Salting in peptides: Conformationally dependent solubilities and phase behavior of a tripeptide zwitterion in electrolyte solution

The dielectrically consistent interaction site model theory is applied to a model system consisting of a zwitterion tripeptide of sequence Gly-Ala-Gly at infinite dilution in a solvent mixture of water and sodium chloride with only the central ∅,ψ dihedrals for conformational degrees of freedom. The peptide is found to be salted into solution by the cosolvent, with its solubility depending strongly on conformation of the central ∅,ψ pair. The distribution of cosolvent relative to the bulk solvent mixture is examined and relatively little specific association is found. Instead, the ionic concentration around the peptide is increased, especially near the phase boundary, and the increased concentration extends up to eight solvent diameters into the bulk. The consequences of such an ionic distribution on thermodynamic measures of association are discussed. Similarities between this model system and the unusual solubility behavior of β-lactoglobulin found in experiments are shown. The molecular distributions calculated are found to be consistent with a separation of the solvent mixture into two non-miscible phases, one of which contains the solute and has a higher cosolvent concentration than the other. Since turbid solutions have been observed in the β-lactoglobulin system, it is suggested that a separation into two liquid phases could be common to both systems.