Experimental and Computational Studies of the Recognition of Amino Acids by Galactosyl-imine and -amine Derivatives:  An Attempt to Understand the Lectin−Carbohydrate Interactions

A galactosyl-naphthyl-imine-based derivative, 1-(β-d-galactopyranosyl-1‘-deoxy-1‘-iminomethyl)-2-hydroxynaphthalene (<b>GNI</b>), and a galactosyl-naphthyl-amine-based derivative, 1-(galactopyranosyl-1‘-deoxy-1‘-aminomethyl)-2-hydroxynaphthalene (<b>GNA</b>), possessing an ONO binding core were studied for their recognition of naturally occurring amino acids using fluorescence and absorption spectroscopy, and the corresponding association constants were derived for the complexes formed. The complexes formed between <b>GNI</b>/<b>GNA</b> and amino acids were supported by electrospray ionization mass spectrometry (ESI/MS). The structures of the complexes were optimized by computational studies using density functional theory, and stabilization energies were computed for the complexes to substantiate the interactions present between <b>GNI</b>/<b>GNA</b> and amino acid. The interactions were found to be primarily hydrogen bonding in nature. These interactions are reminiscent of those present in the lectin-carbohydrate and glycosidase substrate. Thus, the carbohydrate moiety present in <b>GNI</b> shows high specificity toward the −COOH group of the amino acid, which may be relevant to such interactions present between the carbohydrates and the polypeptides.