TY - JOUR

T1 - Unravelling structures of radicals of kynurenic acid formed in the photoinduced reactions with tryptophan and N-acetyl tyrosine

AU - Morozova, Olga B.

AU - Geniman, Maksim P.

AU - Panov, Mikhail S.

AU - Fishman, Natalya N.

AU - Yurkovskaya, Alexandra V.

AU - Sherin, Peter S.

N1 - Funding Information: The computing part of this work was supported by RFBR (project numbers 21-53-12023 and 20-03-00234) and the experimental part by the Ministry of Education and Science of the Russian Federation (contract 075-15-2020-779). Publisher Copyright: © 2022 The Royal Society of Chemistry.

PY - 2022/10/24

Y1 - 2022/10/24

N2 - Kynurenic acid (KNA) in the triplet state reacts with tryptophan (Trp) at neutral pH via proton-coupled electron transfer (PCET), which includes the stepwise transition of both electron and proton from Trp to triplet KNA. In the case of tyrosine (Tyr), the quenching reaction is H-transfer, a simultaneous transfer of electron and proton. In this work, we used the time-resolved chemically induced dynamic nuclear polarization (TR CIDNP) method to unveil the sites of H/H+ transfer within KNA. For this purpose, we obtained the values of 1H hyperfine coupling constants (HFCCs) and g-factors for different tautomeric forms of KNA radicals by the DFT method, then calculated CIDNP intensities using these g-factors and HFCCs according to the Adrian model. The calculated CIDNP intensities for different protons were correlated with their CIDNP intensities in the geminate spectra detected in the photoreactions of KNA with Trp, N-acetyl Trp, and N-acetyl Tyr. Best-fit proportionality relationships between calculated and experimental CIDNP intensities have shown that the KNA anion radical is present in two of the three possible tautomeric forms, which result from the H/H+ movement to the carbonyl oxygen of keto- and oxo-quinolinate forms of KNA, without any visible contribution of the H/H+ transfer to the nitrogen of the enol form. For 4-hydroxyquinoline (4HQN), being the chromophoric core of KNA and exhibiting the same PCET and H-transfer reactions with Trp and Tyr, a single possible tautomeric form of its radical has been revealed as H/H+ transfer to the carbonyl oxygen of the keto-form.

AB - Kynurenic acid (KNA) in the triplet state reacts with tryptophan (Trp) at neutral pH via proton-coupled electron transfer (PCET), which includes the stepwise transition of both electron and proton from Trp to triplet KNA. In the case of tyrosine (Tyr), the quenching reaction is H-transfer, a simultaneous transfer of electron and proton. In this work, we used the time-resolved chemically induced dynamic nuclear polarization (TR CIDNP) method to unveil the sites of H/H+ transfer within KNA. For this purpose, we obtained the values of 1H hyperfine coupling constants (HFCCs) and g-factors for different tautomeric forms of KNA radicals by the DFT method, then calculated CIDNP intensities using these g-factors and HFCCs according to the Adrian model. The calculated CIDNP intensities for different protons were correlated with their CIDNP intensities in the geminate spectra detected in the photoreactions of KNA with Trp, N-acetyl Trp, and N-acetyl Tyr. Best-fit proportionality relationships between calculated and experimental CIDNP intensities have shown that the KNA anion radical is present in two of the three possible tautomeric forms, which result from the H/H+ movement to the carbonyl oxygen of keto- and oxo-quinolinate forms of KNA, without any visible contribution of the H/H+ transfer to the nitrogen of the enol form. For 4-hydroxyquinoline (4HQN), being the chromophoric core of KNA and exhibiting the same PCET and H-transfer reactions with Trp and Tyr, a single possible tautomeric form of its radical has been revealed as H/H+ transfer to the carbonyl oxygen of the keto-form.

KW - Kynurenic Acid

KW - Tryptophan

KW - Protons

KW - Tyrosine

KW - Electrons

UR - http://www.scopus.com/inward/record.url?scp=85142268235&partnerID=8YFLogxK

U2 - 10.1039/d2cp03245g

DO - 10.1039/d2cp03245g

M3 - Article

C2 - 36346380

AN - SCOPUS:85142268235

VL - 24

SP - 27558

EP - 27565

JO - Physical Chemistry Chemical Physics

JF - Physical Chemistry Chemical Physics

SN - 1463-9076

IS - 44

ER -