An electron spin resonance and time-resolved CIDEP study of the oxidation of ascorbic acid by pyruvic acid, duroquinone, and vitamin K1

The photooxidation of vitamin C by pyruvic acid and its derivatives, duroquinone, and vitamin K1, is systematically studied by the combined esr and time-resolved CIDEP technique. Because of the unique property of the triplet pyruvic acid which is different from that of the duroquinone triplet, the choice of these model systems allowed the CIDEP observations of the initial polarization of the ascorbate radical anion and its counter primary radical in either the enhanced absorptive or the emissive mode. The results demonstrate the efficient photooxidation of vitamin C by the triplet vitamin K1 and that the latter triplet possesses similar properties as the triplet quinones. In the photooxidation system involving vitamin C and pyruvic acid, the adjustment of the pH of the aqueous solution afforded an opportunity to observe the photochemical reaction between two anions, the pyruvate anion and the ascorbate anion. Arguments were made in favor of an electron transfer mechanism for the oxidation of vitamin C by triplet vitamin K1 but against the electron transfer mechanism for the oxidation of vitamin C by triplet pyruvate anion. T1 values of the primary radicals estimated from their CIDEP transient responses at low microwave power are reported and it was suggested that the hydrogen-bonding capacity of the ascorbate radical anion in water does not contribute significantly to the relative difference of T1's between itself and the counter pyruvic ketyl radical.