Marcus Cross-Relationship Probed by Time-Resolved CIDNP.

The time-resolved CIDNP method can provide information about degenerate exchange reactions (DEEs) involving short-lived radicals. In the temperature range from 8 to 65 °C, the DEE reactions of the guanosine-5'-monophosphate anion GMP(-H)- with the neutral radical GMP(-H)•, of the N-acetyl tyrosine anion N-AcTyrO- with a neutral radical N-AcTyrO•, and of the tyrosine anion TyrO- with a neutral radical TyrO• were studied. In all the studied cases, the radicals were formed in the reaction of quenching triplet 2,2'-dipyridyl. The reorganization energies were obtained from Arrhenius plots. The rate constant of the reductive electron transfer reaction in the pair GMP(-H)•/TyrO- was determined at T = 25 °C. Rate constants of the GMP(-H)• radical reduction reactions with TyrO- and N-AcTyrO- anions calculated by the Marcus cross-relation differ from the experimental ones by two orders of magnitude. The rate constants of several other electron transfer reactions involving GMP(-H)-/GMP(-H)•, N-AcTyrO-/N-AcTyrO•, and TyrO-/TyrO• pairs calculated by cross-relation agree well with the experimental values. The rate of nuclear paramagnetic relaxation was found for the 3,5 and ß-protons of TyrO• and N-AcTyrO•, the 8-proton of GMP(-H)•, and the 3,4-protons of DPH• at each temperature. In all cases, the dependences of the rate of nuclear paramagnetic relaxation on temperature are described by the Arrhenius dependence.

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

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.