The Structure of Dehydroascorbic Acid in Solution.

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Abstract
The structure of dehydroascorbic acid, an oxidation product of vitamin C in aqueous solution, was examined by 13C and 1H NMR in different solvents. The dimeric molecule was unstable in solution and transformed to other species. In dimethyl formamide and dimethyl sulfoxide at room temperature an equilibrium was set up between the symmetric dimer and an asymmetric anomer. The standard molar free energy difference is determined to be 3.0 .+-. 0.5kJ mol-1 in favor of the asymmetric anomer. Either dimer reacted with H2O to form a bicyclic hydrated monomer. In aqueous solution at room temperature the furanose ring of this momomer opened to a free side chain of ascorbic acid. A hydrated monomer should have higher stability in aqueous solutions, such as in body fluids, than the dimer. In other physiological systems the presence of dimers seems more likely and they are presumably useful as the vitamin''s storage version because of their higher lipid solubility and stability toward oxidative degradation.

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