Anserine Radicals Formed in Photoinduced Reaction with 2,2′-Dipyridyl: Kinetics, Reduction by Aromatic Amino Acids, and Comparison with Methyl Histidine and Carnosine

Abstract

Selectively hyperpolarized NMR spectroscopy allows the indirect detection of optically silent methyl histidine radicals in the reversible photoinduced reactions of the dipeptide anserine (β-alanyl-1-methyl-histidine) with 2,2’-dipyridyl as photosensitizer using time-resolved chemically induced dynamic nuclear polarization (TR CIDNP) method. For comparison, experimental data were obtained for 1-methyl histidine, 3-methyl histidine and carnosine (β-alanyl-histidine). The CIDNP kinetics revealed the formation of the cationic radicals of the studied methyl histidine-containing compounds, resulting from their involvement in the reaction of degenerate electron exchange with the parent molecules, in contrast to the case of carnosine. This finding confirmed electron transfer as the mechanism of photo-induced oxidation of methyl histidine-containing compounds. TR CIDNP was also used to study the reduction of cation-radicals of anserine and methyl histidines by aromatic amino acids tryptophan and tyrosine. From the modeling of CIDNP kinetic data obtained for the three-component system containing photosensitizer, methyl histidine-containing compound, and reducing agent, we determine the rate constants of the reduction reaction. Our results highlight the utility of TR CIDNP to track reactions of histidine radicals at ambient conditions, providing a deeper understanding of the role of histidine-containing dipeptides in biochemical processes, including their potential antioxidant properties and impact on the management of oxidative stress.