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

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 x 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.