The clincher for starting this blog was a glut of deeply interesting papers over the past couple of weeks. In Phys. Rev. Lett. (see here) Steve Granick and colleagues have what they call conclusive evidence for a depletion layer where water meets a hydrophobic surface. This has been a long-standing point of debate, with prior claims ranging from complete drying at the surface, or depletion layers several nm think, to no depletion at all. The issue has also been complicated by the possible presence of nanobubbles of dissolved gases. Granick and co. now report evidence from X-ray reflectivity for a depletion of more than 60% of the bulk density over a layer thickness of 2-4 angstroms. That’s a distance of the order of the diameter of a water molecule, so at least there is no new length scale mysteriously entering the picture. But will this be the last word?
In Biophys. J. (see here), Florin Despa and Stephen Berry have taken on another contentious issue – the origin of the long-range hydrophobic attraction. They say the interaction is electrostatic, caused by induced dipoles on the surfaces of hydrophobic solutes. I’ve only seen the abstract of this paper, but hope to take a good look soon.
Joe Zaccai at the ILL and colleagues have a deeply interesting, not to say perplexing, paper in PNAS in which they report very slow translational diffusion coefficients for water inside the cells of the halophilic archaea Haloarcula marismortui from the Dead Sea. The idea that cell water has different dynamics from bulk water goes back a long way, at least to NMR work by Ray Damadian in the 1970s. But it’s never been shown definitively. Zaccai and colleagues use inelastic neutron scattering to measure relaxation times of the water in situ in the archaeal cells, and say that 75% of it diffuses no less than 250 times slower than bulk water. That’s too slow to be explained away as the dynamics of macromolecular hydration shells. Nothing of the sort is seen in E. coli. So what’s going on? I can’t figure out quite what their hypothesis is – it makes sense to link it to the high salt concentrations (specifically potassium), but beyond that the authors just talk about ‘structured water around K+ ions’ in the presence of proteins, similar to that seen in potassium channels. Hmm… clearly potassium doesn’t ‘structure’ water so significantly in simple salt solutions, so what’s the idea? I’m hoping Joe can enlighten me.