The effects of pH and electrolyte concentration on protein-protein interactions in lysozyme and chymotrypsinogen solutions were investigated by static light scattering (SLS) and small-angle neutron scattering (SANS). Very good agreement between the values of the virial coefficients measured by SLS and SANS was obtained without use of adjustable parameters. At low electrolyte concentration, the virial coefficients depend strongly on pH and change from positive to negative as the pH increases. All coefficients at high salt concentration are slightly negative and depend weakly on pH. For lysozyme, the coefficients always decrease with increasing electrolyte concentration. However, for chymotrypsinogen there is a cross-over point around pH 5.2, above which the virial coefficients decrease with increasing ionic strength, indicating the presence of attractive electrostatic interactions. The data are in agreement with Derjaguin-Landau-Verwey-Overbeek (DLVO)type modeling, accounting for the repulsive and attractive electrostatic, van der Waals, and excluded volume interactions of equivalent colloid spheres. This model, however, is unable to resolve the complex short- ranged orientational interactions. The results of protein precipitation and crystallization experiments are in qualitative correlation with the patterns of the virial coefficients and demonstrate that interaction mapping could help outline new crystallization regions.
Bibliographical noteFunding Information:
This study was supported by grants from the National Science Foundation (BES-9510420) and NASA (NAG8––1346).
We acknowledge the support of National Institute of Standards and Technology of the U.S. Department of Commerce and the National Science Foundation in providing the neutron research facilities and support for the neutron scattering experiments. The assistance of D. Iampietro and L. Ryan in the neutron scattering studies is gratefully acknowledged. We are thankful to P. Pjura for useful discussions and for the introduction to the centrifugal crystallization, to S. Leonard and A. Gittis for x-ray diffraction characterization of the lysozyme crystals, and to D. Asthagiri for kindly providing the program for the protein charge and dipole moment calculations.