An ab initio study of the energetics of protonation of some N-β- d-glucopyranosylamine derivatives

Abstract

By using the ab initio method at the RHF (Restricted Hartree Fock) level, the energetics of protonation have been studied for five N-β- d-glucopyranosylamine derivatives in the gas phase under consideration of two plausible protonation centres. The calculations of the energy and Gibbs free energy of protonation were accomplished by using a Gaussian 6-31G ∗ basis set in which a polarisation function on the L shell orbitals was accounted for. The calculated energies, Δ E prot, and Gibbs free energies, Δ G prot, of protonation showed that with N-( p-chlorophenyl)- and N-( m-chlorophenyl)-β- d-glucopyranosylamines, the preferred centre of protonation in the gas phase was a ring oxygen atom, whereas with the remaining derivatives, N-( p-toluidine)-, N-( m-toluidine)- and N-( m-nitrophenyl)-β- d-glucopyranosylamines, protonation of the nitrogen atom was energetically preferred. The application of SCRF solvation model allowed to find that in the methanolic solutions the protonation of the oxygen atom was preferred only in the case of N-( p-chlorophenyl)-β- d-glucopyranosylamine. The results of ab initio calculations were subsequently confirmed by semiempirical PM3 examination of the mechanism of mutarotation reaction of beta-ring with protonated methanol in the gas phase. These results enabled to suggest different mechanisms underlying mutarotation reaction for the considered derivatives.