What proteins do to water

Why does protein hydration water display anomalous dynamics? There is a huge literature on this, particularly in terms of quantifying the effect. Clearly the effect is heterogeneous and complex, but there is some reason to suppose that in general hydration water exhibits anomalous diffusion, the translational motion following a t**0.6 time dependence rather than Brownian t**0.5.

Francesco Pizzitutti at Saclay, Peter Rossky at Texas at Austin, and their coworkers have tried to figure out what is going on using MD simulations (J. Phys. Chem. B 111, 7584; 2007). They consider two aspects of the problem: the effect of protein topology and of static, energetic effects due to, e.g. pinning (polar water-binding) sites, and dynamic effects due to protein motion. Both of these slow down translation, but rotational retardation seems to come only from energetic (electrostatic) effects: when these are switched off, the water molecules actually reorient faster than in bulk. Translational motion happens by water molecules jumping between sites previously occupied by other waters, but also to sites previously occupied by protein groups – hence the involvement of protein motions. Without this protein motion, the options for water hopping are smaller, and so the diffusional retardation is even greater.

What about collective effects due to the hydrogen-boded network? These do seem to exist, and indeed to be strong – the effect of electrostatic pinning sites can percolate throughout the entire surface layer. That fits with the notion of percolation-dependent hydration dynamics discussed by Oleinikova et al. (Phys. Rev. Lett. 95, 247802; 2005).

This is a very nice paper that helps to prise apart the many factors operating simultaneously.