Studies on L-Ascorbic Acid Derivatives. V. Hydrolysis of L-Ascorbic Acid 3-Phosphate

Abstract

The rate constants for the hydrolysis of L-ascorbic acid 3-phosphate were measured in aqueous solutions at 50-100° from pH 1 to 9.6 and also in strong acid solutions up to 4M hydrochloric acid. The first order kinetics with respect to the unreacted phosphate were obtained by determining the remaining phosphate. The pH rate profile (Fig. 3) for the hydrolysis of L-ascorbic acid 3-phosphate is represented by the rate equation. Kobsd=ka [H+] [H3A]/C+kN [H3A]/C+k1 [H2A-]/C+k2 [HA2-]/C+k3 [A3-]/C. Where [H3A]/C, [H2A-]/C, [HA2-]/C and [A3-]/C are molar fractions of the neutral species, monoanion, dianion and trianion, and ka, kN, k1, k2, and k3 are their associated specific rate constants. From the observed hydrolysis rate and pK values, these specific rate constants were evaluated. The pK values of L-ascorbic acid 3-phosphate were determined to be pK1=0.01, pK2=3.27, and pK3=6.70 at 28.5°. In hydrochloric acid solutions (1-4M), the rate of hydrolysis is proportional to the hydrogen ion concentration. This result coupled with the moderately large solvent isotope effect and with the activation parameters suggests that the hydrolysis of the conjugate acid of L-ascorbic acid 3-phosphate is bimolecular and a water molecule is participated in the transition state. Hydrolysis of the monoanion which is predominant at pH ∼2 involves mainly P-O bond fission. Measurements of the solvolysis product composition indicated that the molar ratio, methyl phosphate/inorganic phosphate, was slightly smaller than the ratio, methanol/water, present in the reaction mixture. Decrease in the rate by the addition of an organic solvent, the large negative value of the activation entropy and the insignificant D2O effect suggested the reaction mechanism as in Chart 2. When the dianion was hydrolyzed in H218O, 18O was introduced mainly into inorganic phosphate. Different from those of common monoalkyl- or monoarylphosphates, the solvolysis in methanol-water mixture afforded almost exclusively inorganic phosphates. These data taken in conjunction with the relatively large solvent isotope effect and with the negatively large entropy value led to the postualation that the hydrolysis of the dianion underwent through a slow proton transfer to the carbonyl oxygen concerted with the nucleophilic attack by hydroxide ion which brought about the rapid scission of the P-O bond. In the case of the trianion an extremely low hydrolysis rate was observed and possible mechanistic discussion was given to account for the stability.