Wednesday, March 26, 2008

Chemistry vs geometry

Pablo Debenedetti and colleagues have carried out precisely the kind of study that is needed to tease apart the various factors that might be at play in hydrophobic association of proteins (N. Giovambattista et al., PNAS 105, 2274-2279; 2008 – paper here). One can anticipate that the potential for effects such as abrupt drying transitions as the two surfaces approach is affected both by surface chemistry – by the distribution of hydrophilic and hydrophobic groups – and by geometry. Certainly, both have been implicated as playing a role in how real proteins behave, as for example in the Berne group’s study of protein associations for BphC and melittin (Liu et al., Nature 437, 159-162; 2005; Zhou et al., Science 305, 1605-1609; 2004). Melittin monomers enclose a tubelike space, for example, whereas BphC is slablike. Moreover, melittin, like many proteins, has a rough surface with concavities. To decouple the effects, Pablo and colleagues have simulated the association of a mutated melittin dimer in which the distribution of hydrophobic and hydrophilic groups is retained but the surface is artificially flattened. The results suggest that the flattened melittin behaves as an intermediate case between ideal, flat hydrophobic and hydrophilic surfaces, and that the drying seen in the case of ‘real’ melittin happens only at very small separations (about one intervening water layer) for the flattened case, being localized to a central region where an apolar residue resides. It can be suppressed by replacing that residue. In other words, drying seen for ideal hydrophobic plates is probably stronger than it is for real proteins, where it is likely to be highly sensitive to small variations in surface chemistry.

Michael Geisler and colleagues at the Technical University of Munich have looked at Hofmeister effects in the adhesion of spider silk proteins to a solid surface, using single-molecule AFM force spectroscopy (Langmuir 24, 1350-1355; 2008 – paper here). They find that the desorption forces follow the Hofmeister series, but can’t yet develop a clear interpretation of what is going on. The hydrophobicity of the silk protein also plays a part: ions that stabilize adhesion do so less when the protein is less hydrophobic, ‘indicating that hydrophobic and Hofmeister effects are closely related’ – but how?

Dusan Bratko and Alenka Luzar have attempted to unravel the much vexed question of how dissolved gases affect the hydrophobic interaction (Langmuir 24, 1247-1253; 2008 – paper here). They have used simulations to look at how various gases influence water structure close to a hydrophobic surface, and solvation forces between two such surfaces. They say that although there does seem to be accumulation of dissolved gas at the interface, it doesn’t have a big effect either on putative water depletion or on solvation forces – something that several experiments seem to bear out. One of the nice aspects of this work is that it enables the authors to make a link between capillary evaporation of pure water induced by hydrophobic confinement and evaporation nucleated by an excess of dissolved gas at the interface – two things that are sometimes not so clearly distinguished. But the simulations can’t follow the possible formation of nanobubbles and the effect this might have on the hydrophobic interaction.

Finally, I have good reason to think that my recent Essay in Nature on water (here) might be seen by some as an endorsement of the ‘new view’ of water structure championed by Anders Nilsson and Lars Pettersson. It’s not, as I think is clear if you read carefully. I merely point out that, first, it is remarkable that such fundamental disagreements about water are still occurring (I know, of course, that Anders and Lars’ idea has been strongly criticized), and secondly, that the implications are rather more far-reaching than might be naively supposed. I must apologize, incidentally, for giving the impression that the experimental work on which their new model is based was done by Lars at Stockholm, rather than by Anders at Stanford.

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