Structure and Antiradical Activity of Hydrogen-Bound Complexes of Protocatechoic Acid with Monosaccharides in Aqueous Media

It was found by the photocolorimetry method that binary compositions of protocatechic acid with monosaccharides (galactose, mannose) exhibit a pronounced antiradical synergistic effect in reaction with 2,2ʹ-diphenyl-1-picrylhydrazyl in acidic media. The synergistic properties of phenol-saccharide mixtures decrease with increasing pH of the medium. Using NMR spectroscopy and density functional theory, it was shown that the mechanism of synergism consists in the formation of intermolecular hydrogen–bonded phenol-monosaccharide complexes. The ionization energies of the donor and donor-acceptor complexes are lower than those of protocatechuic acid and hence the complexes react more actively with the radical compared to the initial antioxidant. Acceptor ion-molecular complexes with ionization energies higher than those of the reagents are less reactive.