Tuesday, November 13, 2007

Sticking hydrophobes with salts and smallness

Do Hofmeister effects after all depend on altering ‘water structure’? Frankly, I doubt it. But a suggestive case is made in a paper by Andrew Thomas and Adrian Elcock (JACS doi:10.1021/ja073097z). Their MD simulations of various salt solutions show that changes in water-water hydrogen bonding appear to be correlated with experimental solubility data for hydrophobic solutes. Strongly salting-out salts, for instance, cause significant decreases in the water-water hydrogen-bonding fraction. Lithium ions, previously considered anomalous in their salting-out behaviour, form linear ionic chains, with correspondingly unusual hydration structures. But is all this behaviour seen in neutron-scattering studies of salt solutions? I don’t recall that it is. In any event, Thomas and Elcock also find that for simulations that include hydrophobes, an increase in hydrophobic association for certain hydrophobes and salts also correlates with solubility data. There’s a way to go yet before we understand all this.

Relevant to this paper is an experimental study by Jared Smith, Rich Saykally and Phillip Geissler (JACS 129, 13847; 2007) on the effects of dissolved halide ions on hydrogen bonding in water. In contrast to the old ideas about structure-making/breaking, they find that the effects on Raman and IR vibrational spectra can be explained by the action of the ions’ electric fields on adjacent water molecules, and that H-bond strengths are altered very little beyond the first hydration shell. In other words, the H-bond network seems rather robust to such perturbations.

Hydrophobic association in pure water is studied by K. G. Ayappa and colleagues at the Indian Institute of Science in Bangalore (Langmuir doi:10.1021/la7022902). They consider the effects of nanoconfinement on the interaction, looking at 2.82-nm diameter water droplets in reverse micelles. They find that the attraction is enhanced by the confinement, which they explain by the lack of sufficient water to solvate and stabilize the solvent-separated solutes. Plausible? I guess so – after all, hydration of lone hydrophobes is thermodynamically favourable. Dave Thirumalai has considered this issue recently (JACS 128, 13490; 2006) – I must remind myself of what he found…

Tuesday, November 6, 2007

Protein-water coupling: confirmations and complications

With far too much to catch up with here, I shall do little more than list things that have crossed my radar screen. Lots happening, all interesting…

Alla Oleinikova, Nikolai Smolin, and Ivan Brovchenko have a paper in Biophys. J. (93, 2986) entitled “Influence of Water Clustering on the Dynamics of Hydration Water at the Surface of a Lysozyme”, in which they use MD simulations to look at the coupling of water and protein dynamics as the degree of hydration changes. In line with their earlier work, they see maximal dynamical coupling when the water coverage corresponds to a percolating water network on the protein surface.

Ivan and Alla have also told me about their forthcoming book, Interfacial and Confined Water, to be published by Elsevier, which will look at water’s behaviour at hydrophilic and hydrophobic surfaces in general but with clearly a pretty strong focus on biomolecules, including these ideas about percolation transitions in the hydrogen-bonded network.

The hydration dynamics at a protein surface are also the topic of a paper from Dongping Zhong and colleagues at Ohio State University (PNAS doi:10.1073/pnas.0707647104). They have used ultrafast spectroscopy to map out the hydration dynamics from place to place on the surface of various mutants of sperm whale myoglobin, and find two distinct dynamical regimes: one with dynamical timescales of 1-8 ps, the other with around 20-200 ps. These regimes are strongly correlated with the protein’s structure and composition, confirming the intimate relationship between hydration dynamics and protein fluctuations.

But at the same time, this story gets more complex. Martin Weik has sent me a forthcoming paper to be published in PNAS (doi:10.1073/pnas.0706566104) called “Coupling of protein and hydration-water dynamics in biological membranes”. Here they use inelastic neutron scattering and MD simulations to look at the relationship between water dynamics and fluctuations of lipids and bacteriorhodopsin in the purple membrane between 120 and 260 K. They find that the two seem to be decoupled, at least below 260 K, in contrast to the situation for soluble proteins and their hydration layers. In other words, there is no coupled ‘glass-like’ transition of the water and membrane protein: the onset of water motion as the temperature is raised through 200 K does not coincide with a dynamical transition of bR. That adds a whole new layer of complexity to the ongoing story of protein-water dynamics: membranes change the game.

Time to change the subject, then. The hydration of DNA tends to get far less attention than that of proteins, but evidently has interesting stories attached. It seems fairly clear now that the regular double helix depends on the presence of water, though that tends to be glossed over in biochemical texts. Hermann Gaub and colleagues have now made that point in a very forceful manner (JACS doi:10.1021/ja074776c). They have used an AFM tip attached to one strand to drag a length of double-stranded DNA from water into a poor (nonpolar) solvent, octane - whereupon the ds-DNA unzips spontaneously. This happens too in MD simulations. That, the authors say, might be exploited by helicases, which need only force the DNA into a hydrophobic binding pocket to make it unwind. A lovely and striking result.