We have continued to investigate the nucleation and crystallization process of white hen egg lysozyme. Light scattering experiments by Saenger and co-workers show a maximum in the growth of prenuclear aggregates in the neighborhood of 0.8 molar salt. The salt concentration dependence for the growth of prenuclear aggregate shows a strong similarity with that of enzyme-substrate kinetics and their salt concentration dependence. Based on extensive work on enzyme-substrate reaction rates, we postulated that the nucleation process is hindered at low salt concentrations due to monopolar electrostatic repulsions and is aided at intermediate salt concentrations by the steering effect of dipolar forces which are shorter ranged. We are investigating the initial nucleation process by numerical solution of the Langevin equation. Our preliminary results indicate that at 1.4 Mm protein and 0.68 M NaCl, the characteristic time for protein-protein association is approximately 1.4 x10 ^-7 seconds. The association time is controlled by monomer concentration and the translational diffusion constant, which varies little with salt concentration. Thus it may be taken as a constant at fixed monomer concentration. On the other hand, the time for reorientation strongly depends on electrostatic steering forces. As the latter forces are screened, reorientation time becomes larger than the time for association and there is not sufficient time for monomers to reorient themselves prior to association. This leads to defective associations and results in amorphous precipitates. The Computer Graphics Laboratory is used for graphic display of protein association and reorientation processes.
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