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 β-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.