Theoretical investigation of the electronic absorption spectra of the glucosides

Abstract

n-alkyl-1X-β-D-glucopyranosides (X = O, S) contain a glucosidic bond joining a glucose moiety and n-alkyl group. We have elucidated the optical properties of the O-/S-glucosidic bond of n-alkyl-β-D-glucopyranosides under aqueous solution by using time-dependent density functional theory (TD-DFT) on the base of the fully relaxed geometries of (model) compounds. For thio-glucopyranosides, a broadening absorption bands with the delocalizations of the sulfur lone pair electrons under UV light are predicted, which is in good agreement with the experimental observations. The main delocalization interaction is found to be the donation of the lone pair of the sulfur atom into σ* orbital of the C-Opyran bond that enhances the S-C-Opyran bond and efficiently raises the electron distribution of the glucose moiety, which is confirmed by the second-order perturbation NBO and electron density difference (EDD) analyses. Such n→σ* character is detected in X = O case either which, however, is basically excluded from any transitions. In addition, we have also investigated the derived X-C-Opyran (X = N(H), P(H)) bond. The n(N)→σ*(C-Opyran) electron donation is found to be less, although still main, delocalization interaction, while there is even no delocalization of n(P)→σ*(C-Opyran) character due to the distant from each other. The absorption characteristics of C-/Si-glucosidic bonds are also discussed.