Methylene blue photosensitized oxidation of cysteine sulfinic acid and other sulfinates: The involvement of singlet oxygen and the azide paradox

The methylene blue photosensitized oxidation of cysteine sulfnic acid is in vestigated. Enhancement of the oxygen consumption rate in deuterium oxide s uggests the involvement of singlet oxygen (O-1(2)) in oxidation, Addition o f the O-1(2) quencher azide produced an unusual enhancement of the oxidatio n rate of all the sulfinates assayed. It is assumed that azide works as a o ne-electron carrier between O-1(2) and the sulfur compounds, Analyses of th e products indicate that the photochemical oxidation of cysteine sulfinic a cid proceeds through two simultaneous mechanisms, The Type II (singlet oxyg en) mechanism is responsible for oxidation of the sulfinic group to the sul fonic group with production of cysteic acid, stable to the photooxidation s ystem, whereas the Type I (electron transfer) mechanism is involved in the degradation of cysteine sulfinic acid to acetaldehyde. Other products detec ted were ammonia, sulfate, and hydrogen peroxide which account for the degr adation of cysteine sulfinic acid and for the excess of oxygen consumption detected during the oxidative reaction. (C) 2000 Academic Press.