Water in tight places

A clutch of studies this week on water in confined geometries. Alan Soper and his coworkers at ISIS have used neutron scattering to look at the structure of water within the hydropholic pores of Vycor glass, the walls of which are lined with surface OH groups [J. Phys. Chem. B 111, 5610 (2007)]. For pores 40 angstroms wide – several dozen molecular diameters – they find significant disruption of the bulk liquid structure. The average number of hydrogen bonds per molecule is decreased from about 3.6 to about 2.2, and they see structural changes extending at least two layers into the liquid owing to the orientational effects of hydrogen-bonding to surface OH. It’s perhaps a little surprising that the effect is so big, and certainly raises questions about how bulk-like cell water is (the average distance between macromolecules in the cytoplasm is just 1-2 nm).

Sow-Hsin Chen at MIT and his colleagues have continued to study deeply supercooled water. Confined in mesoporous silica with pore diameters of 15 angstroms, it can be supercooled to at least 160 K. Deeply supercooled water is predicted to have a density minimum around 70 K below the well-known density maximum at 277 K – an echo of the ice-like low-density liquid phase predicted under pressure, and of the low-density amorphous ice that has been well established already. Using SANS, Chen and colleagues now see this density minimum at around 210 K for D2O [PNAS early edition, www.pnas.org/cgi/doi/10.1073/pnas.0701352104].

In a preprint [arxiv:0705.2348], Simone Melchionna use MD simulations to look at water in hydrophobic pores. They find spontaneous cavity formation for pore diameters of around 2 nm, which they relate to previous predictions and reports of density depletion and drying at hydrophobic surfaces, particularly the Lum-Chandler-Weeks model. In narrower pores (around 1.5 nm or so), this cavitation can result in complete (but intermittent) emptying, as has been proposed in protein ion or water channels as a gating mechanism. (In those cases, the precise nature of the residues in the pore neck seems to be rather crucial.)

Finally, something with a real biomolecule in it. Gerhard Hummer and colleagues have conducted MD simulations of water within the nonpolar cavities of tetrabrachion, a protein of the hyperthermophilic archaebacterium Staphylothermus marinus [JACS asap, doi:10.1021/ja070456h]. This contains several large hydrophobic cavities linked by a central channel, and the crystal structure shows that all contain water. The researchers find that the largest cavity contains 7-9 water molecules both at room temperature and at 365 K, the organism’s optimal growth temperature. But it empties at a slightly higher temperature, around 384 K. The second-largest cavity is filled with five waters at room temperature, as the X-ray structure confirms, but this breaks up at 365 K. Thus, both cavities are close to emptying at 365 K, and Hummer et al. suggest that this emptying might create sockets into which the proteases, which bind to it in its functional state, can plug.