Topical delivery of retinyl ascorbate: 4. Comparative anti-oxidant activity towards DPPH

Abstract

The free radical scavenging properties of retinyl ascorbate (RA-AsA) were determined by monitoring the decomposition of 2,2-diphenyl-1-picrylhydrazyl (DPPH) as a function of time and in comparison with ascorbic acid (AsA), ascorbic acid palmitate (AsA-Pal), retinoic acid (RA), retinol (ROL) and retinol palmitate (Rol-Pal). The rate constant of RA-AsA (mean3±SD) was 4.9±0.3 M−1 s−1, and indicated greater potency as an antioxidant compared to the rest of the test compounds (AsA 3.4±0.4 M−1 s−1, AsA-Pal, 2.9±0.2 M−1 s−1, RA 1.4±0.3 M−1 s−1, ROL 1.3±0.1 M−1 s−1, Rol-Pal exhibited insignificant activity). The decomposition rate constant of DPPH, 5±0.6 × 10−8 M−1 s−1, in ethanol and BHA, 154±3 M−1 s−1 were both used as control. The compound RA-2-carboxy-2-hydroxy-ethanoate was isolated by prep-TLC and was identified, by 13C and 1HNMR spectroscopy, as the major by-product from the reaction of RA-AsA with DPPH, which was also found to be potent antioxidant, 2.1±0.2 M−1 s−1. This suggests that oxidation of AsA moiety did not lead to the production of erythrulose species, which could cause deleterious modifications of cellular proteins.