Chemically induced dynamic nuclear polarization study of the reduction of histidine radical in the reactions with aromatic amino acids

Abstract

In order to model the involvement of histidine radicals in the electron transfer reactions in proteins, we investigated the kinetics of the reduction of hisitidine radicals in the reactions with aromatic amino acid residues by means of chemically induced dynamic nuclear polarization with microsecond time resolution. Histidine radicals were generated in photochemical reaction with triplet-excited 3,3’,4,4’-benzophenone tetracarboxylic acid. The intermolecular electron transfer from N-acetyl derivatives of tyrosine and tryptophan to the radical of N-acetyl histidine was studied in neutral and basic aqueous solutions. It was found that the rate constant of reduction of short-lived histidine radical in the reactions with tyrosine anion is k_r = (1.9±0.5)•10⁸ L mol^–1 s^–1, which is more than two orders of magnitude higher than that of reduction of histidine radical with neutral tyrosine (k_r = (1.2±0.4)•10⁶ L mol^–1 s^–1). In the reduction of histidine radical with N-acetyl tryptophan, the protonated states of reagents do not change, when pH of aqueous solution varies from neutral to basic and the reaction rate coefficient remains constant being k_r = (1.3±0.4)•10⁶ L mol^–1 s^–1.