The animal product hazard factor N-glycolylneuraminic (Neu5Gc) and brain nutrient substance N-acetylneuraminic acid (Neu5Ac) were studied at the M062X/6-311 + G(d,p) geometry optimization level. We considered the electronic structure parameters with different solvents: (benzene ε = 2.27, acetic acid ε = 6.25, ethanol ε = 24.85, lactic acid ε = 22.00, formic acid ε = 51.1, water ε = 78.35). The maximum molecular surface electrostatic potentials, which were 62.77 for Neu5Gc and 60.90 kcal/mol for Neu5Ac, are both located on the carboxyl group hydrogen. The orbital analysis showed that the amide group and carboxyl group confer the sites with susceptibility to nucleophilic and electrophilic attack, respectively. The solvent effect showed that polar solvents, such as formic acid and water, can enhance the two molecules' nucleophilic activity. To better understand the roles of the hydroxyl group in the two molecules, the independent gradient model theory confirmed the four intramolecular hydrogen bonds of Neu5Gc at gas phase, whereas Neu5Ac only has two. The lowest bond dissociation energy in solvent occurs at O7-H, which is 104.03 kcal/mol in water for Neu5Gc and 104.57 kcal/mol in lactic acid for Neu5Ac. The lowest proton affinity value for Neu5Gc (20.34 kcal/mol) and Neu5Ac (20.76 kcal/mol) was both occur at the carboxyl group O6-H under ethanol. The antioxidant mechanisms of the two sialic acid are prone to sequential proton-loss electron transfer under polar or non-polar solvents.
Keywords: activity index; density functional theory; independent gradient model; sialic acid; solvent effects.