Protein crystallizations. The functional dependence of the nucleation rate on the protein concentration and the solubility

Crystallization of proteins is normally carried out in a four-component system of water, a buffer, protein and precipitant, either salt or alcohol. With four components, at fixed pH, temperature and pressure, it is possible to vary the protein concentration and its solubility independently, because the solubility is determined by the precipitant concentration. This degree of freedom is useful in crystallizing rabbit muscle aldolase and lysozyme because the nucleation rate is a function only of the total protein concentration and not of the protein solubility. Large crystals are obtained when both the protein concentration and the protein solubility are small. The small protein concentration suppresses the rate of nucleation, and the small solubility provides sufficient protein to grow large crystals on the few nuclei. Sometimes it is desirable to grow crystals from seeds, especially if a special crystal form is required. In this case also, homogeneous nucleation of an unfavorable form may be avoided for aldolase through the use of low protein concentration.