Crystallization technique of high-quality protein crystals controlling surface free energy

The relationship between protein crystal quality and growth kinetics was assessed by measuring the normal growth rates vs supersaturation of the (110) and (101) faces of dislocation-free tetragonal hen egg white lysozyme crystals at three precipitant concentrations, with NaCl as the precipitant. Assuming a two-dimensional birth and spreading nucleation mechanism, an increase in the surface free energy of the step edges was realized with increasing NaCl concentration, as established by decreases in the full width at halfmaximum values of X-ray diffraction rocking curves obtained from crystals. These results demonstrate that controlling the surface free energy of the step edges is an important aspect of obtaining high-quality protein crystals. This work also proposes a mechanism for the observed improvements in the crystal quality, based on the reduced incorporation of impurities into the steps during crystal growth.