Solution structures of methyl aldopyranosides revealed by vacuum-ultraviolet electronic circular-dichroism spectroscopy

Abstract

Vacuum-ultraviolet (VUV) electronic circular-dichroism (ECD) spectra of methyl β-D-glucopyranoside (methyl β-D-Glc), methyl α- and β-D-galactopyranosides (methyl α- and β-D-Gal), and methyl α- and β-D-xylopyranosides (methyl α- and β-D-Xyl) were measured down to 163 nm in aqueous solution using a s ynchrotron-radiation VUV-ECD spectrophotometer. Five methyl aldopyranosides exhibited characteristic ECD spectra depending on the gauche (G) and trans (T) conformations of the hydroxymethyl group at C-5 and the α-/β-anomer configurations of the methoxy group at C-1. To elucidate the contributions of these structures to the spectrum, the ECD spectra of three rotamers (GT, GG and TG) of methyl β-D-Glc, methyl α-D-Gal and methyl β-D-Gal were calculated using a time-dependent density functional theory and molecular dynamics simulations. These theoretical spectra were very similar to those observed experimentally, indicating that the GT and GG rotamers show negative and positive ECD around 170 nm, respectively, and that the α- and β-anomers exhibit negative and positive ECD around 160 nm, respectively. These spectral differences between the two rotamers and between the two anomers were attributable to changes in steric configurations, including the intramolecular hydrogen bond around the ring oxygen and the methoxy oxygen, respectively. These relationships were supported by the ECD spectra experimentally observed for methyl α-D-Xyl and methyl β-D-Xyl, demonstrating that the VUV-ECD spectroscopy is a powerful technique for characterizing the structures of saccharides in aqueous solution.