Antifreeze: what the sugar does

First, an historical note: I recently discovered that this very nice paper by Charles Tanford on the history of the hydrophobic effect is available online. Much of this stuff appears in his books The Hydrophobic Effect (Wiley, 1980) and Nature’s Robots (OUP, 2001), but it’s a very nice summary of it.

Joe Zaccai has sent me a preprint of a paper just accepted by EMBO Reports that uses neutron scattering to look at water dynamics in vivo in E. coli. It shows that these dynamics are ‘normal’ and bulk-like, contrary to suggestions that water is ‘tamed’ in the cytoplasm. Bertil Halle and his coworkers have a paper in press with PNAS that reports precisely the same conclusion based on NMR data. So together, these papers ought to bury one more water myth.

There’s an interesting study here (JACS 130, 2928-2929; 2008) by Robert Ben and colleagues at Ottawa of the effect of sugar hydration on the antifreeze behaviour of glycoproteins. By substituting various sugars on antifreeze glycoprotein analogues, they find that the sugar conformation and thus hydration is important for inhibition of ice recrystallization. Here’s the punchline: “our data indicate that the compatibility of a hexose with the three-dimensional hydrogen-bonded network of water is inversely proportional to recrystallization-inhibition activity” – a finding they associate with the consequent free-energy change of transferring a water molecule to the ice lattice.

Also in JACS (130, 3120-3126; paper here), Greg Voth and his coworkers Feng Wang and Sergei Izvekov report ab initio MD simulations showing that hydronium ions form unusual cation pairs in concentrated aqueous HCl, stabilized by delocalization of the excess charge of the hydrated proton. This is consistent with Greg’s earlier work showing that hydronium seems to display amphiphilic behaviour – one can regard this as a kind of amphiphilic clustering.

Water does interesting stuff around benzene, which is hydrophobic around the edges but can form hydrogen bonds via the pi orbitals over the ring faces. So how does this translate to C60? Dahlia Weiss, Tanya Raschke and Michael Levitt have addressed that question using MD simulations in a paper here (J. Phys. Chem. B 112, 2981-2990; 2008). They say that the waters in the first hydration shell become more oriented, and have an increased number of hydrogen-bonding contacts, but that hydrogen bonding is disrupted between the first and second hydration shells. In general, the hydration shell is dense and ‘well-structured’ – I’d guess consistent, at a glance, with the kinds of orientational ordering described by Jan Engberts and W. Blokzijl in their 1993 article on hydrophobicity (Angew. Chem. Int. Ed. 32, 1545-1579), as opposed to the old notion of a hydrophobic ‘iceberg’. In this regard, the authors say that “C60 behaves as a large hydrophobic solute.”