Volume effects produced by protein-ion interactions. The binding of bovine plasma albumin with thiocyanate and trichloroacetate ions.

The volume changes produced by the reaction of bovine plasma albumin with thiocyanate and trichloroacetate ions differed substantially from each other and from those produced by sodium dodecyl sulfate (KATZ, S., SHAW, M. E., CHILLAG, S., AND MILLER, J. E. (1972) /. Biol. Chem., 247, 5528-5233). The volume effects attributable to proteinanion interaction, W, resulting from the interaction of thiocyanate anion (SCN-) with 2% albumin, in water, exhibited a minimum of -30 ml per lo5 g of protein at 0.05 M SCN-. At higher anion concentrations there was a linear increase of this parameter reaching a value of 110 ml per lo5 g of protein at 0.5 M SCN-. The corresponding reactions employing trichloroacetate ions generated a positive biphasic 6V isotherm with the value for 6V of 275 ml per lo5 g of protein at 0.5 M anion concentration. Medium effects were investigated by comparing these reactions in water, 0.1 M acetate, pH 5.0, and 0.1 M EDTA, pH 5.0. The AV for dilution of these anions by water and acetate was similar; however, the use of 0.1 M EDTA as solvent caused a diminution of this type of volume effect. On the other hand, the 6V isotherms for albumin-ligand interaction in these buffers differed; e.g. the 6V isotherms for albumin-SCNreaction in water and in EDTA were approximately of the same magnitude, whereas the 6V values in acetate were more negative. The isotherms produced by the reaction of trichloroacetate ion with albumin in water and 0.1 M acetate were virtually superimposable, but in 0.1 M EDTA the 6V values were more positive. The distinctive volume effects produced by the respective systems establish that the association mechanisms are determined primarily by the nature of the protein and anion involved. Evidence supporting this hypothesis is provided by the relatively small effect of buffer ions on these volume parameters. Apparently several types of electrostriction