Complexation of boric acid with vitamin C

Abstract

Mono-chelate (1 : 1) and bis-chelate (1 : 2) anionic complexes of boric acid with vitamin C (L-ascorbic acid, H2A) were isolated from aqueous solutions in salt form with Li+, Na+ and Ca2+ ions. The complexes were characterized by FTIR, 13C and 11B MAS (magic angle spinning) NMR techniques. The spectral data agreed with the calculated structures that H2A complexes with boric acid through its cis-enediol groups by forming five-membered chelate rings. Side-chain OH groups do not participate in complexation as corroborated by the results of the test experiments conducted with iso-propylideneascorbic acid (i-H2A) where the side chain is blocked. Empirical relations, that can be used as diagnostics for the 1 : 1 and 1 : 2 ascorbatoborate complexes, were derived from the observed chemical shift values. Bis-chelate complexes were found to have higher thermal and hydrolytic stabilities than their mono-chelate homologs. The effect of the cation on the stabilization of the complexes was also investigated. The observed relative stability of the 1 : 1 Na–ascorbatoborate complex with respect to the analogous Ca complex, correlated with the recent reports about the role of alkali metal ions in the stabilization of ribose in the prebiotic world.