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.