In this paper the relationship between the ascorbate anion (AH-) and its oxidation products, ascorbyl radical anion (A·-) and dehydroascorbic acid (DHA), are studied by means of theoretical calculations. Additional calculations are performed on α-hydroxytetronate, a model compound of ascorbate lacking the side chain. The method uses density functional theory with the B3LYP functional and a polarizable conductor dielectric model to compute solvation effects. Our results indicate that the model compound reacts with the α-tocopheroxyl radical to regenerate vitamin E with a free energy change of reaction (in water) of -7.4 kcal/mol. This reaction is 2.9 kcal/mol more exergonic than the corresponding reaction involving ascorbate, suggesting that the model compound may make a more effective antioxidant than ascorbate. However, the disproportionation of the ascorbyl radical anion, a reaction that regenerates AH-, is found to be exergonic while the similar reaction involving the model compound is slightly endergonic. The reason for the difference is that the disproportionation of A·- is found to be driven by the formation of the hemiketal structure of dehydroascorbic acid (DHA). (C) 2000 Elsevier Science Inc.

Ascorbyl free radical, Dehydroascorbic acid, Density functional theory, Disproportionation, Free radicals, Solvent effects
Free Radical Biology and Medicine
Ottawa-Carleton Chemistry Institute

DiLabio, G.A. (Gino A.), & Wright, J.S. (2000). Hemiketal formation of dehydroascorbic acid drives ascorbyl radical anion disproportionation. Free Radical Biology and Medicine, 29(5), 480–485. doi:10.1016/S0891-5849(00)00357-9