Ab initio study of the structural properties of ascorbic acid (vitamin C)

Abstract

ABSTRACT:Geometries of the neutral and ionic tautomeric species of ascorbic acid were optimized at the density functional theory (DFT) level using the B3LYP functional. The radical species were evaluated using the unrestricted B3LYP method. Single-point energy calculations were also performed using the Møller-Plesset (MP2) and unrestricted MP2 (UMP2) methods for the closed-shell and open-shell systems, respectively. The effects of aqueous solution were evaluated using the conducting polarized continuum model (CPCM) and polarized continuum model (PCM). The geometries of most stable radicals in the respective groups were also optimized in the water solution using the CPCM model at the UB3LYP level. All calculation were performed using the 6-311ϩϩG(d,p) basis set. The nature of stationary points on the gas phase potential energy surfaces (PESs) was evaluated using vibrational frequency calculations; all geometries characterize local minima. The species obtained by the deprotonation of the O 3 site is the most stable monoanion of ascorbic acid. For the radical species, the structure obtained by the dehydrogenation of the O 3 site is the most stable monoradical. Among the radical anions, the species obtained by the deprotonation of the O 3 site and subsequent dehydrogenation of the O 2 site is the most stable in the gas phase and in an aqueous medium. The computed isotropic hyperfine coupling constants of this species were found to be in good agreement with the experimental data. Our investigation also supports the earlier findings that the oxidized species of ascorbic acid in water solution by the OH• radical is radical anion of the AAO 3• O 2 Ϫ form. The spin densities and molecular electrostatic potentials are also discussed.

Investigation ofL(+)-Ascorbic acid with Raman spectroscopy in visible and UV light

“…Geometry optimization of species in aqueous solution by use of a continuum model offers an attractive alternative. Accordingly, Allen et al (4) have made a comprehensive study of ascorbic acid conformers, including some tautomers, some anions, and radical species in an aqueous conducting polarization continuum model and in the gas phase, using the DFT/B3LYP/6-311++G(d,p) high level method. Costanzo et al (23) have added a further study of much the same content in the gas phase but including molecular dynamics on AH − and the radical anion A * − modeled in aqueous solution.…”

“…Ascorbic acid is also a strong antioxidant, with a high reactivity toward oxygen. The acid participates (4,(24)(25)(26)43). The dihedral (torsional) angles are defined as shown for atoms A-B-C-D, with A nearest the observer, who is looking down the B-C bond.…”

“…in many redox reactions in natural biological systems and is commonly found in small concentrations in fruits and vegetables (2)(3)(4)(5)(6). The acid is soluble in water and is an important nutrition component for humans, primates, and guinea pigs, which all are incapable of synthesizing it from glucose due to their lack of the enzyme gulonolactone oxidase in the glucoronic acid pathway (7).…”

“…Chemical evidence, X-ray structure solutions, and ab initio studies indicate that the deprotonation involves the proton H3 on the O3 site; see Figure 1. The OH group on C3 has an acidity constant of pK a ∼4.1 to ∼4.25, forming a highly stable AH − mono-anion of ascorbic acid (4,(10)(11)(12)(13)(14)(15)(16)(17). Other tautomers are not so stable; for example, the hydrogen on O2 can be moved to O1, forming a tautomer of ascorbic acid (4), but this is not the only possibility.…”

“…The acid/base and oxidation/reduction reactions of ascorbic acid are coupled, and in highly basic solutions and under the presence of oxygen the molecules are easily oxidized to various radicals, forming in the end salts of the dehydroascorbic acid, C 6 H 6 O 6 (ascorbic acid that has lost H2 and H3 protons and two electrons) (19,(21)(22)(23). Quite stable free radicals of ascorbic acid seem to be formed via ionization of the O3 or O2 anions, and also a stable di-radical seems to exist via ionization of the O3 O2 di-anion (4).…”