Spatio-Temporal Patterns in Ferritin Crystal Growth
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Spatio-Temporal Patterns in Ferritin Crystal Growth Olga Gliko and Peter G. Vekilov Department of Chemical Engineering, University of Houston, Houston, TX 77204-4004 ABSTRACT We investigate the unsteady kinetics and the formation of spatio-temporal patterns during the ferritin crystal growth, which is controlled by the rate of supply of material. For this, we apply a novel phase-shifting interferometry technique. We find that the growth rate and local slope fluctuate by up to 100% of their average values as a result of step bunching. The fluctuation amplitudes decrease with higher supersaturation and larger crystal size, as well as with increasing distance from the step sources. Since these are parameters that govern the protein supply field, we conclude that fluctuations are rooted in the coupling of the interfacial processes of growth to the bulk transport in the solution. Analysis of the step velocity dependence on local slope indicates a very weak interaction between the steps. Hence, in transport-controlled systems with non-interacting or weakly interacting steps the step bunches decay and step train tends towards its stable, equidistant state.
INTRODUCTION The loss of stability of equidistant step trains leads to bunches of steps spreading along the crystal face, interspersed with bands of lower step density. The step bunches leave trails of higher defects density in the crystal lattice, and in this way lower the perfection and the utility of the grown crystals [1-3]. The kinetics instabilities and step bunching during the crystallization of the protein lysozyme were studied using a high-resolution interferometry technique [4]. It was concluded that fluctuations are intrinsic and result from the coupled bulk transport and interfacial kinetics processes [3]. According to this mechanism, the strongest instabilities occur when the growth proceeds under equal weights of the transport and kinetics in the overall rate control. Hence, shifts towards purely kinetic, or, conversely, purely diffusive regimes should lead to higher stability. Developed numerical model of coupled bulk transport and nonlinear interfacial kinetics quantitatively reproduces the experimentally observed unsteadiness [5]. If the rationale developed based on lysozyme data holds, in crystallization systems with dominant transport control, such as ferritin, a shift of the working point toward slower transport should dampen the fluctuations. Thus, the aim of this work is to experimentally study the origin of kinetics unsteadiness in ferritin growth. We investigate the dependencies of the amplitude of local slope fluctuations on the parameters affecting transport to the interface: supersaturation, crystal size and location on the crystal faces. EXPERIMENTAL PROCEDURES The crystallizing solution contains between 2–2.5 mg/mL horse spleen ferritin purchased form Sigma and purified to reduce the level of the most common impurity, the covalent dimer of ferritin, to below 5% [6]. We use 2.0% (w/v) CdSO4 as a precipitant, and 0.2 M sodium acetate N7.18.1 D
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