How much water does it hold?

The last couple of days, in connection with the behaviour of water in protein active sites, I have been reading about water and the hydrophobic effect in general, and I found it fascinating how little we really know about both of these in general. For one thing, we still seem to have miles to go in understanding ice, bulk water, and the transition between the two. There was a debate between Richard Saykally of Cal Berkeley and Anders Nilsson of Stanford about the number of hydrogen bonds that a water molecule makes in bulk water. One would think that something as simple as this would have been unraveled by now. But nothing about water is simple. Nilsson published the pretty amazing contention that many water molecules in bulk liquid water at room temperature form only two hydrogen bonds. This would mean that those hydrogen bonds are unusually strong. This paper sparked heated debate, and Saykally was a vocal opponent, countering with his own experiments, and contending that it would be a walk across the street to Stockholm if Nilsson's viewpoint turned out to be true. I am no water expert, but the best value for the energy of an average hydrogen bond in bulk water that I have come across seems to be 1.5 kcal/mol (determined by Saykally), which can definitely be less than that in a protein active site. At least for now, this discussion of "average" hydrogen bonds generally sounds a little slippery to me.

I think that this whole issue of how much a hydrogen bond in bulk water is worth could affect how much gain in energy a water molecule displaced from a protein active site might gain. Friesner et al. in their recent publications indicate that water molecules which cannot form their full complement of hydrogen bonds in a protein active site because of confinement could get an enthalpic advantage if they are pushed out into solvent. I think there's much less ambiguity about the entropic advantage that such a water molecule could have. Then there's also Dunitz's whole argument about entropy-enthalpy compenstation (see previous post) which could factor in...I am still really groping about for coherence in this landscape.

As far as hydrophobic interactions are concerned, while the general idea that the hydrophobic effect is entropically driven seems correct, it also seems situation dependent, especially varying with the temperature. One of those basic important things to note is that the enthalpy of transfer for a nonpolar solute to water is close to zero, and can even be slightly favourable. But when two nonpolar surfaces in water aggregate, it's the entropy of disordering waters clustered around the solute that is very favourable. As usual, even these seemingly simple issues are draped in subtleties. Among others, Themis Lazaridis has explored this very interestingly in a review (2001), in which he cements the "classical" view of the hydrophobic effect. He also counters arguments about the similar energy of cavity creation in water compared to some other solvents by saying that while this may be true, the decomposition of factors contributing to the energy might be different for the two cases.

And finally, and I should have posted about this a long time back, Water In Biology, a great blog all about the molecular intricacies of water, from Philip Ball, staff writer for Nature and author of many excellent books, including H2O: A biography of water.

Water really seems like a testament to that quote by T.S. Eliot about coming back to the beginning again and again, and newly getting to know a place every time. There are just so many things about it we don't understand.