8th Liquid Matter Conference September 6-10, 2011 Wien, Austria ...

Fri 911:10-14:00P4.49Liquid-liquid phase separation in protein solutionscontrolled by multivalent salts and temperatureFelix Roosen-Runge, 1 Christodoulos Christodoulou, 2 Fajun Zhang, 2 Marcell Wolf, 2Roland Roth, 3 and Frank Schreiber 21 Universität Tübingen, Institut für Angewandte Physik, 72076, Tübingen, Germany2 Universität Tübingen, Tübingen, Germany3 Universität Erlangen-Nürnberg, Erlangen, GermanySolutions of most globular proteins are charge-stabilized. Charge screening, salting-in and saltingoutbehavior via addition of salts are available ways controlling the phase behavior of the solutions.However, multivalent salts have been found to induce even more complex effects in protein solutions.In particular, reentrant condensation is observed for several globular proteins in the presenceof multivalent salts [1, 2], i.e. the protein solution is stable at low and high salt concentration butphase-separates at intermediate salt concentration. As conclusive explanation, specific ion bindingto the protein residues cause an inversion of the protein global charge. Importantly, besides theexpected amorphous aggregation at low charge-stabilization, also a liquid-liquid phase separationoccurs for several proteins at intermediate salt concentration. The full phase diagram of the proteinsolution extends to three control parameter - temperature, protein and salt concentration. Thecoexistence region in the salt-protein plane is a closed convex area for each fixed temperature and,within the accessible temperature range, decreases monotonically upon cooling down until a lowercritical solution temperature. Considering the protein-ion complexes as effective particles withshort-range attractions whose strength varies with the protein charge, the full phase behavior canbe explained. For several protein-ion complexes, the binodal and spinodal lines of the liquid-liquidphase separation are measured using clouding point measurements and static light scattering. Interestingly,the observed lower critical solution point suggests a relevant role of hydration to thefree energy of the protein solution. [1] F. Zhang et al. , Phys. Rev. Lett. 101, 148101 (2008) [2] F.Zhang et al. , Proteins 78(16), 3450-3457 (2010)49