Preferential interaction coefficients of proteins in aqueous arginine solutions and their molecular origins

Preferential interaction coefficients provide a thermodynamic measure to quantify the interactions between cosolutes and a protein. Preferential interactions of cosolutes can be measured experimentally using dialysis/densimetry and vapor pressure osmometry (VPO) techniques. The cosolute arginine is a widely used aggregation suppressor with a seemingly unique behavior. Its role in protein aggregation has been studied extensively, although a complete mechanistic understanding of its behavior is lacking. Moreover, due to experimental limitations, experimental preferential interaction data for arginine has only been reported at low concentrations. Schneider and Trout ( J. Phys. Chem. B 2009 , 113 , 7 ) have reported experimental preferential interaction data for argHCl (up to 0.7 m), and their study raised several interesting questions about the preferential interaction of arginine with proteins. Arginine is attracted to proteins at low concentrations but it was highly excluded at high concentrations. Furthermore, the preferential interaction coefficient values were found to vary as a square of the concentration, which is different from commonly observed linear relationship for other cosolutes like urea, glycerol, guanidinium hydrochloride, etc. In this study, preferential interaction coefficients of argHCl have been estimated computationally for two proteins (lysozyme and α-chymotripsinogen A) for a large concentration range (up to 2.8 m). On the basis of these results, the molecular level interactions responsible for the nonlinear exclusion of arginine from the protein surface are identified.