TY - JOUR

T1 - Proton-coupled electron transfer as the mechanism of reaction between triplet state of kynurenic acid and tryptophan

AU - Zhuravleva, Yuliya S.

AU - Morozova, Olga B.

AU - Tsentalovich, Yuri P.

AU - Sherin, Peter S.

N1 - Publisher Copyright: © 2020 Elsevier B.V. Copyright: Copyright 2020 Elsevier B.V., All rights reserved.

PY - 2020/6/1

Y1 - 2020/6/1

N2 - Kynurenic acid (KNA−) is an endogenous UV-A chromophore of the human eye lens able to sensitize photodamages to eye lens proteins. In the present work, we studied in details the mechanism of reaction between KNA− triplet state (TKNA−) and tryptophan (Trp), the most effective quencher of TKNA− among aromatic and sulfur-containing amino acids, by nanosecond laser flash photolysis. Previous studies suggested electron transfer as the reaction mechanism without its direct spectroscopic confirmation. Time-resolved kinetics and spectral data of this work clearly evidence the formation of neutral tryptophanyl radicals immediately after the TKNA− quenching by Trp that indicates either hydrogen transfer or proton-coupled electron transfer (PCET) as the reaction mechanism. Low kinetic isotope effect for the quenching rate constant, kH2O/kD2O = 1.3, speaks in favor of PCET mechanism of the quenching reaction, which includes the diffusion-controlled electron transfer as the primary step and the proton transfer in the radical cage as the final step.

AB - Kynurenic acid (KNA−) is an endogenous UV-A chromophore of the human eye lens able to sensitize photodamages to eye lens proteins. In the present work, we studied in details the mechanism of reaction between KNA− triplet state (TKNA−) and tryptophan (Trp), the most effective quencher of TKNA− among aromatic and sulfur-containing amino acids, by nanosecond laser flash photolysis. Previous studies suggested electron transfer as the reaction mechanism without its direct spectroscopic confirmation. Time-resolved kinetics and spectral data of this work clearly evidence the formation of neutral tryptophanyl radicals immediately after the TKNA− quenching by Trp that indicates either hydrogen transfer or proton-coupled electron transfer (PCET) as the reaction mechanism. Low kinetic isotope effect for the quenching rate constant, kH2O/kD2O = 1.3, speaks in favor of PCET mechanism of the quenching reaction, which includes the diffusion-controlled electron transfer as the primary step and the proton transfer in the radical cage as the final step.

KW - Kynurenic acid

KW - Proton-coupled electron transfer

KW - Radicals

KW - Triplet state

KW - Tryptophan

KW - OXIDATION

KW - TIME-RESOLVED CIDNP

KW - TYROSINE

KW - UV FILTER

KW - AMINO-ACIDS

KW - KINETICS

KW - PHOTOCHEMISTRY

KW - PROTEINS

KW - WATER

KW - PHOTOOXIDATION

UR - http://www.scopus.com/inward/record.url?scp=85083668224&partnerID=8YFLogxK

U2 - 10.1016/j.jphotochem.2020.112522

DO - 10.1016/j.jphotochem.2020.112522

M3 - Article

AN - SCOPUS:85083668224

VL - 396

JO - Journal of Photochemistry and Photobiology A: Chemistry

JF - Journal of Photochemistry and Photobiology A: Chemistry

SN - 1010-6030

M1 - 112522

ER -