A bulk water-dependent desolvation energy model for analyzing the effects of secondary solutes on biological equilibria

Dual Effect of Secondary Solutes on Binding Equilibria: Contributions from Solute-Reactant Interactions and Solute-Water Interactions

This study examines the role of water in binding equilibria with a special focus on secondary solutes (cosolutes) that influence the equilibrium but are not constituents of the final product. Using a thermodynamic framework that includes an explicit term for the release of water molecules upon binding, this investigation reveals how solutes may alter equilibria by changing the activity of the reactants, reflected in ΔG°(obs), and by changing the chemical potential of the solvent, reflected in ΔGS. The framework is applied to four experimental binding systems that differ in the degree of electrostatic contributions. The model systems include the chelation of Ca2+ by EDTA and three host-guest reactions; the pairings of p-sulfonatocalix[4]arene with tetramethylammonium ion, cucurbit[7]uril with N-acetyl-phenylalanine-amide, and β-cyclodextrin with adamantane carboxylate are tested. Each reaction pair is examined by isothermal titration calorimetry at 25 °C in the presence of a common osmolyte, sucrose, and a common chaotrope, urea. Molar solutions of trehalose and phosphate were also tested with selected models. In general, cosolutes that enhance binding tend to reduce the solvation free energy penalty and cosolutes that weaken binding tend to increase the solvation free energy penalty. Notably, the nonpolar-nonpolar interaction between adamantane carboxylate and β-cyclodextrin is characterized by a ΔGS value near zero. The results with β-cyclodextrin, in particular, prompt further discussions of the hydrophobic effect and the biocompatible properties of trehalose. Other investigators are encouraged to test and refine the approach taken here to further our understanding of solvent effects on molecular recognition.

The water association band as a marker of hydrogen bonds in trehalose amorphous matrices

The water association band is a suitable marker of residual water behavior in bioprotective trehalose matrices.

Experimental support for a desolvation energy term in governing equations for binding equilibria

This study introduces a new thermodynamic framework for aqueous reaction equilibria that treats water as a coreactant in the development of a general binding equation. The approach features an explicit consideration for the change in hydration that occurs when two solvated surfaces come into contact. As an outcome of this framework, the standard-state free energy of binding is defined by the summation of two terms: the traditional term (-RT ln Ki) plus a desolvation free-energy term that is weighted by the number of complexes formed at equilibrium. The new formalism suggests that the equilibrium ratio, Ki, is not a constant and that the observed concentration dependence of Ki may be used to obtain the molar desolvation energy and the standard-state free energy at infinite dilution. The governing equation is supported by results from isothermal titration calorimetry using the chelation of calcium(II) by EDTA as a model binding reaction. This work may have far-reaching implications for solution thermodynamics, including an explanation for the oft-noted discrepancy between the enthalpy values obtained by calorimetry and those from the van't Hoff approach.

A desolvation model for trifluoroethanol-induced aggregation of enhanced green fluorescent protein

Studies of amyloid disease-associated proteins in aqueous solutions containing 2,2,2-trifluoroethanol (TFE) have shown that the formation of structural intermediates is often correlated with enhanced protein aggregation. Here, enhanced green fluorescent protein (EGFP) is used as a model protein system to investigate the causal relationship between TFE-induced structural transitions and aggregation. Using circular dichroism spectroscopy, light scattering measurements, and transmission electron microscopy imaging, we demonstrate that population of a partially α-helical, monomeric intermediate is roughly correlated with the growth of β-sheet-rich, flexible fibrils for acid-denatured EGFP. By fitting our circular dichroism data to a model in which TFE-water mixtures are assumed to be ideal solutions, we show that increasing entropic costs of protein solvation in TFE-water mixtures may both cause the population of the intermediate state and increase aggregate production. Tertiary structure and electrostatic repulsion also impede aggregation. We conclude that initiation of EGFP aggregation in TFE likely involves overcoming of multiple protective factors, rather than stabilization of aggregation-prone structural elements.

Changes in apparent molar water volume and DKP solubility yield insights on the Hofmeister effect

This study examines the properties of a 4 × 2 matrix of aqueous cations and anions at concentrations up to 8.0 M. The apparent molar water volume, as calculated by subtracting the mass and volume of the ions from the corresponding solution density, was found to exceed the molar volume of ice in many concentrated electrolyte solutions, underscoring the nonideal behavior of these systems. The solvent properties of water were also analyzed by measuring the solubility of diketopiperazine (DKP) in 2.000 M salt solutions prepared from the same ion combinations. Solution rankings for DKP solubility were found to parallel the Hofmeister series for both cations and anions, whereas molar water volume concurred with the cation series only. The results are discussed within the framework of a desolvation energy model that attributes solute-specific changes in equilibria to solute-dependent changes in the free energy of bulk water.