram form
that depict the distribution of 697 conformations with regard to the
distance (center-to-center) of separation of Pyr and DMPD groups (top)
and with regard to their relative energies (bottom).
been assessed (e.g., single electron transfer between Ru(I) and
Ru(III) terminal groups, observed in pulse radiolysis experi-
ments). Peptide oligomers that measure up to 40 Å in end-to-
end separation of metal centers have been investigated with
remarkable results that show that electron transfer can proceed
over very long distances, albeit slowly (as low as k ) ca. 104
s-1) with moderate driving force, ∆Get ) -1.5 eV. The most
rapid of these electron transfers (for 6, n ) 1) occurs with a
rate constant approaching 109 s-1; the observation of a
maintenance in electron-transfer rate for very long peptides in
this series has been ascribed to the development of helical
structure for the polyprolines for which additional electronic
interaction between chain segments is important (vide infra).3
Other groups have employed spans of prolines separating
organic donors and acceptors (e.g., tryptophan and tyrosine
moieties, amino acid residues commonly found in proteins).4
Using pulse radiolysis methods, Faraggi and Klapper4 have
surveyed oligomers with n ) 1-5 with results that show a
systematic falloff with the number of spacer residues. Rate
constants associated with shifts of charge (hole or radical sites
created at Trp moieties transferred to remotely placed Tyr) range
103-105 s-1. Similar results have been provided by Bobrowski
et al.,4 regarding proline oligomer linkages of these amino acids.
Mechanisms proposed to account for the rate constant trends
have differed slightly for these two groups, with a through-
bond electronic coupling mechanism cited in the work of Faraggi
and Klapper4 and a combination of mechanisms, through-bond
In sum, the behavior of the oligomers can be modeled simply
in terms of a distribution of a large pool of random conforma-
tions into two families (Scheme 2). Some orientations that
promote electronic coupling most effectively along peptide
chains are associated with the weak CT absorption (hνCT) that
is observed for the series (Figure 2). This family of foldamers

Electron Transfer across Amino Acid Spacers
J. Phys. Chem. B, Vol. 103, No. 3, 1999 579
Figure 8. Distributions of conformations determined from simulations for 4 with reference to peptide backbone dihedral angles (reference structure
shown).
is the source of the radical-ion phototransients that appear early
in the picosecond time domain on laser photolysis at 410 nm
and have decayed for the most part by 1 ns. These intermediates
may be reasonably represented by the average distances of
separation of Pyr and DMPD groups indicated in the simulation
(e.g., about 7 and 11 Å for 2 and 4, respectively), corresponding
roughly to solvent-separated radical-ion pairs.17,32
As the result of a random sampling of peptide backbone and
other angles, a second family of peptide conformations is less
suited for long-range interaction via through-bond (or through
space) electronic coupling. Following photoexcitation associated
with local pyrene absorption (hνLE, ca. 330 nm), these species
survive well into the mampsecpmd regime and are responsible
for normal Pyr fluorescence. The implication is that the majority
of these species undergo forward electron transfer on a time
scale that is slower than that associated with the rates of return
of radical-ion pairs to the ground state; the two distinguishable
slow and fast processes (eqs 1 and 3) represent behavior
associated with normal and inverted Marcus regimes, respec-
tively.

580 J. Phys. Chem. B, Vol. 103, No. 3, 1999
Jones et al.
SCHEME 2
flexible chain of hydroxyethylglutamines.38 The estimates of
relative diffusion coefficients for chain ends that were derived
in this study resulted from emission lifetime data that fell in
the 5-15 ns range for the shortest oligomers examined (n ) 4,
5), a time scale that is long relative to the present findings of
charge recombination times for the oligomers, 2-4.39 That is,
the values of average k-et are too large to be influenced
significantly by a conformational gating that is at least partially
rate limiting (i.e., the charge recombination rates reflect the
behavior of a relatively static ensemble). Finally, the role of
partially folded peptide chains has received special scrutiny in
terms of specific modes of electronic coupling via hydrogen
bonding.40 For example, in command of more conformational
space, the tripeptide 4 will have increased opportunity to
participate in specific intramolecular H-bond patterns that may
amplify the usual through (peptide) σ bond coupling mechanism.
The feature of the kinetics data that remains is the dependence
of charge recombination rate on peptide length. A falloff in
transient decay times with the number of intervening amino
acids is clearly apparent for 1-3; although not having precise
physical meaning, average rate constants for return electron
transfer (i.e., the reciprocal average decay times) are, respec-
tively, 12 × 1010, 2.3 × 1010, and 0.31 × 1010 M-1 s-1. It is
tempting to record simply that the trend shows a regular
dependence of a near 10-fold decrease in rate with the imposition
of each Ala spacer (3 σ bonds or about 4.5 Å). The standard
view8,21 is that the charge recombination rate constant should
be related to Hab and the Franck-Condon weighted density of
states (FCWD) for electron transfer and that the electronic
coupling term scales exponentially with distance with coefficient
-â (eqs 4 and 5). For the series 1-3, the computed â value is
Acknowledgment. Support of this research by the Depart-
ment of Energy, Office of Basic Energy Sciences, Division of
Chemical Sciences, is gratefully acknowledged. We also wish
to thank Xin Zhou and Valentine Vullev for technical assistance.
References and Notes
(1) Paper no. 9 from Boston University in the series PhotoactiVe
Peptides.
(2) Isied, S. S. In Principles of Electron-Transfer Reactions: Applica-
tions in Inorganic and Organometallic Chemistry and Biology; Isied, S.
S., Ed.; Advances in Chemistry Series 253; American Chemical Society:
Washington, DC, 1997. Slate, C. A.; Striplin, D. R.; Moss, J. A.; Chen, P.;
Erickson, B. W.; Meyer, T. J. J. Am. Chem. Soc. 1998, 120, 4885.
(3) Ogawa, M. Y.; Moreira, I.; Wishart, J. F.; Isied, S. S. Chem. Phys.
1993, 176, 589. Mishra, A. K.; Isied, S. S.; Ogawa, M. Y.; Wishart, J. F.
Chem. ReV. 1992, 92, 381.
(4) Chandrasekar, R.; Faraggi, M.; Klapper, M. H. J. Am. Chem. Soc.
1994, 116, 1414. DeFelippis, M. R.; Faraggi, M.; Klapper, M. H. Ibid. 1990,
112, 5640. Bobrowski, K.; Holcman, J.; Poznanski, J.; Ciurak, M.;
Wierzchowski, K. L. J. Phys. Chem. 1992, 96, 10036.
(5) Fox, M. A.; Galoppini, E. J. Am. Chem. Soc. 1997, 119, 23; 1996,
118, 2299. Werner, U.; Wiessner, A.; Kuehnle, W.; Staerk H. J. Photochem.
Photobiol. A: Chem. 1995, 85, 77. Mihara, H.; Tanaka, Y.; Fujimoto, T.;
Nishino, N. J. Chem. Soc., Perkin Trans. 2 1995, 1915; Chem. Lett. 1996,
135. Garcia-Echeverria, C. J. Am. Chem. Soc. 1994, 116, 6031. Sisido, M.;
Tanaka, R.; Inai, Y.; Imanishi, Y. Ibid. 1989, 111, 6790. Sisido, M.; Inai,
Y.; Imanishi, Y. Macromolecules 1990, 23, 1665. Tamiaki, H.; Maruyama,
K. Chem. Lett. 1993, 1499.
(6) Jones, G., II; Lu, L. N.; Vullev, V.; Gosztola, D. J.; Greenfield, S.
R.; Wasielewski, M. R. Bioorg. Med. Chem. Lett. 1995, 5, 2385. Jones, G.,
II; Feng, Z.; Oh, C. J. Phys. Chem. 1995, 99, 3883. Jones, G., II; Farahat,
C. W.; Oh, C. Ibid. 1994, 98, 6906. Jones, G., II; Farahat, C. W. Res. Chem.
Intermediates 1994, 20, 855.
(7) Jones, G., II; Lu, L. N. J. Org. Chem. 1998, 63, 8938.
(8) Newton, M. AdV. Chem. Phys. in press. Beratan, D. N.; Onuchic,
J. N. Chapter 2 in ref 9. Onuchic, J. N.; De Andrade, P. C. P.; Beratan, D.
N. J. Chem. Phys. 1991, 95, 1131; 1990, 92, 722.
(9) Bendall, D. S., Ed. Protein Electron Transfer; BIOS Scientific
Publishers Ltd.: Oxford, U.K., 1996.
(10) Kozlov, G. V. Ogawa, M. Y. J. Am. Chem. Soc. 1997, 119, 8377.
McLendon, G. L.; Wishart, J. F.; Baillard, E. R.; Corin, A. F. Proc. Natl.
Acad. Sci. U.S.A. 1996, 93, 9521. Rabanal, F.; DeGrado, W. F.; Dutton, P.
L. J. Am. Chem. Soc. 1996, 118, 473.
(11) Lloyd-Williams, P.; Albericio, F.; Giralt, E. Chemical Approaches
to the Synthesis of Peptides and Proteins; CRC Press: Boca Raton, FL,
1997; Bodanszky, M. In Principles of Peptide Synthesis, 2nd ed.; Springer-
Verlag: New York, 1993.
(12) Ezzell, S. A.; McCormick, C. L. Macromolecules 1992, 25, 1881.
(13) Jones, G., II; Oh, C. J. Phys. Chem. 1994, 98, 2367.
(14) Wynne, K.; Reid, G. D.; Hochstrasser, R. M. Opt. Lett. 1994, 19,
895.
2
ket ) 4π2/h|Hab| FCWD
(4)
(5)
2
2
|Hab| ) |Hab(max)| exp(-ârDA
)
0.22, an indicator of very effective electronic coupling for these
short links, but a number that is based on the simplest assumed
distance relation. For comparison, the range of â values observed
for oligoproline linked D-A systems is 0.2-1.0 Å-1 3,4
.
That the real distance dependence for this series is more
complicated is clear on comparing di- and tripeptides, 3 and 4.
Within the error limits of the decay time experiments, these
two peptides behave in a remarkably similar fashion in terms
of quantum yields of fluorescence and radical-ion-pair decay
times. Several factors could be at play in the assistance to charge
recombination for the longer peptide 4. According to the
Marcus-Hush semiclassical theory,8,32 the solvent reorganiza-
tion energy (λout) that is approximated by the dielectric
continuum model is a subtle function of distance. According to
Liang and Newton,35 this parameter is predicted to show a
weaker dependence on donor-acceptor separation for electron
transfer in the inVerted region.36
Another potential influence for the present series is the
“harpoon” mechanism proposed by Verhoeven et al.,37 under
which the average through-space distance between end groups
is influenced by electrostatic attraction, a feature that requires
more conformational freedom (i.e., linked ion pairs are driven
to favorable distances for charge recombination for the longer
peptide). This kind of mechanism will, however, depend to a
degree on the time scale of the Brownian motion of chain ends
for peptide oligomers. Some sense of this time scale is provided
in a study of energy transfer between naphthalene and (dim-
ethylamino)naphthylsulfonyl (dansyl) chromophores across a
(15) Clark, T. In Handbook of Computational Chemistry; John Wiley:
New York, 1985.
(16) Foster, R. Organic Charge-transfer Complexes; Academic Press:
New York, 1969.
(17) Jones, G., II. In Photoinduced Electron Transfer, Part A; Fox, M.
A., Chanon, M., Eds.; Elsevier: New York, 1988.
(18) Hush, N. S. Coord. Chem. ReV. 1985, 64, 135. Mulliken, R. S. J.
Am. Chem. Soc. 1952, 64, 811.

Electron Transfer across Amino Acid Spacers
J. Phys. Chem. B, Vol. 103, No. 3, 1999 581
(19) Creutz, C.; Newton, M. D.; Sutin, N. J. Photochem. Photobiol. A:
Chem. 1994, 82, 47.
with λmax ) 550 nm and ꢀ550 ) ca. 9500 M-1 cm-1 Farahat, C. W. Ph.D.
Dissertation, Boston University, 1992, Chapter 3.
(28) An alternate mechanism of charge recombination (formation of
singlet radical-ion pairs followed by decay, not to the ground state, but to
a local triplet) has been established in a separate nanosecond flash photolysis
study of Pyr-modified Ala oligomers containing electron donor tryptophans
(Trp);7 e.g., for Pyr-Ala-TrpOEt, formation of local triplet states of Pyr
was observed in the microsecond time domain, a result not obtained for
the present Pyr/DMPD series, consistent with the relative positioning in
energy of radical-ion pairs for the latter slightly below the Pyr triplet.
(29) Tamiaki, H.; Maruyama, K. J. Chem. Soc., Perkin Trans. 1 1991,
817.
(20) A Stern-Volmer plot of fluorescence quenching of 5 by AcDMPD
provided a value for the slope of 3.4 × 103 M-1; given a fluorescence
lifetime for 5 in DMF of 20 ns, a rate constant for bimolecular quenching
of 1.7 × 1011 M-1 s-1 is computed. Since the latter value is well above
that assumed for diffusion-limited quenching, a static quenching mechanism
is required (i.e., ground-state CT complexation).
(21) Curtiss, L. A.; Naleway, C. A.; Miller, J. R. J. Phys. Chem. 1995,
99, 1182. Jordan, K. D.; Paddon Row, M. N. Chem. ReV. 1992, 92, 395.
Shephard, M. J.; Paddon-Row, M. N.; Jordan, K. D. Chem. Phys. 1993,
176, 289.
(22) For examples and leading references regarding through-bond
intramolecular CT systems, see: Verhoeven, J. W.; Wegewijs, B.; Scherer,
T.; Rettschnick, R. P. H.; Warman, J. M.; Jaeger, W.; Schneider, S. J. Phys.
Org. Chem. 1996, 9, 387. Penfield, K. W.; Miller, J. R.; Paddon-Row, M.
N.; Cotsaris, E.; Oliver, A. M.; Hush, N. S. J. Am. Chem. Soc. 1987, 109,
5061.
(23) See, for example: Petrov, N. Kh.; Borisenko, V. N.; Alfimov, M.
V.; Tiebig, T.; Staerk, H. J. Phys. Chem. 1996, 100, 6368. Cao, H.; Fujiwara,
Y.; Haino, T.; Fukazawa, Y.; Tung, C. H.; Tanimoto, Y. Bull. Chem. Soc.
Jpn. 1996, 69, 2801.
(24) Average fluorescence decay times for 3-5 range 6-20 ns; further
details regarding fluorescence properties of the conjugates for various
solvents will be separately reported.25
(25) Jones, G., II; Zhou, X.; Lu, N. L.; Fu, H. Manuscript in preparation.
(26) Grellman, K. H.; Watkins, A. R.; Weller, A. J. Lumin. 1970, 12,
678. Lianos, P.; Georghiou, S. Photochem. Photobiol. 1979, 29, 13.
(27) In nanosecond flash photolysis experiments involving electron-
transfer quenching of the quinone oxidant, chloranil triplet, the radical-
cation of Ac-DMPD has been observed as a broadly absorbing transient
(30) Schanze, K.; Cabana, L. A. J. Phys. Chem. 1990, 94, 2740.
(31) Fox, M. A.; Miller, J. R. J. Org. Chem. 1991, 56, 5380.
(32) Kavarnos, G. J. Top. Curr. Chem. 1990, 156, 23.
(33) Jones, G., II; Lu, N. L.; Mari, F. Manuscript in preparation.
(34) Wright, P. E. Annu. ReV. Biophys. Biophys. Chem. 1991, 20, 519.
Cummings, A. L.; Eyring, E. M. Biopolymers 1975, 14, 2107.
(35) Liang, C.; Newton, M. D. J. Phys. Chem. 1993, 97, 3199.
(36) Using the dielectric continuum treatment, solvent reorganization
energies fall in the range, λout ) 0.83-1.10 eV (DMF solvent) for through-
space distances of Pyr/DMPD separation of 8.0-12.0 Å.
(37) Lauteslager, X. Y.; Wegewijs, B.; Verhoeven, J. W.; Brouwer, A.
M. J. Photochem. Photobiol. A 1996, 98, 121.
(38) Haas, E.; Katchalski-Katzir, E.; Steinberg, I. Z. Biopolymers 1978,
17, 11.
(39) An interesting finding of the energy transfer study38 is that the
relative rate of diffusive motion for chain ends is increasingly inhibited for
shorter peptide links.
(40) Kurnikov, I. V.; Zusman, L. D.; Kurnikova, M. G.; Farid, R. S.;
Beratan, D. N. J. Am. Chem. Soc. 1997, 119, 5690, and references cited
therein.