Water Shell Undergoes Structural Changes

Water undergoes structural changes in the thin layer surrounding hydrophobic molecules, researchers at Purdue University report (Nature, DOI: 10.1038/nature11570). These previously predicted, but unconfirmed, changes depend on the size of the hydrophobic molecules. Despite this hydration shell’s importance—it plays a role in many processes, including protein folding and molecular self-assembly—its structure remains poorly understood.

Dor Ben-Amotz, Joel G. Davis, and coworkers used Raman spectroscopy and a mathematical method called multivariate curve resolution to analyze the hydration shell around linear alcohols ranging from methanol to heptanol. Raman signals from changes in the shell “are hard to find because they lie under a huge signal from the bulk water,” Ben-Amotz says.

“Around short oil molecules and at low temperatures, we see that water is highly structured, with more tetrahedral order and stronger hydrogen bonds than in surrounding bulk water,” Ben-Amotz says. That structure disappears at higher temperatures, at which the hydration shell looks more like bulk water.

But for molecules longer than 1 nm, the researchers see a different structure at high temperature—one with less tetrahedral order and weaker hydrogen bonding than in bulk water. “This new structure depends on the length of the chain—the longer the chain, the more dramatically the new structure comes in,” Ben-Amotz says.

Such length-dependent structural changes had been predicted theoretically by David Chandler and coworkers at the University of California, Berkeley, and in subsequent theoretical work by Shekhar Garde at Rensselaer Polytechnic Institute and others. But they had not been demonstrated experimentally until now.

The Ben-Amotz study and related earlier work on hydration free energy by Gilbert C. Walker and Isaac T. S. Li of the University of Toronto “are excellent examples of the convergence of understanding from molecular theory and simulations and state-of-the-art experiments,” Garde says. Walker calls the Ben-Amotz data “strong evidence” in support of the predictions made by Chandler’s and Garde’s groups.

The next step, Ben-Amotz says, is to see how the new structures affect interactions between molecules. “Lots of biological molecules have hydrophobic patches and chains that are larger than 1 nm,” he says. Such structures “influence everything from the folding of proteins and the formation of biological membranes to the binding of drugs to hydrophobic pockets.”