Courtesy of Martin Chaplin
Water molecules cluster to form hydrogen-bonded bicyclo-octamers (H2O)8 (top left) that can link together into larger structures (top right). Ideally they form 280-member icosahedral clusters, (H2O)280, (below), shown looking down the two-fold, three-fold, and five-fold axes of symmetry. Only the oxygen atoms of the constituent water molecules are shown (except at top left).
Researchers are beginning to glimpse water's secret social life. Evidence is mounting that water in living systems naturally gathers into frameworks of 14, 17, 21, 196, 280, or more molecules. Some say that the clusters' apparent existence necessitates redesigning simulation models of life processes. And support is growing behind the idea that these intricate structures play key roles in operations ranging from molecular binding to turning on and off basic cell processes.
Such huge clusters certainly exist under some conditions, according to Richard Saykally, professor of chemistry at University of California, Berkeley. Saykally has spent years studying isolated water clusters using laser spectroscopy techniques that he developed. "There is no theoretical or practical limit on the size that these clusters could grow to," he says, adding that their life spans are limited only by their collisions with other molecules, an event that, within the stormy cell interior, usually occurs every few picoseconds.
Using infrared spectroscopy, two research groups recently added to the evidence that clusters of dozens or even hundreds of water molecules exist in nature.12 Many chemists are intrigued by the uses nature might have for such structures. A team led by William Royer at the University of Massachusetts Medical School, Worcester, has shown experimentally that 17-molecule water clusters can serve as a communications medium between protein subunits.3 Chemist Martha Teeter at Boston College has found that clusters of 30 or more water molecules mediate some protein binding.4 Martin Chaplin at London's South Bank University posits an even more radical model for how cluster dynamics may make it possible for cells to maintain ion gradients without spending energy.5 Further, he contends that collapsing water structures may serve as signaling switches in the cell.
Charlatans have seized on the principles. Stephen Lower, a retired chemistry professor at Simon Fraser University in British Columbia, maintains a Web site to catalog claims such as seven-sided water containing "mono-atomic minerals" to "clarify" the nervous system, structured water conducting "healing alpha-theta frequencies," and other quackery.6 Such claims distort what is a far more subtle and theoretical pursuit. Lower says that models such as Chaplin's are "consistent with X-ray diffraction data and meet the tests of a good scientific model, in that they provide plausible explanations of the unusual properties of water."
Teeter has been exploring the interplay between water and protein for 20 years. She notes that often the binding of two protein molecules seems to be mediated by clustered water. "People have found that the crystal structure of trypsin and trypsin inhibitor don't fit together perfectly," she says. "The amino acid side chains conflict." To form a tight complex, these side chains must change their conformations. Mobile water structures along the proteins' surfaces link the two proteins by binding to each. Her calculations indicate that these water structures are organized as fragmented dodecahedrons (12-sided figures), 9–15 Ångstroms long, enough to accommodate 30 or more molecules. Teeter has seen similar events in her studies of myoglobin.
A CAGEY PROPOSITION
Chaplin's ideas are far more sweeping. He proffers a cluster-based solution to a long-standing problem: how cells concentrate potassium ions on one side of their membranes and sodium ions on the other.
Hydrogen bonds oscillate, growing stronger and weaker dozens of times every picosecond, he notes. And, as others have proposed, stronger bonds put an ordered, equal distance between each molecule, reducing the water's density and expanding its volume. Weaker bonds relax the structure and let water molecules pack closer together.
Teeter and others have shown that proteins tend to put water molecules in order. Because proteins make up half or more of the structures inside cells, a significant amount of the water inside a cell will tend to be clustered at any given moment, while the water outside a cell is likely less so.
Sodium and potassium ions, with their single, positive charges, balance the negative charges of the horde of phosphates inside a cell. But even though they carry the same charge, potassium is physically larger than sodium and has a lesser charge density. Sodium's higher charge density tends to bind water tightly, limiting the formation of less dense structures, but potassium, with its lower charge density, gravitates toward clusters. That gives potassium an affinity for the less dense water structures common inside a cell, while sodium tends to associate with the disordered water more common outside.
"Experiments have found potassium ions inside 20-molecule water clathrates," Chaplin points out. "Sodium hasn't been found in that arrangement." In Chaplin's model, sodium ions migrate out of the cell while potassium ions are drawn in. "The general idea is that they're pumped," Chaplin says, "but if you work out how much energy it would take to do all that pumping, it's about four times more energy than the cell can generate."
According to Chaplin, it's a simple rule of attraction: "The potassium prefers the structured, less dense water inside the cell; the sodium doesn't." Chaplin adds that potassium ions will be drawn to the negatively charged carboxyl groups that proteins carry. Fixed to carboxyls, the potassiums' water cages attract other molecules, growing outward – in many cases, far enough to back into clusters mushrooming from other proteins. The clusters then knit together.
As the potassium-seeded clusters grow, the cell's interior becomes more structured, more viscous, with less room for things to move freely. Cellular processes grow sluggish. Then, with a twitch of a muscle or other signal, the cell's membrane is momentarily depolarized enough to allow sodium ions to flood the cell's interior, breaking up the clusters and beginning the processes of protein function all over again.
ON TO SOMETHING?
Initial reactions to Chaplin's idea are guarded. Angel Garcia, a staff scientist at Los Alamos National Laboratory who specializes in the hydration and dynamics of biomolecules, doubts its validity, partly because of the disparate time scales on which various cellular processes occur, such as the rotation of proteins and the structuring and unstructuring of water.
Fabio Bruni, a physicist at Italy's Università di Roma Tre who studies water and its relation to proteins, contends that Chaplin overinterprets data produced by a flawed algorithm used to analyze a 1967 X-ray diffraction study7 that has since been discredited. Chaplin asserts that the key data from the 1967 study have been verified, not discredited, by the more recent work.8 But Bruni argues that large water clusters are not needed. Proton transfer in cells can be explained by movements of individual water molecules. "The rather static clusters Chaplin proposes are at odds with a [model of] dynamical clusters based on molecules' random connectivity" for which other experiments have shown evidence, Bruni says. Chaplin maintains that his clusters are dynamic, not static, and he argues that Bruni's combination of data from diffraction studies and molecular modeling leads to skewed conclusions that disregard evidence for clusters provided by Teeter and others.
The disagreement is part of a larger problem, Chaplin says. "Scientists ... have tried for 30 years to produce a model of water that agrees with experimental data and therefore can be used to model biological systems, but none ... have been able to predict the properties of water very well," he says. "This is due in part to the limitations inherent in simulations made up of relatively small numbers of single water molecules." Simulations need to accommodate larger numbers of water molecules and incorporate multibody interactions, he adds.
Teeter says that Chaplin may be on to something; his general ideas are consistent with Teeter's experimental data. Chaplin's concepts also gain support from studies by Frank Mayer and his coworkers at Germany's University of Göttingen. The group showed in experiments that low-density water, the kind created by clustering, is not only less reactive and more viscous than bulk water, but also stimulates enzyme activity.10
Indeed, the new infrared evidence, coupled with larger ideas such as Chaplin's, prods researchers to incorporate an expanded view of water into their work. "Outside the cell, water is lumpy and basically wild," notes Guenter Albrecht-Buehler, the Robert Laughlin Rea Professor of cell biology at Northwestern University Medical School. "Inside, it's a masterpiece of organization. Water's ability to form these structures may well be a driving force behind how cells do what they do and why."