Experimental and theoretical study of racemization, stability and tautomerism of vitamin C stereoisomers.

Experimental degradation of vitamin C (VC) in water was monitored using specific rotation, FT-IR and (1)H NMR at room temperature and at 80-90°C. The optical activity was reduced (100%) to zero after 6 and 63 days at 80-90°C (≅25% in less than 24h) and room temperature, respectively. (1)H NMR and FT-IR was performed after these periods which indicated the formation of a complex mixture at higher stirring time and elevated temperature. These observations indicated that vitamin C is unstable in solution at room temperature or at elevated temperature. The solid VC did not lose optical activity after several mounts at room temperature. The instability of VC was related to isomerization (loss of optical activity) of the l-enantiomer either to d-enantiomer (racemization), degradation to undesired products or tautomerization to other isomers. Theoretical investigation (at B3LYP/6-311++G(d,p) and MP2/6-311++G(d,p) levels of theory in vacuo and aqueous phases) formulated tautomerization for RS-stereoisomers of ascorbic acids (AA). The mechanism for deactivation of VC was examined.