Marcus Cross-Relationship Probed by Time-Resolved CIDNP

Abstract

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.

Keywords

• chemically induced nuclear polarization (CIDNP)
• guanosine monophosphate
• tyrosine anion
• short-lived radicals
• degenerate electronic exchange
• Marcus theory