Frontier orbitals and transition states in the oxidation and degradation of L-ascorbic acid: a DFT study.
DFT calculations were carried out to investigate reaction paths of L-ascorbic acid (AAH2), hydroxyl radicals and water clusters. Frontier-orbital analyses were also performed to examine the regioselectivity of the OH˙ addition. Transition states of the electrolytic dissociation of AAH2 and intermediate carboxylic acids were found to have very small activation energies through proton transfers along hydrogen bonds. The ionized species (anions) are subject to the electrophilic attack of OH˙. The elementary processes of AAH2 → A˙(-) → dehydroascorbic acid → diketogulonic acid → threonic, oxalic, xylonic and lyxonic acids were investigated and discussed. The processes involved in the conversion of dehydroascorbic acid into a bicyclic hemiketal were also examined as a side-chain participating reaction. The oxidation and degradation of vitamin C up to threonic acid were described mainly as a donor (AAH2)-acceptor (OH˙) reaction.
Fukui Institute for Fundamental Chemistry, Kyoto University, Takano-Nishihiraki-cho 34-4, Sakyou-ku, Kyoto 606-8103, Japan. yamabes@fukui.kyoto-u.ac.jp.
Shinichi
S
Yamabe
Noriko
N
Tsuchida
Shoko
S
Yamazaki
Shigeyoshi
S
Sakaki
D001205:Ascorbic Acid / Q000737:chemistry*; D002087:Butyrates / Q000737:chemistry; D002245:Carbon Dioxide / Q000737:chemistry; D003683:Dehydroascorbic Acid / Q000737:chemistry; D005609:Free Radicals / Q000737:chemistry; D006860:Hydrogen Bonding; D008958:Models, Molecular; D008968:Molecular Conformation; D019815:Oxalic Acid / Q000737:chemistry; D010084:Oxidation-Reduction; D011789:Quantum Theory; D013400:Sugar Acids / Q000737:chemistry
D002087:Butyrates; D005609:Free Radicals; D013400:Sugar Acids; D002245:Carbon Dioxide; C015252:gulonic acid; D019815:Oxalic Acid; C011369:threonic acid; D001205:Ascorbic Acid; D003683:Dehydroascorbic Acid