The second osmotic virial coefficients of seven proteins-ovalbumin, ribonuclease A, bovine serum albumin, a-lactalbumin, myoglobin, cytochrome c, and catalase-were measured in salt solutions.Comparison of the interaction trends in terms of the dimensionless second virial coefficient b 2 shows that, at low salt concentrations, protein-protein interactions can be either attractive or repulsive, possibly due to the anisotropy of the protein charge distribution. At high salt concentrations, the behavior depends on the salt: In sodium chloride, protein interactions generally show little salt dependence up to very high salt concentrations, whereas in ammonium sulfate, proteins show a sharp drop in b 2 with increasing salt concentration beyond a particular threshold. The experimental phase behavior of the proteins corroborates these observations in that precipitation always follows the drop in b 2 . When the proteins crystallize, they do so at slightly lower salt concentrations than seen for precipitation. The b 2 measurements were extended to other salts for ovalbumin and catalase. The trends follow the Hofmeister series, and the effect of the salt can be interpreted as a water-mediated effect between the protein and salt molecules. The b 2 trends quantify protein-protein interactions and provide some understanding of the corresponding phase behavior. The results explain both why ammonium sulfate is among the best crystallization agents, as well as some of the difficulties that can be encountered in protein crystallization. Keywords: protein interactions; protein crystallization; osmotic second virial coefficient; self-interaction chromatographyThe relation between the molecular structure of a protein and the interaction potential between two such molecules is the foundation of a thermodynamic approach to understanding and predicting the behavior of protein solutions.Ultimately, such a relation enables calculation of the phase behavior and thus understanding of the formation of different phases, such as crystals, dense liquid phases, gels, and aggregates. In practice, the interactions between proteins in solutions are modulated by additives such as salts, hydrophilic polymers, or small organic molecules that may be present in formulations or added to induce crystallization. The focus of the present work is limited to exploring the role of different salts in tuning protein interactions.The interactions between proteins include contributions from at least electrostatic and van der Waals forces, hydration effects, hydrogen bonding, salt bridging, and Reprint requests to: Abraham M.

This work is an extension of our previous studies carried out to investigate the possible catalytic role of minerals in the abiotic synthesis of biologically important molecules. In the presence of montmorillonite, a member of the phyllosilicate group minerals that are abundant on Earth and identified on Mars, activated RNA monomers, namely 5k-phosphorimidazolides of nucleosides (ImpNs), undergo condensation reactions in aqueous electrolyte solution producing oligomers with similar structures to short RNA fragments. Analysis of the linear trimer isomers formed in the reaction of a mixture of activated adenosine and cytidine monomers (ImpA and ImpC, respectively) employing high-performance liquid chromatography, selective enzymatic hydrolysis and matrix-assisted laser desorption/ionization mass spectroscopy molecular weight measurements demonstrate that montmorillonite catalysis facilitates the formation of hetero-isomers containing 56 % A-and 44 % C-monomer incorporated in their structure. The results also show that 56 % of the monomer units are linked together by RNA-like 3k,5k-phosphodiester bonds. These results follow the same trend observed in our most recent work studying the reaction of activated adenosine and uridine monomers, and support Bernal's hypothesis proposing the possible catalytic role of minerals in the abiotic processes in the course of chemical evolution.