Ab initio molecular dynamics study of ascorbic acid in aqueous solution

The ascorbic radical anion A*− in aqueous solution was studied using ab initio molecular dynamics based on density functional theory. Calculations of the spin density indicate that, both in vacuum and in solution, the unpaired electron is largely shared between the two oxygens, which, in the fully reduced acid AH2, constitute the acid hydroxyl groups, and the two carbon atoms connecting them. Of these two oxygens in RADAN, the one carrying in the reduced AN form the remaining proton is found to be the site with the largest unpaired electron density and also the site with (marginally) the higher affinity for hydrogen bonds. The hydrophilic character is almost completely lost upon oxidation of A*− to A. Reduction to AH− strengthens the hydrogen bonding of the deprotonated oxygen and weakens the hydrogen bonding of the protonated oxygen atom.