DECOMPOSITION OF ALPHA-LIPOIC ACID-DERIVATIVES BY PHOTOIRRADIATION-FORMATION OF DIHYDROLIPOIC ACID FROM ALPHA-LIPOIC ACID

Due to its strained five-membered ring, alpha-lipoic acid (LA) has an absorption band around 330 nm, which is used to quantify its concentra tion. In order to obtain information for the homolytic rupture of the S-S bond and the formation of dihydrolipoic acid (DHLA), the photochem ical reaction of lipoic acid was examined in the presence or absence o f ascorbic acid upon exposure to UVA light. The absorption band of alp ha-lipoic acid at around 330 nm disappeared by photoirradiation, which corresponds to the rupture of S-S bond of the 1,2-dithiolane ring in lipoic acid. HPLC-Electrochemical Detection (ECD) analysis of lipoic a cid showed significant formation of dihydrolipoic acid and other thiol s. The formation of thiols from the photoreaction of lipoic acid was a lso confirmed by the Ellman method. The formation of thiols from lipoi c acid was completely time-dependent and the formation of the thiols i ncreased up to 55%. Similar results were obtained in the photochemical reactions of short-chain analogues, bisnor- and tetranor lipoic acid. On the other hand, beta-lipoic acid was quite stable, no photodecompo sition of beta-lipoic acid was observed in the UV region. The formatio n of thiols including dihydrolipoic acid from lipoic acid can be expla ined by considering the rupture of S-S bond, which results in the form ation of the dithiyl radicals of alpha-lipoic acid. It is proposed tha t intra- and intermolecular hydrogen abstraction of the dithyl radical produces thiols including dihydrolipoic acid as final products.