Elucidation of Factors for Chiral Conversion of α-Lipoic Acid in Dietary Supplement

Abstract

We elucidated factors for the chiral conversion of α-lipoic acid enantiomers in α-lipoic acid-containing dietary supplements. Samples were cleaned up by the solid-phase dispersive extraction method using Oasis MAX and MCX as the solid-phase gel. The α-lipoic acid enantiomers were reciprocally converted by heating at 180°C, and finally became a racemate. The chiral conversion rate changed depending on sample purity, particularly the presence or absence of coexisting components in the dietary supplements. As candidates for coexisting components, neutral nonionic and highly polar substances were suggested, such as sugars. We found that chiral conversion was promoted by heating in the presence of glucose. Oftentimes, a relatively large amount of S-(–)-α-lipoic acid is detected in dietary supplements claiming to contain R-(+)-α-lipoic acid on the bottle label. We speculate that the proportion of S-form may have been increased by coexisting components such as glucose when heat treatment is performed during the manufacturing process.