Revisit of the Photoirradiation of α-Lipoic Acid—Role of Hydrogen Sulfide Produced in the Reaction

Abstract

α-Lipoic acid (LA) has the specific absorption band at 330 nm and is quite vulnerable to UV irradiation, affording a variety of compounds including polymeric materials and hydrogen sulfide. A better understanding of the photochemical reaction of LA has already been carried out focusing mainly on the reaction product analysis derived from LA. We re-investigated the photochemical reaction of LA focusing our attention on the fate of hydrogen sulfide (H2S) produced in the photochemical reaction procedure. The photoirradiation of LA in the presence of oxidized glutathione (GSSG) formed glutathione trisulfide (GSSSG) and a reduced form of glutathione (GSH). Similar results were obtained in the co-presence of cystine and dimethyl disulfide. The concentration of H2S was reaching the maximum concentration, which was gradually decreasing within 10 min after photoirradiation, while the concentration of GSSSG was increasing with the decrease of H2S concentration. The structural confirmation of GSSSG and the plausible mechanism for the formation of GSSSG are proposed based on the time-dependent and pH-dependent profile of the photoirradiation.