Cation-Induced Hydration Effects Cause Lower Critical Solution Temperature Behavior in Protein Solutions

Abstract

The phase behavior of protein solutions is important for numerous phenomena in biology and soft matter. We report a lower critical solution temperature (LCST) phase behavior of aqueous solutions of a globular protein induced by multivalent metal ions around physiological temperatures. The LCST behavior manifests itself via a liquid-liquid phase separation of the protein-salt solution upon heating. Isothermal titration calorimetry and zeta-potential measurements indicate that here cation-protein binding is an endothermic, entropy-driven process. We offer a mechanistic explanation of the LCST. First, cations bind to protein surface groups driven by entropy changes of hydration water. Second, the bound cations bridge to other protein molecules, inducing an entropy-driven attraction causing the LCST. Our findings have general implications for condensation, LCST, and hydration behavior of (bio)polymer solutions as well as the understanding of biological effects of (heavy) metal ions and their hydration.

Anomalous Protein–Protein Interactions in Multivalent Salt Solution

“…This work, at a microscopic level, echoes experimental results showing the existence of reentrant condensation in HSA solutions in the presence of trivalent ions [18][19][20] and is also in agreement with recent calorimetry experiments. 45 Different models were used: an atomic model, a coarse-grained model, and a colloidal The colloidal model was used to study the interactions of two spherical colloids representing HSA-like molecules. This approach enabled us to include all ions explicitly in a two-body system, which is more difficult and computationally costly to realize with coarsegrained models at high ion concentrations.…”

“…In a recent work 45 calorimetry measurements showed that the binding of trivalent ions to the protein surface, and the resulting protein-protein attraction are entropically driven.…”

Kinetics of liquid–liquid phase separation in protein solutions exhibiting LCST phase behavior studied by time-resolved USAXS and VSANS

“…The LLPS binodal has been determined for BSA with YCl 3 , which leads to a lower critical solution temperature (LCST) phase behavior. 43 Here we use USAXS and VSANS to study the kinetics of LCST-LLPS in the dense phases of the BSA-YCl 3 system. First we aim to establish a new method for studying the kinetics of the phase transition based on the advantages of USAXS and VSANS.…”

Multivalent ions and biomolecules: Attempting a comprehensive perspective

“…[19] Importantly, the driving force for this binding is not enthalpy alone, but hydration entropy. In particular, a physicochemical characterisation of the binding reveals a lower critical solution temperature, [187] and the water coordination around Y 3 + is reduced upon binding to the protein, [188] both of which evidence the release of hydration water molecules with substantial related entropy gains. Finally, we refer the interested reader to a recent overview on the roles of lanthanides in biochemistry by Daumann.…”

“…[265] Multivalent cations such as Y 3 + are able to form cation bridges between negatively charged areas on protein molecules, [19] thus introducing an effective short-ranged attraction between the proteins. Interestingly, under appropriate experimental conditions, LLPS occurs [257,266] with a lower critical solution temperature (LCST-LLPS), i. e., representing an entropy-driven transition, [187] most likely related to the release of hydration water around the multivalent cation. Given that entropic considerations usually favour phase separation for globular, folded proteins upon a temperature decrease, [295] this behaviour is rather unusual and most likely linked to release of hydration water.…”