Heavy water hydration of mannose: the anomeric effect in solvation, laid bare

The presence and consequences of the anomeric effect have been explored and directly exposed, through an investigation of the vibrational spectroscopy of the doubly and triply hydrated a and b anomers of phenyl D-mannopyranoside, (PhMan) isolated under molecular beam conditions in the gas phase. The experiments have been aided by the simple trick of substituting D2O for H2O, which has the advantage of isotopically isolating the carbohydrate (OH) bands from the water (OD) bands. Recording the double resonance, IR-UV ion dip spectra of the hydrated complexes, a- and b-PhMan·(D2O)2,3 in a series of 'proof of principle' experiments, revealed that these heavy water molecules engage the key endocyclic oxygen atom, O5, allowing the anomeric effect to be probed through a combination of vibrational spectroscopy and quantum chemical calculations. Importantly, in the dihydrates, both anomers adopt the same conformation and the two water molecules occupy the same template. One of them acts as a remarkably sensitive reporter, able to sense and expose subtle stereoelectronic changes through the resulting changes in its hydrogen-bonded interaction with the substrate. © The Royal Society of Chemistry 2011

Heavy water hydration of mannose: the anomeric effect in solvation, laid bare

The presence and consequences of the anomeric effect have been explored and directly exposed, through an investigation of the vibrational spectroscopy of the doubly and triply hydrated a and b anomers of phenyl D-mannopyranoside, (PhMan) isolated under molecular beam conditions in the gas phase. The experiments have been aided by the simple trick of substituting D2O for H2O, which has the advantage of isotopically isolating the carbohydrate (OH) bands from the water (OD) bands. Recording the double resonance, IR-UV ion dip spectra of the hydrated complexes, a- and b-PhMan·(D2O)2,3 in a series of 'proof of principle' experiments, revealed that these heavy water molecules engage the key endocyclic oxygen atom, O5, allowing the anomeric effect to be probed through a combination of vibrational spectroscopy and quantum chemical calculations. Importantly, in the dihydrates, both anomers adopt the same conformation and the two water molecules occupy the same template. One of them acts as a remarkably sensitive reporter, able to sense and expose subtle stereoelectronic changes through the resulting changes in its hydrogen-bonded interaction with the substrate. © The Royal Society of Chemistry 2011

Carbohydrate hydration: heavy water complexes of α and β anomers of glucose, galactose, fucose and xylose.

The singly and doubly hydrated complexes of the α and β anomers of a systematically varied set of monosaccharides, O-phenyl D-gluco-, D-galacto-, L-fuco- and D-xylopyranoside, have been generated in a cold molecular beam and probed through infrared-ultraviolet double resonance ion-dip (IRID) spectroscopy coupled with quantum mechanical calculations. A new 'twist' has been introduced by isotopic substitution, replacing H(2)O by D(2)O to separate the carbohydrate (OH) and hydrate (OD) vibrational signatures and also to relieve spectral congestion. The new spectroscopic and computational results have exposed subtle aspects of the intermolecular interactions which influence the finer details of their preferred structures, including the competing controls exerted by co-operative hydrogen bonding, bi-furcated and OH-π hydrogen bonding, stereoelectronic changes associated with the anomeric effect, and dispersion interactions. They also reassert the operation of general 'working rules' governing conformational change and regioselectivity in both singly and doubly hydrated monosaccharides

Isotopic hydration of cellobiose: vibrational spectroscopy and dynamical simulations.

The conformation and structural dynamics of cellobiose, one of the fundamental building blocks in nature, its C4' epimer, lactose, and their microhydrated complexes, isolated in the gas phase, have been explored through a combination of experiment and theory. Their structures at low temperature have been determined through double resonance, IR-UV vibrational spectroscopy conducted under molecular beam conditions, substituting D(2)O for H(2)O to separate isotopically, the carbohydrate (OH) bands from the hydration (OD) bands. Car-Parrinello (CP2K) simulations, employing dispersion corrected density functional potentials and conducted "on-the-fly" from ∼20 to ∼300 K, have been used to explore the consequences of raising the temperature. Comparisons between the experimental data, anharmonic vibrational self-consistent field calculations based upon ab initio potentials, and the CP2K simulations have established the role of anharmonicity; the reliability of classical molecular dynamics predictions of the vibrational spectra of carbohydrates and the accuracy of the dispersion corrected (BLYP-D) force fields employed; the structural consequences of increasing hydration; and the dynamical consequences of increasing temperature. The isolated and hydrated cellobiose and lactose units both present remarkably rigid structures: their glycosidic linkages adopt a "cis" (anti-ϕ and syn-ψ) conformation bound by inter-ring hydrogen bonds. This conformation is maintained when the temperature is increased to ∼300 K and it continues to be maintained when the cellobiose (or lactose) unit is hydrated by one or two explicitly bound water molecules. Despite individual fluctuations in the intra- and intermolecular hydrogen bonding pattern and some local structural motions, the water molecules remain locally bound and the isolated carbohydrates remain trapped within the cis potential well. The Car-Parrinello dynamical simulations do not suggest any accessible pathway to the trans conformations that are formed in aqueous solution and are widespread in nature