Abstract: As model for a reaction mimicking a chemical DNA repair process, the reduction of short-lived thymine radicals by tryptophan and N-acetyl tryptophan in a wide pH range in aqueous solution was investigated. To generate the radicals pulsed laser irradiation was implemented to a solution containing the photosensitizer 3,3′,4,4′-tetracarboxy benzophenone (TCBP), thymine, and tryptophan or N-acetyl tryptophan. As a preliminary step, the photoinduced oxidation of thymine by triplet-excited 3,3′,4,4′-tetracarboxy benzophenone was studied and thymine radical intermediates were characterized by means of time-resolved chemically induced dynamic nuclear polarization (CIDNP). It was shown that proton-coupled electron transfer (PCET) from neutral thymine to the triplet excited photosensitizer in neutral aqueous solution (pH 6.3) leads to the formation of a neutral thymine radical; its formation is preceded by the formation of an elusive cationic thymine radical on the geminate stage of the reaction. In contrast, in basic solution (pH 11.7) a thymine anion reacts with the triplet excited photosensitizer by electron transfer resulting in the formation of a TCBP radical anion and a neutral thymine radical. The latter deprotonates rapidly, and a thymine radical anion is formed at these conditions. To determine the rate of the reduction reaction between tryptophan and its N-acetyl derivative and the neutral thymine radical (pH 6.3) or the thymine radical anion (pH 11.7) the CIDNP kinetics obtained in the photoreaction of TCBP and thymine with and without adding a reducing agent were simultaneously analyzed quantitatively. As a result, the second order rate constants of the reduction reaction, kr, were determined: 1.1 × 109 M−1s−1 at pH 6.3 and 1.3 × 107 M−1s−1 at pH 11.7 for N-acetyl tryptophan; 1.1 × 109 M−1s−1 at pH 6.3 and 4.1 × 107 M−1s−1 at pH 11.7 for tryptophan.

Cite: Fishman N.N. , Morozova O.B. , Vieth H-M. , Yurkovskaya A.V.
Reduction of thymine radicals by Tryptophan: A study of CIDNP kinetics
Journal of Photochemistry and Photobiology A: Chemistry. 2022. V.426. 113761 :1-11. DOI: 10.1016/j.jphotochem.2021.113761 WOS Scopus OpenAlex

Dates:

Submitted:	Oct 8, 2021
Published online:	Jan 1, 2022
Published print:	Jan 12, 2022

Identifiers:

Web of science:	WOS:000781810400003
Scopus:	2-s2.0-85122635608
OpenAlex:	W4205936670

Citing:

DB	Citing
Scopus	3
Web of science	2

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