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

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, k_H2O/k_D2O = 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.