More Hofmeister headaches

The debate rumbles on over Hofmeister effects. In a paper in Scholarly Research Exchange [doi:10.3814/2008/761829 – paper here], Terence Evens and Randall Niedz of the US Horticultural Research Laboratory in Florida say that many previous studies of ion-specific effects on protein precipitation are flawed because they fail to take into account the dependence of pH on the type and concentration of ions in solution, treating it as an independent variable. More generally, they say that individual ion effects can’t be deduced in any straightforward way from the effects of specific salts. In a nutshell, this seems to be the key message: ‘Is the sulphate ion more effective at protein precipitation than the chloride ion? It depends on the protein. It depends on protein concentration. It depends on the concentration of the respective ions. It depends on the proportions and concentrations of the other cations and anions in solution. It depends on the dissolved gases. It may or may not depend on the pH. It depends on temperature. These dependencies are conflated, confounded, lost or ignored in traditional Hofmeister series, but are fundamentally essential to realizing a deeper understanding of ion-specific effects.’ Discuss, as they say. It’s certainly a rather discouraging message on what is already a bewildering problem, but Evens and Niedz present results for ovalbumin and BSA that seem to bear out this complexity.

In a related vein, Shekhar Garde and colleagues at RPI have examined the thermodyanmcis of hydrophobic hydration, association and folding for a hydrophobic polymer in sodium chloride solution and aqueous trimethylamine oxide (TMAO), an osmolyte [M. V. Athawale et al., J. Phys. Chem B 112, 5661; 2008 – paper here]. They’ve found previously that NaCl weakens hydrophobic hydration and enhances association, while TMAO has little effect (Ghosh et al., J. Phys. Chem. B 109, 642; 2005 and Athawale et al., Biophys. J. 89, 858; 2005). Here they carry out temperature-dependent simulations to figure out if the effects are entropic or enthalpic. For TMAO, there is almost precise enthalpic-entropic compensation. For NaCl, changes in solvent-solvent, solvent-salt and salt-salt energy lead to a dominant enthalpic contribution at small length scales (that is, for small solutes), but the strengthening of hydrophobic interactions is entropic in origin at large length scales, being governed by the need to form a solvent-solute interface. This seems to offer further evidence that there is no single ‘explanation’ of Hofmeister-type effects.

Meanwhile, Agustín Colussi and colleagues at Caltech have returned to a more basic level of the problem: the fractionation of ions at the air-water interface (a loose proxy for the air-hydrophobe interface) [J. Cheng et al., J Phys. Chem. B 112, 7157; 2008 – paper here]. They have shown previously [J. Cheng et al., J. Phys. Chem. B 110, 25598; 2006] that aggregation of anions at the interface seems to increase with increasing ion radius. They now extend their experimental study to the cases of the large anion PF6- and the highly polarizable IO3-, and look also at the effect of adding methanol, which will migrate to the surface and cap it with methyl groups. The same relationship with ion radius is found, and the fractionation barely depends on the methanol content. The authors conclude that this fractionation happens not because the ions have any affinity with the surface but because they are expelled from the bulk.

Now forget the salts. Esben Thormann and colleagues at the University of Southern Denmark have looked again at a familiar model system: a polystyrene particle several microns across stuck to an AFM tip and brought close to hydrophobic and hydrophilic surfaces [E. Thormann et al., Langmuir doi:10.1021/la8005162 – paper here]. For approaching surfaces in the hydrophilic case, all looks fine: the interactions are described by DLVO theory. But for the hydrophobic case, bridging air bubbles form, as has often been hypothesized, leading to jump-in at a separation of around 10 nm due to the action of the meniscus. When the particle is retracted, the bubble becomes elongated until it ruptures at about 70 nm. In both cases there are also force plateaus at separations of up to a few hundred nm, which the researchers interpret in terms of bridging polymer molecules pulled out from the particle surface. All this argues for caution in regarding the system as a model of the biological case.

Let’s stick with these model surfaces for a bit. Some time ago I mentioned some ‘curious’ results of Andrei Sommer and colleagues at the University of Ulm on irradiation of water films on diamond. I found some difficulty there figuring out what the underlying hypothesis was. Andrei has now sent me more material on this. The basic motivation for the work is the fact, known for some time but unexplained, that the surfaces of diamond are somewhat conductive. Andrei and colleagues believe this is due to proton migration in thin surface films of water, which are formed in humid conditions. Their experiments [A. P. Sommer et al., Cryst. Growth. Design 7, 2298; 2007] show that for hydrogen-terminated diamond, the conductivity drcreases with increasing humidity. They think this is because the highly ordered water films that form at low humidity are disrupted, degrading proton motion, as the films get thicker. This idea challenges the widely accepted model for the surface conductivity, called the transfer doping model [M. I. Landstrass & K. V. Ravi, Appl. Phys. Lett. 55, 975; 1989], which would predict increased conductivity with increased humidity. Andrei and colleagues have recently debated this point with John Angus and colleagues in Science [V. Chakrapani et al., Science 318, 1424; 2007].

From the perspective of water in biology, Andrei suggests the key point is that the highly ordered (indeed, essentially crystalline) water nanofilms he identifies on the (hydrophobic) diamond surface offer “a unique platform for the systematic investigation of nanoscopic water layers.” In a forthcoming paper for Crystal Growth and Design, he and his colleagues Dan Zhu and Hans Fecht argue that these layers might even provide a platform for the origin of life, as I understand it by potentially templating the evolution of organic monolayers. Apparently Albert Szent-Györgyi suggested something similar in the 1970s, proposing such a role for crystalline interfacial water layers. Diamonds can be extremely ancient, and also extraterrestrial. All this is very intriguing, although as someone now programmed to approach with scepticism the notion of enhanced ordering of water at hydrophobic surfaces I think I would like to see some more direct evidence that the water molecules on diamond are indeed truly ordered, especially if the claim is that this extends beyond a monolayer. But I think they’re working on that.

Heme catalases convert hydrogen peroxide to water and oxygen. One type of such enzyme, so-called Clade 3 of the most abundant (monofunctional) class, contains a tightly bound NADPH molecule which seems to protect one of the intermediates of the ferryloxo group against deactivation to a catalytically inactive form. Reiner Sustmann at Duisburg-Essen and colleagues propose in a new paper (W. Sicking et al., JACS 130, 7345-7356; 2008 – paper here) that a bound water molecule plays a critical part in this process, both by supplying a hydroxyl group that binds temporarily to the porphyrin group and then assists the fast two-electron reduction of the intermediate ferryloxo species by NADPH via a series of proton shifts, to restore the catalase resting state and avoid diversion of the reaction towards the deactivated state. A nice example of the multiple, sophisticated roles that bound water can play in active sites.

Lei Zhou and Steven Siegelbaum at Columbia University present a new coarse-grained approach for conducting normal-mode analysis of the dynamics of proteins, which has a lower computational cost than trying to extract the dynamics from a full MD simulation with explicit water [Biophys. J. 94, 3461; 2008 – paper here]. They say that this method is more accurate than are existing coarse-grained NMA techniques, and gives good agreement with experimental results from quasieleastic neutron and light scattering.

In my last blog entry I referred to recent work on the excited-state dynamics of the green fluorescent protein. Dan Huppert and colleagues at Tel Aviv University have looked at essentially the same aspect of the problem: the role of the proton-transfer process [R. Gepshtein et al., Langmuir 112, 7203; 2008 – paper here]. They say that the non-exponential dynamics seem to stem from the distance-dependence of the proton transfer between the chromophore and a bound water molecule that acts as the acceptor. This distance has a relatively large spread of about 0.2 angstroms in GFP.

Finally, thanks to everyone who helped make my Chem. Rev. article a most-accessed paper for the period Jan-Mar 2008.