New clues to how proteins dissolve and crystallize

Fresh evidence for the 'Law of Matching Water Affinities'

X-ray spectroscopy shows that a protein acetate group (molecule at center) prefers binding with sodium (blue curve) over potassium (red curve); the green sphere represents a cation, with surrounding water... Click here for more information.

BERKELEY, CA -- In the late 19th century the Czech scientist Franz Hofmeister observed that some salts (ionic compounds) aided the solution of proteins in egg white, some caused the proteins to destabilize and precipitate, and others ranged in activity between these poles.

Hofmeister proceeded to rank "salt-out" (destabilizing) ions versus "salt-in" ions according to the magnitude of their effects. The resulting "Hofmeister series" governs the strengths of ions in inducing protein unfolding, bubble coalescence, and many other phenomena, and remains vital to protein chemistry and other biological and chemical studies to this day. But its mechanism has never been properly understood.

A team led by Richard Saykally of the Department of Energy's Lawrence Berkeley National Laboratory has now used Berkeley Lab's Advanced Light Source to study how biologically important, positively charged ions (cations) interact with negatively charged groups found in proteins (anions) to form salts. The team's results, which appear in Proceedings of the National Academy of Sciences, lend strong experimental support to a critical part of a proposed new explanation for Hofmeister effects, known as the Law of Matching Water Affinities.

The Law of Matching Water Affinities

The selective binding of cations to carboxylates was made possible by the Saykally group's liquid microjet experiment on beamline 8.0.1 of DOE's Advanced Light Source. Click here for more information.

"The Law of Matching Water Affinities, recently proposed by Kim Collins, says that the least soluble ion pairs are formed by ions that are closest to each other in their hydration energy -- that is, how strongly they hold onto water," says Saykally, who is a faculty scientist in Berkeley Lab's Chemical Sciences Division and a professor of chemistry at the University of California at Berkeley. "This is a classic example of an ion-specific effect: Hofmeister effects depend on the identity of ions rather than just on their concentration."