Abstract
Photoinduced oxidation of anserine and carnosine by triplet 3,3',4,4'-tetracarboxy benzophenone (TCBP) has been investigated in aqueous solution using time-resolved laser flash photolysis and chemically induced dynamic nuclear polarization (CIDNP). Rate constants of oxidation via triplet quenching were obtained over a wide pH range. The formation of radical pairs as a result of quenching was proven by the observation of CIDNP effects. For comparison with anserine, pH dependences of the quenching rate constant and CIDNP were obtained for the photoreaction of TCBP with 1- and 3-methyl histidine, while the results for carnosine were compared with those previously obtained for histidine. The obtained pH dependences were described in terms of quenching rate constants and relative CIDNP enhancement factors for five reactant pairs corresponding to their exact protonation states. In the case of carnosine and histidine, maxima of the observed quenching rate constant and CIDNP amplitude coincide, while in the case of methylated compounds, these maxima diverge. In the case of anserine, under biologically relevant conditions at pH ∼ 6 ÷ 7, an additional pair of reactants was discovered that provides an anomalously high CIDNP enhancement factor, exceeding that of other radical pairs by at least 2 orders of magnitude. This pair consists of the anserine cation radical with protonated amino group and the TCBP anion radical with two deprotonated carboxyl groupsa chemical structure that potentially facilitates the formation of a pair of transient radicals with an extended lifetime, resulting in a significant increase in CIDNP.