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.
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This is Joe Zaccai's "quick reply" (which he stresses is not vetted by his co-authors):
Frank Gabel, in our lab, measured water diffusion in concentrated NaCl and KCl solutions by the same neutron scattering tecnique and you are quite right it is only slowed down by a small factor compared to bulk water, as expected. We are not suggesting, therefore, that it is the K+ ion by itself that structures the water in the H. marismortui cells but rather some 'coordinated' ion-protein-water complex favouring strong H-bonds. As mentioned in the PNAS paper, proteins from extreme halophiles had already been shown by several thermodynamics-based experimental methods (densimetry, AUC, zero-angle scattering of light, X-rays and neutrons) and "Heini Eisenberg type" analysis based on preferential interactions to have exceptional salt-ion and water binding properties. I would be very interested in suggestions of model compounds that mimic such interactions.
A model to explain how K+, Na+, water and halophilic proteins might interact in the Dead Sea organism, Haloarcula (Halobacterium) marismortui, was published in Biomembranes 1973, 7, 219-251. It should be emphasized that this organism, whose cells contain 4 M K+, grows optimally in medium containing 3.5 – 4M NaCl and that one-third of the aminoacid residues of cell proteins are either aspartate or glutamate, i.e. the proteins are very acidic.
The model postulates a 2-compartment system of 1 – 3 layers of ordered water molecules close to the protein surface, in which the K+ is contained, and “ordinary” water containing Na+ further away. Since the proteins are so acidic, there is need for considerable amounts of cations to balance the negative charges on the proteins.
Experimental evidence has been presented for the 2-compartmental system (J. Membrane Biol. 1971, 6, 259) and for the importance of negatively-charged proteins for retention of K+ (see the Biomembranes journal mentioned above). It was also found that the cells were able to retain K+ when Na+ in the outside solution was replaced by a cation with a crystal radius below 1.1 Å, irrespective of charge. In addition, the cells were able to take up and retain Rb+ or Cs+ instead of K+. The results were explained on the basis of the structure-making or breaking effects of cations on water.
We feel that the recent work of Roderick MacKinnon on K+-channels may support our model. Important parts of these K+-channels are referred to as the “central cavity” and the “selectivity filter”. The central cavity contains about 50 water molecules, most of which cannot be seen in X-ray studies. However, in the centre there is a single K+-ion held by the d- charges of oxygen atoms belonging to 8 water molecules; these latter must be ordered since they can be detected by X-rays. The selectivity filter also contains a single K+ held by oxygen atoms, this time supplied by carbonyl groups from the side-chains of aminoacid residues. In each case the K+ resides near the centre of a distorted cube (Nature 2001, 414, 43-48). In these experiments the K+ ion could not be replaced by Na+ ,a most unusual case of specificity between K+ and Na+.
A general conclusion is that large specificities between Na+ and K+ may develop in very dry systems where part of the water itself becomes structured.
A question arises as to whether Haloarcula marismortui is really unique in having water with a low rate diffusion, or whether it merely seems so, owing to the very few organisms that have been examined so far.
Ben-Zion and Margaret Ginzburg
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