Spherical water

More on the issue of urea denaturation of proteins comes from Paul Cremer and colleagues at Texas A&M (Chen et al., JACS 129, 15104; 2007). They have used vibrational sum frequency spectroscopy to look at the orientation of urea molecules at the surface of bovine serum albumin. They find that the orientation depends on the surface charge on the protein: it flips from pointing the amines towards the surface at high pH (negative surface charge) to pointing the carbonyls at low pH (positive surface charge). That’s interesting. But the authors’ argument that this supports an indirect model for denaturation, where the urea primarily acts via a restructuring of the hydration sphere rather than direct surface bonding, seems rather vague and not at all obvious at this point.

Sergey Buldyrev at Yeshiva University and coworkers (Kumar, Debenedetti, Rossky, Stanley) have found water-like behaviour in a rather un-water-like system (PNAS, doi:10.1073/pnas.0708427104). They’ve looked at a fluid of spherically symmetric particles with two interaction length scales – a hard core and a soft repulsive ‘ramp’ – and see water-like anomalies such as expansion on cooling, as well as cold-induced ‘denaturation’ of a hard-sphere polymer chain. This suggests that this dual-scale characteristic might be the most fundamental feature that gives water its ‘uniqueness’.

Ivan Brovchenko’s paper on water percolation effects in DNA polymorphism, which I mentioned several weeks back, has now been published in JACS: doi:10.1021/ja0732882.