J. Phys. Chem.
of 10.2 and 9.9. The
values found for tryptophan were pH-
independent below the pK of the radical (pK
4.7) and decreased
by 60 mV/pH at higher pH.
For p-methoxyphenol, pulse radiolysis st~dies'~J~
redox potentials of 0.32, 0.40, and 0.46 V at pH 13.5 and 0.6 V
at pH 7. The cyclic voltammetry studies gave
the error limits of the pulse radiolysis results. For tyrosine, cyclic
values of 0.93 and 0.72 V, respectively, at
and 13 compared to values of 0.85 and 0.64 V, respectively,
obtained by pulse radio1ysis.I Considering some of the assumptions
used to derive the latter values, the agreement is acceptable. For
tryptophan, cyclic voltammetry gave higher
values than ob-
tained from any of the pulse radiolysis studies and in neutral
solution the discrepancy is pronounced. In particular, the
measured by cyclic voltammetry at pH 7 is 1.015 V compared
to the pulse radiolysis values of 0.64 V obtained by Jovanovic et
al.' and 0.87 V reported by Butler et al.*
In neutral solution, redox potentials measured for substituted
indoles and phenols (Table
are clearly related to Hammett
coefficients for the substituent. However, the amino acid side chain
present in both tryptophan and tyrosine raises the
molecule above that expected for a simple alkyl side chain.15 This
is an important finding that merits further study; it appears to
confirm the hypothesis' that the amino acid side chain can exert
a strong influence on the redox properties of indole. Also, it
indicates the dangers of estimating redox potentials for amino acids
from simple model compounds and Hammett coefficients.*
Comparison of the data obtained for tryptophan and simple indoles
infers that the redox potential for tryptophan cannot
(13) Steenken, S.; Neta, P.
J. Phys. Chem.
Comparison of the measured
values for tryptophan and tyrosine
with the alkyl-substituted model compounds given in Table
infers that the
amino acid side chain raises
by about 160 mV.
be 0.64 V, as obtained recently by pulse radiolysis,' and the cyclic
voltammetric value of 1.015 V seems to be the correct figure.
It is clear from Figure 2 that the efficiency and direction of
electron transfer between tryptophan and tyrosine are related to
pH. That tyrosine radicals oxidize tryptophan at pH
3 and pH
12 has been demonstrated by pulse radiolysis studies.',* It has
also been demonstrated that tryptophan radicals will oxidize
tyrosine in neutral solution, although the bimolecular rate constants
are low.' According to Figure 2, there is a small thermodynamic
driving force for electron transfer in neutral solution.
-8 kJ mol-'
Such a modest driving force would explain the relative slowness
of electron transfer at pH