Structural flexibility and hydrogen bonding of aqueous vitamin C: Raman and UV spectroscopic and computational...

Raman and UV spectroscopic experiments, supported by quantum mechanical modeling of the conformations, vibrational and electronic absorption spectra, and optical rotation of aqueous vitamin C, reveal qualitative differences in the conformational behavior and hydrogen bonding of its neutral form, ascorbic acid AH₂, and the deprotonated form, the ascorbate anion AH^-. AH₂, which predominates in solutions with concentrations c ≥ 10^-3 M, can adopt about eighty conformations with similar free energies. The three most populated conformers, which together account for about forty percent of the ensemble, lack intramolecular hydrogen bonds (IHBs) between the hydroxyl groups of the 1,2-dihydroxyethyl moiety and the dihydroxyfuranone fragment. AH^-, the major form of vitamin C in solutions with c ≈ 10^-5 M, exists predominantly in a conformation stabilized by a strong charge-assisted IHB between the terminal OH group of the folded 1,2-dihydroxyethyl fragment and the negatively charged CO^- group of the deprotonated dihydroxyfuranone moiety. Water strengthens this IHB, in contrast to analogous IHBs in neutral ascorbic acid, which are weakened or disrupted by water. Besides compacting the aqueous ascorbate conformation, the IHB strengthens and modifies conjugation in the furanone ring. This is reflected in a bathochromic shift of the lowest-energy UV absorption band by ca. 18 nm, an enhancement of the Raman bands, and changes in the bond lengths of the furanone moiety. The unusually strong conformational sensitivity of the UV spectra, as well as the above-mentioned influence of water, indicates a strong impact of both intra- and intermolecular hydrogen bonding on conjugation in the AH^-. This represents an atypical case of a marked change in conjugation efficiency without a change in the coplanarity of the conjugated fragments and suggests the non-trivial character of conjugation in aqueous ascorbate.