A lot about interfaces

Janamejaya Chowdhary and Branka Ladanyi at Colorado State have used MD simulations to look at the dynamics of H-bonds at a water-hydrocarbon interface (J. Phys. Chem. B ASAP doi:10.1021/jp; paper here). They find that the reorientation of the O-H bond is anisotropic, and quantify the effects of cooperativity in the dynamics.

Robert Woods and colleagues at the University of Georgia study how bound water mediates the binding of concanavalin A to its target carbohydrate ligand (R. Kadirvelraj et al., JACS ASAP; paper here). Or rather, they look at a modified ligand of the natural trisaccharide, with a hydroxylethyl side chain that may or may not displace a conserved water in binding of the natural ligand. The crystal structure reported here shows that this water is retained, though its position is distorted. This helps to explain the previous thermodynamic data on ligand specificity for Con A, showing that there is no entropic component for the synthetic ligand arising from water displacement.

Roger Tam and colleagues in Ottawa have looked at the inhibition of ice recrystallization by mono- and disaccharides (JACS ASAP; paper here). Specifically, they look for correlates of ice-growth inhibition in the degree of hydration of the sugars, and find that, rather than using the total number of tightly bound water molecules, a better predictor of inhibiting ability is a hydration index in which the hydration number is divided by the molar volume. The researchers conclude that the inhibition arises from a disruption of water ‘pre-ordering’ at the ice-water interface.

Joe Zaccai and colleagues have measured water dynamics in human red blood cells using quasielastic incoherent neutron scattering (A. M. Stadler et al., JACS ASAP; paper here). In line with their previous work on E. coli, they find that most (90%) of the cell water has similar translational diffusion to the bulk, while about 10% is slower, this presumably being the water hydrating haemoglobin.

Sherwin Singer and colleagues at Ohio State have looked at the hydration dynamics of myoglobin using MD simulations (T. Li et al., J. Phys. Chem. B 10.1021/jp803042u; paper here). Specifically, they look at the time-dependent fluorescence Stokes shift after photoexcitation of the Trp-7 residue, a measure of the relaxation dynamics of the chromophore’s environment. The question is whether the water dynamics are due to constraint of the water by interactions with the protein, or whether they are controlled by the dynamics of the protein itself. This distinction should be revealed by arresting the protein in the simulations. Singer and colleagues find that doing so significantly changes the Stokes shift, suggesting that the intrinsic protein flexibility is important. They caution, however, that this does not necessarily imply that the water dynamics exhibit no intrinsic slow component of relaxation; rather, the protein and water dynamics are so intimately coupled that either slow water dynamics or slow protein dynamics (or both) could alter the Stokes shift.

Shekhar Garde and colleagues at RPI have conducted simulations of hydrophobically induced polymer collapse near to the interface with air or a hydrophobic wall (S. N. Jamadagni et al., J. Phys. Chem. B 10.1021/jp806528m – paper here here). They find that the driving force for collapse is smaller at the water-alkane interface, and all but vanishes at the air-vapour interface, where the polymer remains unfolded. They think that both the weaker hydration of the polymer and the enhanced density fluctuations of water at the interface produce faster conformational switches in the folded chain. The results throws up lots of interesting questions, most obviously of course what this implies for the conformational flexibility of two peptide chains approaching one another via the hydrophobic interaction.

Hangjun Lu and colleagues at Zhejiang Normal Univerity have looked at how an external charge of +1e near a carbon nanotube will affect the filling and emptying by water (H. Lu et al., J. Phys. Chem. B 10.1021/jp802263v – paper here here). It seems that the charge stabilizes the water-filled state when it is at the midpoint of the nanotube, but much less so if it is moved towards the ends. The implication is that this is a method that might be exploited by protein channels to control water transport via the positioning of ionized residues.

The freezing-point depression of water that hydrates phospholipid membranes has been studied using NMR by Dong-Kuk Lee at Seoul National University of Technology and coworkers (D.-K. Lee et al., Langmuir 24, 13598 (2008) – paper here). They find that water molecules still show liquid-like signatures below -20 C in bilayers, and that the freezing behaviour is depressed still further by cholesterol, a known cryoprotectant.

I have a kind of follow-up to my Chem. Rev. article in a forthcoming issue of ChemPhysChem, which has now appeared online (here). This will form part of a special issue on the subject of water at interfaces, stemming from a meeting of the DFG Forschergruppe 436 in Dortmund last summer.