Short‐lived anserine radicals, generated in the photoinduced reaction with triplet 3,3′,4,4′‐tetracarboxy benzophenone, were studied using time‐resolved chemically induced dynamic nuclear polarization (CIDNP). Significant differences were found in the CIDNP kinetics in neutral and basic aqueous solutions. At pH 6, oxidized form of anserine with positively charged amino group is cation radical NH 3 + Ans • + . This was proved by observation of fast CIDNP decay typical to the case of degenerate electron exchange (DEE) with the parent molecule. The absence of such a decay in basic solution indicated that primary anserine radicals with neutral amino group, NH 2 Ans • + , undergo fast () transformation into the secondary radicals (NH 2 Ans • + )′ stabilized by some interaction of functional groups, which violates the conditions for DEE. Transition from one type of kinetics to another occurs at a pH well below the p K a of the amino group of anserine (9.5). Analysis of kinetic data allowed us to estimate p K a ≈ 6.4, which refers to the amino group of NH 3 + Ans • + radicals. Radicals (NH 2 Ans • + )′ were found to be resistant to the reduction by tryptophan or tyrosine, while radicals NH 3 + Ans • + are readily reduced by both aromatic amino acids with the rate constants of several units of 10 8 M −1 s −1 .
Morozova et al. (Wed,) studied this question.
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