Dependence of nucleation kinetics and crystal morphology of a model protein system on ionic strength

Nucleation rate data for hen egg-white lysozyme crystallization were obtain ed using a particle counter. Tetragonal lysozyme crystals were expected to form at the temperature and solution conditions of these experiments: 4 deg reesC, pH 4.5 with 0.1 M sodium acetate buffer and 2-6% NaCl (w/v). The rat es varied as expected, as smooth monotonic functions of supersaturation at 2%, 3% and 6% NaCl. However, at 5% NaCl, a great deal of scatter in the dat a was observed. At 2% and 3% NaCl, all the batches contained crystals with tetragonal morphology. At 6% NaCl, almost all of the vials contained the wh ite powder with few or no tetragonal crystals. At 5% NaCl concentration, a mixture of tetragonal crystals and powder formed in varying proportions in all the vials as observed by visual inspection. The powdery material was ex amined using optical microscopy and was seen to consist of needles with reg ular structure and sharp, faceted edges. Powder diffraction data from these needles was inconsistent with experimental powder diffraction data from te tragonal lysozyme crystals. It is possible that at high salt and protein co ncentrations liquid-liquid separation occurred and yielded a crystal polymo rph. (C) 2001 Elsevier Science B.V. All rights reserved.