Conformational effects in sugar ions: spectroscopic investigations in the gas phase and in solution

We present direct investigations of the conformational preferences of sugars with a positively charged substituent at their anomeric centre, C-1, which display in solution, a preference for an equatorial conformation-an apparent reversal of the normal anomeric effect. The investigations focus on the protonated monosaccharide, D-xylopyranosyl imidazolium in its alpha and beta forms, first probed in a range of different solvents through NMR measurements and then in the gas phase, free of solvent or counterion interactions, through infrared multiphoton dissociation spectroscopy. The results, when compared and discussed in the light of density functional theory, ab initio and natural bond orbital calculations, expose the possible origins of the reversed conformational preference and provide a better understanding of the factors controlling conformational choice.