STUDIES IN THE PHYSICAL CHEMISTRY OF AMINO ACIDS, PEPTIDES, AND RELATED SUBSTANCES : XI. THE SOLUBILITY OF CYSTINE IN THE PRESENCE OF IONS AND ANOTHER DIPOLAR ION.

1. As an introduction to the relations that obtain in biochemical systems containing several components, some ionic, some dipolar ionic, the solubility of cystine has been investigated in the presence of glycine and neutral salts. 2. Both glycine and sodium chloride increase cystine solubility at all concentrations. The interaction between cystine and ions is, however, diminished with increase in glycine concentration, and the interaction between cystine and glycine with increase in ionic strength. 3. Sodium sulfate also increases the solubility of cystine, but at concentrations greater than one molal its solvent action is smaller than its salting-out effect, which is greater at all concentrations than that of sodium chloride, and greater the higher the glycine concentration. 4. These interactions are defined by an equation giving the solubility ratio of cystine in terms of salting-out constants, constants related to the electric moments of cystine, and to the ionic strength and dielectric constant of the solution. 5. The higher the concentration of glycine and therefore the dielectric constant of the solution, the smaller that part of the interaction between ions and dipolar ions which depends upon Coulomb forces and the greater appears the salting-out effect. 6. Conversely, the greater the ionic strength and the salting-out effect the smaller the interaction between dipolar ions in solution.