Vacuum-Ultraviolet Electronic Circular Dichroism Study of Methyl α-D-Glucopyranoside in Aqueous Solution by Time-Dependent Density Functional Theory

The vacuum-ultraviolet (VUV) electronic circular dichroism (ECD) spectrum of methyl alpha-D-glucopyranoside (methyl alpha-D-Glc) was measured down to 163 nm in aqueous solution using a synchrotron-radiation VUV-ECD spectrophotometer. The spectrum exhibited two characteristic ECD peaks around 170 nm, which depend on the trans (T) and gauche (G) configurations of the hydroxymethyl group at C-5. To elucidate the influences of the T and G configurations on the spectrum, the ECD spectra of three rotamers (alpha-GT, alpha-GG, and alpha-TG) of methyl alpha-D-Glc were calculated using time-dependent density functional theory (TDDFT) combined with molecular dynamics simulation. A linear combination of the ECD spectra of these three rotamers, which differ markedly from each other, produced a methyl alpha-D-Glc spectrum similar to that observed experimentally. The spectrum was assignable to the n-sigma* transitions of the ring oxygen and methoxy oxygen with minor contributions from the hydroxyl oxygen. The differences in alpha-GT, alpha-GG, and alpha-TG spectra were attributed to fluctuations of the configurations of the hydroxymethyl group at C-S and the hydroxyl group at C-4, which strongly affected the orientations of intramolecular hydrogen bonds around the ring oxygen. These findings demonstrate that combining VUV-ECD and TDDFT is useful for structural characterization of saccharides in aqueous solution.