Antioxidant reactions of dihydrolipoic acid and lipoamide with triplet duroquinone

The oxidation of the antioxidants dihydrolipoate and lipoamide by triplet duroquinone ((3)DQ) has been studied by laser flash photolysis and time-resolved resonance Raman (TR3) spectroscopy. Reaction of (3)DQ with lipoamide by electron transfer [k(H2O)/k(D2O similar to 1] was more rapid than with dihydrolipoate, in which a proton is also involved [k(H2O)/k(D2O similar to 2]. For dihydrolipoate at neutral pH the undeprotonated form was the major reactive species with k similar to 10(9) dm(3) mol(-1) s(-1). At higher pH values the reaction of ionised (thiolate) forms was observed with k greater than or equal to 4 x 10(9) dm(3) mol(-1) s(-1). The electron transfer mechanism of reaction between 3DQ and lipoamide was confirmed by TR3 spectra in which formation of the durosemiquinone radical anion and lipoamide disulfide radical cation (RSS+.) was observed. (C) 1998 Academic Press.