A Renaissance For Hofmeister

IT STARTED in the late 1800s with egg whites. At the German Medical School, in Prague, the prolific research chemist Franz Hofmeister was dissolving gloppy egg-white proteins in solutions of different ions, duly noting that some, such as sulfate and fluoride, caused the proteins to precipitate readily, whereas others, such as iodide and isocyanate, did not.

Hofmeister also found that ions varied in their effects on other fundamental properties of ionic solutions, such as their ability to unfold proteins and affect surface tension. He ranked the ions in order of these effects, also noting that anions have greater solution effects than cations. At the left of the so-called Hofmeister series are what are called kosmotropes (order makers), which tend to precipitate proteins and prevent unfolding, and to the right are chaotropes (disorder makers), which increase the unfolding or denaturation of protein.

The Hofmeister series soon became a fundamental framework with which to study many kinds of biochemical systems, which frequently involve saline solutions. Although the orders of some of the ions change places under different conditions, the general trends have become textbook knowledge.

Hofmeister was as important to discovering these ubiquitous ion-solution effects "as Mendel was to genetics," says colloid and surface chemist Barry Ninham, professor at Australian National University, in Canberra.

But the mechanisms underlying the Hofmeister effects have remained murky. The thought has been that the ions somehow affect bulk water structure, which in turn affects the chemistry of substances dissolved in it.

Now, this time-honored concept has been turned on its head. During the past decade—and particularly in the past few years—many of the presumed reasons for the Hofmeister effects have been called into question, and in the minds of many scientists, they have been shown to be just plain wrong. Ions likely have very little effect on water beyond the first sphere of molecules that surround them, for example. And where ions once were thought to shy away from surfaces, scientists are now finding that some congregate there.

These findings have enormous implications for all walks of chemistry—from reactions on aerosol droplet surfaces in the atmosphere to protein behavior in cells. Protein crystallization, for example, currently an often frustrating empirically based art, could become a predictive science if we knew how different solutions affect the protein crystallization process.

SO WHAT is really going on with ions in solution? The search for an answer has led to the development of an enormous new facet of chemistry. A growing portfolio of research in thermodynamics, spectroscopy, molecular dynamics, and theory is aimed at picking apart the complex mechanisms of how ions affect water, both in bulk and on surfaces, and the molecules dissolved in it.

The Hofmeister renaissance is evident in an explosion of research papers, particularly in the Journal of Physical Chemistry B and the Journal of the American Chemical Society . Hofmeister citations have gone from a few per year more than a decade ago to over a thousand per year now. Current Opinion in Colloid & Interface Science devoted an entire issue to the Hofmeister series in 2004.

Ninham sees the renewed interest in the subject as a serious effort to bridge the gap between physical chemistry and biochemistry. "There's a huge disjunction between biological and physical sciences," he says.

Part of the reason for the explosion in interest has been the increased sophistication of science in general: new, more sensitive spectroscopic techniques and new ways to interpret spectra; more powerful computers; and more ingenious computational methods.

THE NEW RESEARCH on Hofmeister effects in bulk water represents some of the more striking turnarounds in thinking about the series. The explanation accepted for nearly 100 years has been that ions have long-range influence on water's hydrogen-bond networks. Ions such as iodide weaken water's network of hydrogen bonds and are known as "structure breakers." Sulfate ions, by contrast, strengthen the network and are called "structure makers."

When scientists first began exploring the issue with spectroscopy, they found more evidence for long-range bulk effects. For example, spectra of solutions with certain ions resembled those of water under extreme pressure, implying that the ions had induced large structural changes throughout the bulk water.

One explanation that had been invoked for these structure-making and -breaking observations, notes chemistry professor Paul S. Cremer of Texas A&M University, was that water is better ordered around, say, sulfate ions than around perchlorate ions, which lie to the structure-breaking side of the series. Sulfate "jealously guards" the water molecules surrounding it and out-competes any protein in the solution, Cremer explains. Without a sturdy water network surrounding them, proteins would then precipitate out, so the rationale went. But in light of recent results, "there's no question it's dead wrong," Cremer says.

Since the 1990s, Ninham has been sounding the alarm about problems with the reigning explanations for Hofmeister effects. And since then, nails have been steadily hammered in the coffin of the structure maker/breaker dogma.

A seminal study in 2003 by Huib J. Bakker, professor at the University of Amsterdam and the FOM Institute of Atomic & Molecular Physics, measured the relative "stiffness" of hydrogen bond networks in liquid water dosed with various ions. Using infrared pump-probe spectroscopy, they studied the rotational dynamics of water molecules in the different solutions, demonstrating that the ions had no effect on water behavior beyond the first sphere of surrounding water molecules.

Soon thereafter, chemistry professor Gary J. Pielak at the University of North Carolina, Chapel Hill, weighed in with thermodynamic studies showing no correlation between the effect of ions in the Hofmeister series on protein stability and water structure.

And recent studies combining Raman spectroscopy and theory from University of California, Berkeley, chemistry professors Richard J. Saykally and Phillip L. Geissler and graduate student Jared D. Smith show that ions' effects on the spectra of water's OH vibrations can be attributed to the ions' electric fields acting on neighboring molecules, not to the long-hypothesized long-range effects on bulk water structure (J. Am. Chem. Soc., DOI: 10.1021/ja071933z).

All of those data call for a re-asking of an old question: What produces the Hofmeister effects? Scientists in large numbers are turning their attention to interfaces. Huge amounts of important chemistry occur at surfaces and macromolecular interfaces-for example, between air and water and between macromolecules and water.

In 2000, Pavel Jungwirth, associate professor of molecular physics at the Institute of Organic Chemistry & Biochemistry, Academy of Sciences of the Czech Republic, in Prague, and chemistry professor Douglas J. Tobias at UC Irvine created a stir with molecular dynamics simulations that showed, in direct opposition to previous thinking, that the heavier halide ions can adsorb to water-air interfaces. UC Irvine chemistry professor John C. Hemminger later provided experimental evidence for that idea using electron spectroscopy (Science 2005, 307, 5709).

Further research is showing that different ions' tendencies to gravitate toward the air-water interface follows the Hofmeister series. Those discoveries could help atmospheric chemists, for example, who are interested in the reactions that occur on aerosol droplets.

Cremer's group is probing for a general ion/water/macromolecule interaction mechanism that might explain the Hofmeister series. To do so, they rely on a simple model system consisting of poly(N-isopropylacrylamide) floating on water. This polymer behaves a lot like a protein, but its structure is much simpler. Cremer's spectroscopic and thermodynamic studies of the polymer on the surface of water show that an ion's ability to orient water molecules adjacent to the polymer on the water surface follows the Hofmeister series (J. Am. Chem. Soc. 2007, 129, 12272). This mechanism might be general to all proteins near their isoelectric point (the pH at which the molecule has no net charge) in ionic solutions, he suggests.

But despite the bustle of research activity, this second wind for Hofmeister's series has just begun, scientists say. "A universal picture is starting to emerge, but we've got a ways to go before we get to the bottom of it," Saykally says.

And that bodes well for interesting times ahead, Tobias says, adding: "We've showed we still don't know some things that are critically important about one of the simplest systems imaginable—salt water."