Nanometer-scale ion aggregates in aqueous electrolyte solutions: guanidinium sulfate and guanidinium thiocyanate.

Neutron diffraction experiments and molecular dynamics simulations are used to study the structure of aqueous solutions of two electrolytes: guanidinium sulfate (a mild protein conformation stabilizer) and guanidinium thiocyanate (a powerful denaturant). The MD simulations find the unexpected result that in the Gdm2SO4 solution the ions aggregated into mesoscopic (nanometer-scale) clusters, while no such aggregation is found in the GdmSCN solution. The neutron diffraction studies, the most direct experimental probe of solution structure, provide corroborating evidence that the predicted very strong ion pairing does occur in solutions of 1.5 m Gdm2SO4 but not in 3 m solutions of GdmSCN. A mechanism is proposed as to how this mesoscopic solution structure affects solution denaturant properties and suggests an explanation for the Hofmeister ordering of these solutions in terms of this ion pairing and the ability of sulfate to reverse the denaturant power of guanidinium.