Modulation of the rate of reversible electron transfer in oxidized tryptophan and tyrosine containing peptides in acidic aqueous solution.

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 pKa 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, kf, and from the tryptophan residue to the neutral tyrosyl radical, kr. 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, kf. This effect was manifested as an increase of 2 orders of magnitude in kf for a change in the linkage order between residues in the dipeptide: kf = 4 × 10(3) s(-1) for the oxidized Tyr-Trp increased to kf = 5.5 × 10(5) s(-1) 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.