C O M M U N I C A T I O N S
Table 1. Rate and Equilibrium Constants for Added Sodium Dibasic Phosphate and Histidinea
(M-1)
k
(s-1)
k
-1
(M- s-1)
1
k
(M- s-1)
1
K
′
(M-1)
k
red (M- s-1)
1
base
pK
a
K
A
1
2
A
5
5
3
3
7
7
4
4
Na2HPO4
histidine
7.2
6.6
30.0 ( 0.1
26.3 ( 0.3
3.3 ( 0.1 × 10
1.4 ( 0.1 × 10
5.0 ( 0.4 × 10
1.9 ( 0.1 × 10
1.7 ( 0.3 × 10
1.4 ( 0.1 × 10
22.2 ( 0.1
37.8 ( 0.2
2.3 ( 0.5 × 10
6.9 ( 0.3 × 10
a
According to Scheme 1, at room temperature in 0.8 M NaCl.
reaction can be described as occurring by multisite electron-proton
5
transfer. It is important as a possible model for the tyrosine-
histidine pair in photosystem II.5 There is precedence for the
appearance of this pathway in earlier observations on oxidation of
phenols by the triplet excited state of C60 in the presence of added
N-bases.15
,7
Recent results on the oxidation of hydrogen-bonded phenols in
acetonitrile also support a concerted reaction,16 as do voltammetric
1
7
results on hydrogen-bonded phenols in nonpolar solvent. By
contrast, in closely related studies, intramolecular oxidation of a
Figure 2. (A) Cyclic voltammograms of 20 µM (solid) and 40 µM Os-
2+
phenol linked to a Ru(bpy)
to changes in the external pH and not to added buffer.
3
derivative has been found to respond
2
+
(
(
(
(
bpy)3 in the presence of 0.1 mM tyrosine in 50 mM phosphate buffer
1
8
2
-
-
pH ) 8.0, [HPO4 ]/H2PO4 ] ) 15/1) at room temperatue in 0.8 M NaCl.
B) Dependence of kobs on the reduction potential of the oxidant at pH 7.5
closed) and pH 6.0 (open) for the series Os(bpy)3 , Fe(bpy)3 , Ru(4,4′-
The observation of competing pathways suggests that enzymes
may have the ability to tune kinetic pathways based on solvent
accessibility of the oxidized tyrosine.12 Such tunability may be
critical in regulating enzyme kinetics and in allowing enzyme
mechanisms to respond to proton gradients.
2
+
2+
2
+
2+
2+
dimethyl-bpy)3 , Ru(bpy)2(4,4′-dimethyl-bpy) , and Ru(bpy)3
Scheme 1
Acknowledgment. This research was supported by the Uni-
versity of North Carolina. We thank Dr. Stephen Feldberg for
numerous helpful discussions.
Supporting Information Available: Parameters, mechanisms, and
methods of digital simulation as well as rate constants. This material
is available free of charge via the Internet at http://pubs.acs.org.
References
are observed in both, allowing for independent evaluation of kred
(
1) Cukier, R. I.; Nocera, D. G. Annu. ReV. Phys. Chem. 1998, 49, 337-369.
2-
and K′ . (4) When the reaction with added HPO
in D O, H O/D
2 2 2
4
was performed
O isotope effects of 1.7 ( 0.1 and 1.0 ( 0.1 for
and K′ were observed and 2.1 ( 0.6 and 1.2 ( 0.4 for kred and
A
(2) Mayer, J. M. Annu. ReV. Phys. Chem. 2004, 55, 363-390.
(3) Hammes-Schiffer, S. Acc. Chem. Res. 2001, 34, 273-281.
(
4) Brudvig, G. W.; Thorp, H. H.; Crabtree, R. H. Acc. Chem. Res. 1991, 24,
311.
K
A
A
2-
-
k
1
(Figures S3 and S4). (5) Bulk electrolysis in HPO
4
/H PO
2 4
(5) Alstrum-Acevedo, J. H.; Brennaman, M. K.; Meyer, T. J. Inorg. Chem.
2
005, 44, 6802-6827. Meyer, T. J.; Huynh, M.-H. V.; Thorp, H. H.
occurs with n ) 1, consistent with radical coupling following one-
electron oxidation.
Angew. Chem., Int. Ed., submitted for publication, 2006.
(6) Stubbe, J.; Nocera, D. G.; Yee, C. S.; Chang, M. C. Y. Chem. ReV. 2003,
1
03, 2167-2202.
Direct oxidation of tyrosine with added phosphate at the electrode
in the absence of catalyst is undetectable over the entire pH range
studied. The catalytic effect of added base is considerable. Oxidation
(
7) Tommos, C.; Babcock, G. T. Acc. Chem. Res. 1998, 31, 18-25.
(8) Johnston, D. H.; Glasgow, K. C.; Thorp, H. H. J. Am. Chem. Soc. 1995,
1
17, 8933-8938.
(
9) Napier, M. E.; Hull, D. O.; Thorp, H. H. J. Am. Chem. Soc. 2005, 127,
2+
of TyrOH by Os(bpy)
3
, followed by spectrophotometric monitor-
11952-11953.
(
10) Sistare, M. F.; Holmberg, R. C.; Thorp, H. H. J. Phys. Chem. B 1999,
ing in 0.8 M NaCl at room temperature (pH ) 7) occurs with k ∼
103, 10718-10728.
2
-1 -1
2
1
1
.7 × 10 M s , which is slower by ∼10 than kred
with added HPO or histidine at neutral pH.
These results demonstrate that catalyzed oxidation of tyrosine
in water occurs at a significant rate following association with
A
K ′ in Scheme
(
11) Armistead, P. M.; Thorp, H. H. Anal. Chem. 2000, 72, 3764-3770.
2-
(12) 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, 3181-3182.
4
(13) Bock, C. R.; Connor, J. A.; Gutierrez, A. R.; Meyer, T. J.; Whitten, D.
G.; Sullivan, B. P.; Nagle, J. K. J. Am. Chem. Soc. 1979, 101, 4815.
-
2-
histidine or the base form of a H
2
PO
4
/HPO
4
buffer, presumably
(
14) Marcus, R. A.; Sutin, N. Biochim. Biophys. Acta 1985, 811, 265-322.
forming H-bonded association complexes such as ArOH-OP(OH)-
(15) Biczok, L.; Gupta, N; Linschitz, H. J. Am. Chem. Soc. 1997, 119, 12601-
2-
12609. See also: Shukla, D.; Young, R. H.; Farid, S. J. Phys. Chem. A
O
2
. Once formed, the association complexes can react either via
2004, 108, 10386-10394.
concerted loss of electrons and protons (EPT), kred, or via rate-
limiting proton transfer followed by electron transfer (PT-ET)
oxidation of the phenoxide anion, Scheme 1.
(16) Rhile, I. J.; Mayer, J. M. J. Am. Chem. Soc. 2004, 126, 12718-12719.
17) Costenin, C.; Robert, M.; Saveant, J. J. Am. Chem.. Soc. 2006, 128, 4552-
(
4553.
(18) Sj o¨ din, M.; Styring, S.; Wolpher, H.; Xu, Y.; Sun, L.; Hammarstr o¨ m, L.
J. Am. Chem. Soc. 2005, 127, 3855-3863.
In the EPT pathway, electron and proton transfers occur to
3+
2-
separate acceptors, (Os(bpy)
3
and HPO
4
in Scheme 1), and the
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