Different Tools to Study Interaction Potentials in γ-Crystallin Solutions: Relevance to Crystal Growth

Osmotic pressure, small-angle X-ray scattering and quasi-elastic light scattering were used to study the medium-range interaction potentials between macromolecules in solution. These potentials determine macromolecular crystallization. Calf eye lens γ-crystallins were used as a model system with the charge, and therefore the interactions, varied with pH. The second virial coefficient was determined under the same conditions with each of the three techniques. Osmotic pressure and quasi-elastic light scattering can be used conveniently in the laboratory to rapidly test the type of interactions (either attractive or repulsive) present in the solution. The measurement is direct with osmotic pressure, whereas with quasi-elastic light scattering, the directly measured coefficient is a combination of thermodynamic and hydrodynamic terms. X-rays, which require more sophisticated equipment such as synchrotron radiation facilities, can provide more detailed information on the interparticle potentials when models are used. At low ionic strength, two potentials were found necessary to account for the temperature and pH phase diagram as a function of protein concentration. The first potential is the van der Waals attractive potential that was previously shown to account for the fluid-fluid phase separation at low temperature. The second potential is an electrostatic coulombic repulsive potential which is a function of the protein charge and thus of the pH. The interaction trail could be followed at protein concentrations as low as 10 mg ml⁻¹. The results as a whole are expected to be valid for all compact low molecular weight proteins at low ionic strength.