EFFECTS OF CONVECTIVE SOLUTE AND IMPURITY TRANSPORT IN PROTEIN CRYSTAL-GROWTH

High-resolution optical interferometry was used to investigate the eff ects of forced solution convection on the crystal growth kinetics of t he model protein lysozyme. Most experiments were conducted with 99.99% pure protein solutions. To study impurity effects, similar to 1% of l ysozyme dimer (covalently bound) was added in some cases. We show that the unsteady kinetics, corresponding to bunching of growth steps, can be characterized by the Fourier components of time traces of the grow th rate. Specific Fourier spectra are uniquely determined by the solut ion conditions (composition, temperature, and flow rate) and the growt h layer source activity. We found that the average step velocity and g rowth rate increase by similar to 10% with increasing flow rate, as a result of the enhanced solute supply to the interface. More importantl y, faster convective transport results in lower fluctuation amplitudes . This observation supports our rationale for system-dependent effects of transport on the structural perfection of protein crystals. We als o found that solution flow rates >500 mu m/s result in stronger fluctu ations while the average growth rate is decreased. This can lead to gr owth cessation at low supersaturations. With the intentionally contami nated solutions, these undesirable phenomena occurred at about half th e flow rates required in pure solutions. Thus, we conclude that they a re due to enhanced convective supply of impurities that are incorporat ed into the crystal during growth. Furthermore, we found that the impu rity effects are reduced at higher crystal growth rates. Since the exp osure time of terraces is inversely proportional to the growth rate, t his observation suggests that the increased kinetics instability resul ts from impurity adsorption on the interface. Finally, we provide evid ence relating earlier observations of ''slow protein crystal growth ki netics'' to step bunch formation in response to nonsteady step generat ion.