Thermochemistry of Microhydration of Sodiated and Potassiated Monosaccharides

The thermochemical properties ΔHon , ΔSon, and ΔGon for the hydration of sodiated and potassiated monosaccharides (Ara = arabinose, Xyl = xylose, Rib = ribose, Glc = glucose, and Gal = galactose) have been experimentally studied in the gas phase at 10 mbar by equilibria measurements using an electrospray high-pressure mass spectrometer equipped with a pulsed ion beam reaction chamber. The hydration enthalpies for sodiated complexes were found to be between −46.4 and −57.7 kJ/mol for the first, and −42.7 and −52.3 kJ/mol for the second water molecule. For potassiated complexes, the water binding enthalpies were similar for all studied systems and varied between −48.5 and −52.7 kJ/mol. The thermochemical values for each system correspond to a mixture of the α and β anomeric forms of monosaccharide structures involved in their cationized complexes

Carbohydrate-aromatic interactions: A computational and IR spectroscopic investigation of the complex, methyl alpha-L-fucopyranoside center dot toluene, isolated in the gas phase

a b s t r a c t A carbohydrate-aromatic complex, methyl a-L-fucopyranoside Á toluene, which provides a model for probing the physical basis of carbohydrate-protein 'stacking' interactions, has been created in a molecular beam and probed through IR ion dip spectroscopy in the CH and OH regions. The results are interpreted in the light of DFT calculations using the MO5-2X functional. They indicate the creation of stacked structures with the aromatic molecule bonded either to the upper or to the lower face of the pyranoside ring, through CH 3,4 -p (upper) or CH 1 -p (lower) interactions leading to binding energies 618 kJ mol À1

Conformational change and selectivity in explicitly hydrated carbohydrates

The combination of vibrational spectroscopy, conducted in a supersonic jet expansion, with computation through molecular mechanics, density functional theory (DFT) and ab initio calculation, has provided a new approach to the conformational and structural assignment of carbohydrates and their molecular complexes. This article reviews the new insights it has provided on the regioselectivity and conformational choice in singly and multiply hydrated monosaccharides. It reveals a systematic pattern of conformational preference and binding site selectivity, driven by the provision of optimal, co-operative hydrogen-bonded networks in the hydrated sugars. Water binding is invariably 'focused' around the hydroxymethyl group (when present); the bound water molecules (on multiply hydrated mannose) are located exclusively on its hydrophilic face while the hydrophobic face remains 'dry'; and there is a correlation between the locale of the preferred binding sites and those involved in protein-carbohydrate molecular recognition. © 2009 Elsevier Ltd. All rights reserved

Conformational change and selectivity in explicitly hydrated carbohydrates

The combination of vibrational spectroscopy, conducted in a supersonic jet expansion, with computation through molecular mechanics, density functional theory (DFT) and ab initio calculation, has provided a new approach to the conformational and structural assignment of carbohydrates and their molecular complexes. This article reviews the new insights it has provided on the regioselectivity and conformational choice in singly and multiply hydrated monosaccharides. It reveals a systematic pattern of conformational preference and binding site selectivity, driven by the provision of optimal, co-operative hydrogen-bonded networks in the hydrated sugars. Water binding is invariably 'focused' around the hydroxymethyl group (when present); the bound water molecules (on multiply hydrated mannose) are located exclusively on its hydrophilic face while the hydrophobic face remains 'dry'; and there is a correlation between the locale of the preferred binding sites and those involved in protein-carbohydrate molecular recognition. © 2009 Elsevier Ltd. All rights reserved

Peptide secondary structures in the gas phase: consensus motif of N-linked glycoproteins.

The possibility of secondary structure acting as a primary determinant in nature's choice of the consensus sequon, NXS/T in all N-linked glycoproteins, has been addressed by determining the intrinsic secondary structures of the capped oligopeptide, Ac-NGS-NHBn, and two "mutants", Ac-QGS-NHBn and Ac-NPS-NHBn, by use of infrared laser ion dip spectroscopy in the gas phase coupled with ab initio and density functional theory calculation. Their global minimum energy conformations, exclusively or preferentially populated in all three peptides, display marked differences. NGS adopts an open, S-shaped backbone conformation rather than the C(10) "Asx" turn structure that all previous measurements have identified in solution; the difference can be related to the high dipole moment of the "Asx" conformation and structural selection in a polar environment. QGS adopts a similar but more rigid backbone structure, supported by markedly stronger hydrogen bonds. NPS adopts an Asx turn coupled with a C(10) beta-turn backbone conformation, a structure also adopted in a crystal environment. These and other more subtle structural differences, particularly those involving interactions with the carboxamide side chain, provide strong evidence for the operation of structural constraints, and a potential insight into the unique reactivity of the asparagine side chain toward enzymatic glycosylation

Carbohydrate molecular recognition: a spectroscopic investigation of carbohydrate-aromatic interactions.

The physical basis of carbohydrate molecular recognition at aromatic protein binding sites is explored by creating molecular complexes between a series of selected monosaccharides and toluene (as a truncated model for phenylalanine). They are formed at low temperatures under molecular beam conditions, and detected and characterized through mass-selected, infrared ion depletion spectroscopy-a strategy which exploits the extraordinary sensitivity of their vibrational signatures to the local hydrogen-bonded environment of their OH groups. The trial set of carbohydrates, alpha- and beta-anomers of glucose, galactose and fucose, reflects ligand fragments in naturally occurring protein-carbohydrate complexes and also allows an investigation of the effect of systematic structural changes, including the shape and extent of 'apolar' patches on the pyranose ring, removal of the OH on the exocyclic hydroxymethyl group, and removal of the aglycon. Bound complexes invariably form, establishing the general existence of intrinsic intermolecular potential minima. In most of the cases explored, comparison between recorded and computed vibrational spectra of the bound and free carbohydrates in the absence of solvent water molecules reveal that dispersion forces involving CH-pi interactions, which promote little if any distortion of the bound carbohydrate, predominate although complexes bound through specific OH-pi hydrogen-bonded interactions have also been identified. Since the complexes form at low temperatures in the absence of water, entropic contributions associated with the reorganization of surrounding water molecules, the essence of the proposed 'hydrophobic interaction', cannot contribute and other modes of binding drive the recognition of sugars by aromatic residues. Excitingly, some of the proposed structures mirror those found in naturally occurring protein-carbohydrate binding sites