Hemiketal formation of dehydroascorbic acid drives ascorbyl radical anion disproportionation

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 alpha-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 alpha-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.