Structural changes of orthorhombic α-D-galactose crystal by using Raman spectroscopy at high pressure

Abstract

D-galactose crystal, a monosaccharide of biological importance, was investigated by using Raman spectra in a diamond-anvil cell at the spectra range of 3600-50 cm−1 for pressure up to 30.6 GPa. The spectra show that D-galactose experienced about three phase modifications, around 4.9 GPa, in the range of 4.9–8.7 GPa, and between 8.7 and 14.9 GPa, respectively. The three anomalies are characterized by the disappearance of lattice modes, changes of internal modes and the splitting as well as the appearance of C–H moieties. The complicated behaviors of internal modes are associated with the wagging and scissoring of CH2; stretching vibrations of CH, CH2. The changes in the vibration modes under high pressure may be associated with the modifications of hydrogen bond, especially the O3–H⋯O4 hydrogen bond. At around 22.4 GPa, the intensity of Raman spectra changes significantly, which may lead to another phase transition and further confirmation is needed. Besides, for most modes observed, changes of the slopes of the wavenumber versus pressure plots are also the proofs of the crystal structure modifications.