Laser flash photolysis and time resolved CIDNP study of photoreaction of 2,2′-dipyridyl with N-acetyl tyrosine in aqueous solutions

Abstract

Laser flash photolysis and time-resolved chemically induced dynamic nuclear polarization (CIDNP) methods have been applied to the investigation of the photoreaction of 2,2′-dipyridyl and N-acetyl tyrosine. The quenching of the triplet dipyridyl by tyrosine proceeds via an electron transfer under acidic (pH < 5) and strong basic (pH > 10.5) conditions, and a hydrogen transfer in neutral and moderately basic (6 < pH < 9.5) solutions. The rate constant of the electron transfer is close to the diffusion-controlled limit k q = (2–3.5) × 10 9 M −1 s −1, whereas the rate constant of the hydrogen transfer is significantly lower: k q = 7 × 10 7 M −1 s −1 in non-buffered solutions and k q = 2 × 10 8 M −1 s −1 in buffered solutions. The pattern of the CIDNP spectrum does not change upon pH variation, which has been attributed to the fast deprotonation of tyrosyl cation radicals formed in acidic solutions. The kinetics of nuclear polarization obtained at different pH values allows for the determination of the rate constant of the degenerate electron exchange between tyrosyl radical and anion of tyrosine k ex = (9 ± 1.5) × 10 7 M −1 s −1, and the nuclear relaxation times for tyrosyl radical: T 1 = 63 ± 16 μs for H3,5 and T 1 = 200 ± 60 μs for β-protons.