New Insight into an Old Puzzle – the Hofmeister Universality Probed by AFM

Hofmeister and his PhD student, Lewith, discovered 126 years ago that different ions destabilize proteins to a markedly different extent. They ranked ions according to their precipitation power in a series known today as the Hofmeister series. Since then, scientists discovered dozens of additional ion-specific phenomena including surface tension, ion transport through biological and inanimate membranes and channels, colloidal stability, enzyme activity, bacterial growth, and more. Remarkably, with only few exceptions, the same Hofmeister series was discovered to characterize the effect of ions on this myriad of ostensibly different phenomena, strongly suggesting the existence of an underlying common microscopic mechanism. The universality reflected in the Hofmeister series has turned this problem into one of the fundamental puzzles in biophysics and the physics of soft matter. The search for an underlying mechanism has motivated extensive research and important discoveries but the Hofmeister universality proved more challenging than naively anticipated.

In the past few years, our lab has been employing Atomic Force Spectroscopy to measure the effect of different ions on the short range force acting between two surfaces in solution. The full force vs. distance curves obtained this way gave significant new insight into the Hofmeister puzzle and suggested, in combination with recent optical measurements, a surprisingly simple picture of the underlying physics.