ScienceShot: A Mineral Mystery in the Driest Place on Earth
Martin Reich

At the Smallest Scale, Water Is a Sloppy Liquid

Water may seem like a dull liquid. But at the molecular scale, there's a party going on. New simulations reveal that water molecules actually form two different types of structures that break apart and recombine at lightning speeds. Such complexity just might be the reason why life as we know it sprang forth in a wet environment.

As simple as an individual water molecule is—two atoms of hydrogen bonded with one of oxygen—it forms weak bonds to its neighbors creating more-complex structures. That's allowed it to serve as the medium for the growth and evolution of the most complex molecules in the universe, including enzymes, proteins, and the mother of all known living creatures, DNA. But why water and not, say, hydrogen peroxide or even ammonia? Scientists have been wrestling with this quandary for over a century. Indeed, 5 years ago, Science called this one of the 125 most important unresolved scientific issues.

A new study might have uncovered an essential clue. Researchers used computer models to probe how water molecules form structures, a phenomenon that has resisted visual examination so far. Working with standard desktop computers, the team applied models originally designed to study complex systems, such as the Internet, the spread of viruses, and the folding of proteins, to investigate the configurations of water at the smallest scales.

What the researchers observed, they reported online in The Journal of Physical Chemistry B, is that water molecules bond with one another in a surprisingly complex and dynamic way. Any given volume of water contains two types of molecular structures—one a blobby, loosely packed agglomeration and the other a tight, regular arrangement resembling a crystal lattice. But both structures tend to break apart and recombine frequently, on the order of extremely tiny fractions of a second. The result is a chaotic mix of water molecules. Within that mix, the hydrogen atoms form connections that function like hooks, onto which carbon or nitrogen atoms can presumably grab to form the beginnings of complex organic molecules. And the process can dramatically influence the motion of even more-complex biological systems, such as proteins, by helping their assembly. As far as anyone knows, no other liquid demonstrates this property.

The finding introduces "a framework to understand how water with its [hidden structure] influences protein function at the fundamental level," says physicist and co-author Francesco Rao of the University of Freiburg in Germany. The models present a "very crude" first look at these structures, adds physical chemist and co-author Peter Hamm of the University of Zürich in Switzerland. "It is becoming clearer and clearer," he says, that "water is more than just a solvent, but actually an integral part of the functional structure of proteins."

It's "a fascinating and provocative paper," says physical chemist James Skinner of the University of Wisconsin, Madison. The study, he says, helps to illuminate subtle but important details about molecular motions in water.

The demonstration by a computer model that water exists in two different microscopic constructions is a "wonderful discovery," adds physicist H. Eugene Stanley of Boston University. Although earlier laboratory experiments have suggested this possibility, he says, the authors are the first to model the process in detail. It's a step toward unraveling why this liquid can produce 15 types of ice, for example. More important, Stanley says, "We will never understand biology until we understand water."