1H CIDNP study of the kinetics and mechanism of the reversible photoinduced oxidation of tryptophyl-tryptophan dipeptide in aqueous solutions

Abstract

The pulsed method of chemically induced dynamic nuclear polarization (CIDNP) with the microsecond time resolution was used to study the nuclear polarization kinetics of Trp-Trp (tryptophyl-tryptophan) dipeptide protons in the photoreaction between the dipeptide and 2,2′-dipyridyl in aqueous solutions at pH from 2 to 10. The dependence of the selectivity of the reversible photoinduced oxidation of the dipeptide by 2,2′-dipyridyl in the triplet-excited state was found. When the pH of the solutions was below pKa of the terminal amino group, polarization was observed only for the protons of the C-terminal residue. The threshold effect of protonation of the terminal amino group of the dipeptide on the selectivity of both the deactivation of the 2,2′-dipyridyl triplet state and the degenerate intermolecular electron exchange was revealed. No intramolecular electron migration in the oxidized dipeptide was detected. At the pH exceeding pKa of the terminal amino group of the dipeptide, the quenching of the triplet excited state of 2,2′-dipyridyl is not selective. This is manifested as equal intensities of the nuclear polarization signals of the both residues of tryptophan in the 1H NMR spectrum of the dipeptide containing the CIDNP signals of the geminate reaction products. The rate constant of the degenerate electron exchange and nuclear paramagnetic relaxation times of the protons of the neutral radical and radical cation of the dipeptide were determined by the numerical simulation of the nuclear polarization kinetics.