Hydrogen Bonding and Cooperativity in Isolated and Hydrated Sugars: Mannose, Galactose, Glucose, and Lactose

The conformation of phenyl-substituted monosaccharides (mannose, galactose, and glucose)
and their singly hydrated complexes has been investigated in the gas phase by means of a combination
of mass selected, conformer specific ultraviolet and infrared double resonance hole burning spectroscopy
experiments, and ab initio quantum chemistry calculations. In each case, the water molecule inserts into
the carbohydrate at a position where it can replace a weak intramolecular interaction by two stronger
intermolecular hydrogen bonds. The insertion can produce significant changes in the conformational
preferences of the carbohydrates, and there is a clear preference for structures where cooperative effects
enhance the stability of the monosaccharide conformers to which the water molecule chooses to bind. The
conclusions drawn from the study of monosaccharide−water complexes are extended to the disaccharide
lactose and discussed in the light of the underlying mechanisms that may be involved in the binding of
carbohydrate assemblies to proteins and the involvement, or not, of key structural water molecules.