From the gas phase to aqueous solution: Vibrational spectroscopy, Raman optical activity and conformational structure of carbohydrates

Abstract

Structural investigations of isolated and hydrated glucose, galactose and lactose structures in the gas phase based upon infra-red ion dip (IRID) spectroscopy conducted at low temperatures, are linked with parallel investigations conducted in aqueous solution at 298 K based upon measurements of their vibrational Raman and Raman optical activity (ROA) spectra. ‘Basis sets’ of computed (gas phase) Raman and ROA spectra associated with their low-lying conformational structures are used to construct ‘weighted sums’ which can approximate their experimental Raman and ROA spectra recorded in solution and provide estimates of their conformational population distributions in aqueous solution at 298 K; they are compared with estimates based upon analysis of NMR measurements and molecular mechanics and molecular dynamics calculations. The altered conformational preferences in the singly hydrated complexes of glucose and galactose isolated in the gas phase, appear to be sustained in aqueous solution, supporting the view that explicit hydration provides a key influence on their conformational preferences in solution. On the other hand, the conformational preference of the isolated β(1 → 4) disaccharide, lactose which resists conformational alteration when singly hydrated, is altered when it is transferred to aqueous solution at 298 K. The computational evidence suggests the control is exerted by entropic effects associated with a loosening of the structure around the glycosidic linkage.