|ALS Capabilities Reveal How Like Can Attract Like|
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.
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.
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.
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