Modulation
of the Rate of Reversible Electron Transfer
in Oxidized Tryptophan and Tyrosine Containing Peptides in Acidic
Aqueous Solution
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Abstract
Time-resolved chemically induced
dynamic nuclear polarization (CIDNP)
was used to investigate reversible intramolecular electron transfer
(IET) in short-lived oxidized peptides, which had different structures
and contained tryptophan and tyrosine residues, in an acidic aqueous
solution with a pH below the p<i>K</i><sub>a</sub> of the
tryptophanyl cation radical. The CIDNP kinetic data were obtained
at the microsecond scale and were analyzed in detail to calculate
the rate constants for electron transfer in both directions: from
the tyrosine residue to the tryptophanyl cation radical, <i>k</i><sub>f</sub>, and from the tryptophan residue to the neutral tyrosyl
radical, <i>k</i><sub>r</sub>. The charge of the terminal
amino group and the presence of glycine and proline spacers were shown
to strongly affect the rate constants of the reaction under study.
Among these functional groups, the presence and the location of the
positive charge on the amino group in close proximity to the cationic
indolyl radical had the strongest effect on the rate constant of the
forward IET from the tyrosine residue to the tryptophanyl radical
cation, <i>k</i><sub>f</sub>. This effect was manifested
as an increase of 2 orders of magnitude in <i>k</i><sub>f</sub> for a change in the linkage order between residues in the
dipeptide: <i>k</i><sub>f</sub> = 4 × 10<sup>3</sup> s<sup>–1</sup> for the oxidized Tyr-Trp increased to <i>k</i><sub>f</sub> = 5.5 × 10<sup>5</sup> s<sup>–1</sup> in oxidized Trp-Tyr. The reverse rate constant for IET was less
sensitive to the amino group charge. Moreover, the presence of glycine
or proline spacers in the peptides with a tryptophan residue at the
N-terminus not only reduced the IET rate constant but also shifted
the equilibrium of the IET in the reaction under study toward the
formation of tyrosyl radicals with respect to the peptide Trp-Tyr.
That is, the glycine or proline spacers affected the difference in
the reduction potential of the tryptophanyl and tyrosyl radicals