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ALS Capabilities Reveal How Like Can Attract Like Print

A Berkeley Lab research team working at the ALS has observed an unusual pairing that seems to go against a universal scientific truth—that opposite charges attract and like charges repel. Led by Berkeley Lab chemist Richard Saykally and theorist David Prendergast, researchers demonstrated that, when hydrated in water, positively charged ions (cations) can actually pair up with one another.

A New Law of Water Affinities

Late 19th century Czech scientist Franz Hofmeister’s research on ionic compounds resulted in the well known "Hofmeister series," which governs the strengths of ions in inducing protein unfolding, bubble coalescence, and many other phenomena. It remains vital to protein chemistry and other biological and chemical studies to this day. But its mechanism has never been properly understood.

"The Law of Matching Water Affinities” finally came along to shed some light on the issue in 2006. The law says that the least-soluble ion pairs are formed by ions that are closest to each other in their hydration energy — that is, in how strongly they hold onto water. In other words, Hofmeister effects depend on the identity of ions rather than just on their concentration.

Hofmeister himself discovered that sodium salts out egg-white protein more efficiently than potassium, as does calcium. It's a difference with profound biological significance. Computer simulations and quantum calculations of how sodium and potassium bind to proteins were performed by researchers in 2006. Their work indicated that the large difference between the binding efficiency of the two cations (which are otherwise similar in many ways) were consonant with the Law of Matching Water Affinities. The simulations and calculations supported the Law's theoretical predictions.

Researchers working at ALS Beamline 8.0.1 then developed an approach that provided a new class of experimental support. Incorporating liquid microjet technology into the high-vacuum environment of a synchrotron x-ray experiment has allowed the group to perform near-edge x-ray absorption fine-structure (NEXAFS) measurements on liquid electrolyte samples that would otherwise be difficult or impractical to measure with synchrotron radiation.

Through a combination of x-ray spectroscopy, liquid microjet technology, and first principles theory, researchers observed and characterized contact pairing between guanidinium cations in aqueous solution. This cation-to-cation pairing was predicted earlier, but it has never before been definitively observed. The guanidinium cation pairing is significant because it may mean that other similar cation systems can pair this way as well—and that could have implications for biology. For example, guanidinium is the side chain of the amino acid arginine.

Calculated spatial distribution function for a 1.8M GdmCl solution showing carbon (yellow, 0.002 atoms Å−3) and water (green, 0.025 atoms Å−3) density maps around the Gdm+ ion, demonstrating cation–cation stacked pairing and the preferential binding sites of water.  Water molecules interacting weakly with the Gdm+ π electrons are expelled from between the ions as the ions approach.

Guanidinium is an ionic compound of hydrogen, nitrogen, and carbon atoms whose salt—guanidinium chloride—is widely used by scientists to denature proteins for protein-folding studies. This practice dates back to the late 19th century when the Czech scientist Franz Hofmeister observed that cations such as guanidinium can pair with anions (negatively charged ions) in proteins to cause them to precipitate. The Hofmeister effect, which ranks ions on their ability to "salt out" proteins, became a staple of protein research even though its mechanism has never been fully understood.

In 2006, Kim Collins of the University of Maryland proposed a "Law of Matching Water Affinities" to help explain "Hofmeister effects." Collins's proposal holds that the tendency of a cation and anion to form a contact pair is governed by how closely their hydration energies match, meaning how strongly the ions hold onto molecules of water. Saykally, who is a faculty senior scientist in Berkeley Lab's Chemical Sciences Division and a professor of chemistry at the University of California Berkeley, devised a means of studying both the Law of Matching Water Affinities and Hofmeister effects. In 2000, he and his group, led by graduate student Kevin Wilson, incorporated liquid microjet technology into the high-vacuum experimental environment of ALS beamlines and used the combination to perform the first x-ray absorption spectroscopy measurements on liquid samples. This technique has since become a widely used research practice.

Rich Saykally, who is a faculty senior scientist in Berkeley Lab’s Chemical Sciences Division and a professor of chemistry at the University of California, Berkeley, devised a means of studying both the Law of Matching Water Affinities and Hofmeister effects.

With the liquid microjet technology, a sample rapidly flows through a fused silica capillary shaped to a finely tipped nozzle with an opening only a few micrometers in diameter. The resulting liquid beam travels a few centimeters in a vacuum chamber and is intersected by an x-ray beam before being collected and frozen out. In analyzing their current results, which were obtained at ALS Beamline 8.0.1, researchers concluded that the counterintuitive cation–cation pairing observed is driven by water-binding energy, as predicted by theory.

The chemical information that one can extract from such experimental data alone is limited, so researchers interpreted the spectra with a combination of molecular dynamics simulations and a first principles theory method for calculating x-ray spectra. Development of this first principles theory method was led by Prendergast, a staff scientist in the Theory of Nanostructures Facility at Berkeley Lab’s Molecular Foundry.

The researchers found that the guanidinium ions form strong donor hydrogen bonds in the plane of the molecule, but only weak acceptor hydrogen bonds with the π electrons orthogonal to the plane. When fluctuations bring the solvated ions near each other, the van der Waals attraction between the π electron clouds squeezes out the weakly held water molecules, which move into the bulk solution and form much stronger hydrogen bonds with other water molecules. This release of the weakly interacting water molecules drives the contact pairing between the guanidinium cations.

 

 


 

Research conducted by: O. Shih, G.C. Dallinger, J.W. Smith, K.C. Duffey, and R.C. Cohen (Univ. of California, Berkeley); A.H. England and R.J. Saykally (Univ. of California, Berkeley, and Berkeley Lab); and D. Prendergast (Berkeley Lab).

Research funding: U.S. Department of Energy (DOE), Office of of Basic Energy Sciences (BES), and the National Science Foundation. Operation of the ALS is supported by DOE BES.

Publication about this research: O. Shih, A.H. England, G.C. Dallinger, J.W. Smith, K.C. Duffey, R.C. Cohen, D. Prendergast, and R.J. Saykally, "Cation-cation contact pairing in water: Guanidinium," Journal of Chemical Physics 139, 035104 (2013).

ALS Science Highlight #286

 

ALSNews Vol. 351