The Photochemistry of Lipoic Acid: Photoionization and Observation of a Triplet Excited State of a Disulfide

Abstract

Abstract Under short‐wavelength UV irradiation, lipoic acid (LipSS) and its reduced form, dihydrolipoic acid (DHLA), undergo photoionization processes through a bi‐ or monophotonic pathway. After ionization, the LipSS radical cation (LipSS . + ) and radical anion (LipSS . − ) are generated. LipSS . − can be converted to equimolar amounts of LipSS and DHLA through second‐order decay. Triplet acetone can be quenched by LipSS and DHLA with a rate close to the diffusion‐controlled limit. The mechanism was further confirmed by continuous irradiation experiments. When LipSS is directly irradiated with UVA light, the first excited triplet state of LipSS is observed, with a lifetime τ =75 ns. Characteristic reactions include triplet energy transfer to oxygen and β‐carotene and addition to isoprene. The lifetime of triplet LipSS is also shortened by addition of water and methanol.