The photochemistry of lipoic acid: photoionization and observation of a triplet excited state of a disulfide.
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 tau=75 ns. Characteristic reactions include triplet energy transfer to oxygen and beta-carotene and addition to isoprene. The lifetime of triplet LipSS is also shortened by addition of water and methanol.
Bucher
Götz
G
Lehrstuhl für Organische Chemie II, Ruhr-Universität, 44780 Bochum, Germany. goetz.bucher@ruhr-uni-bochum.de
Lu
Changyuan
C
Sander
Wolfram
W
D000096:Acetone / Q000737:chemistry; D000975:Antioxidants / Q000737:chemistry / Q000528:radiation effects; D003500:Cyclization; D004220:Disulfides / Q000737:chemistry*; D005609:Free Radicals / Q000737:chemistry*; D007834:Lasers; D015394:Molecular Structure; D010084:Oxidation-Reduction; D010782:Photolysis; D008063:Thioctic Acid / Q000031:analogs & derivatives* / Q000737:chemistry / Q000528:radiation effects; D013997:Time Factors; D014466:Ultraviolet Rays
D000975:Antioxidants; D004220:Disulfides; D005609:Free Radicals; D000096:Acetone; D008063:Thioctic Acid; C015144:dihydrolipoic acid