Paul J. Shlichta

Heterogeneous and epitaxial nucleation of protein crystals on mineral surfaces.

Fifty different mineral samples were tested as potential heterogeneous or epitaxial nucleants for four commonly crystallized proteins. It was found, by conventional protein crystallization techniques, that for each protein there was a set of mineral substrates that promoted nucleation of crystals at lower critical levels of supersaturation than required for spontaneous growth. Numerous examples, involving all four proteins, were observed of modification of crystal habit and, in some cases, unit cell properties promoted by the presence of the mineral nucleants. In at least one case, the growth of lysozyme on the mineral apophyllite, it was shown by lattice analysis and x-ray diffraction that the nucleation and growth of the protein crystal on the mineral was likely to involve a direct lattice match.

Convection phenomena during the growth of sodium chlorate crystals from solution

Abstract Schlieren observations of sodium chlorate crystals suspended in supersaturated solutions showed that, as supersaturation or crystal size increased, the boundary layer on the side faces remained stable while the top boundary layer and emerging plume changed from stable to partially oscillatory, with localized eddies or pulses rising periodically from portions on the top face. The transition was a function of the Grashof number Grtop (in which the characteristics length was defined as the area/perimeter of the top face) and the crystal height z, the critical Gr∗top being equal to 70 + (17 mm-1)z. Grtop correlated well with plume width, number of streamlines, eddy density, and plume velocity. The oscillation period, however, correlated best with the weight-increase growth rate. The mass transfer rate, as measured by the Sherwood number, correlated with Grtop according to the relation Sh=0.27 (Grtop) 1 4 . The linear growth rate varied as the inverse 1 4 power of the characteristic length and the 5 4 power of t he supersaturation. The vertical faces grew more rapidly than the horizontal faces, the ratio (1.1 to 1.6) having no apparent correlation with Grtop but increasing with decreasing supersaturation. A rough estimate of boundary layer flow versus growth rate for near-critical stable convection indicated that growth of the side faces depleted over half the excess solute in the boundary layer. External impulses initiated transient instabilities and/or oscillations in stable plumes, the threshold impulse energy being a function of Gr∗top minus Grtop. The duration of the transients increased with impulse energy until a maximum decay time was reached. Impulse energies greatly in excess of the threshold value caused transient separation of the boundary layer on the top face of in extreme cases over the entire crystal surface. Plume wavering, due to background-induced fluid motion, was observed only at the lowest supersaturations.

Facilitation of the growth of protein crystals by heterogeneous/epitaxial nucleation☆

Abstract A preliminary exploration has been made of the effect of inorganic crystals on the nucleation of protein crystals. Four representative proteins were crystallized by the vapor diffusion technique in the presence of fifty species of inorganic crystals. A substantial decrease in critical supersaturation was caused by thirty nucleants for canavalin and by seven to fifteen nucleants for the other proteins. Several cases of anomalous morphology were observed, as well as one instance of epitaxy, i.e. lysozyme on apophyllite. It has thus been shown that inorganic crystals can be used as heterogeneous nuclei or epitaxial substrates for facilitating reliable and controlled nucleation in automated protein crystal growth experiments.

The use of heterogeneous and epitaxial nucleants to promote the growth of protein crystals

Abstract Fifty different mineral samples were tested as potential heterogeneous or epitaxial nucleants for four commonly crystallized proteins. It was found, using conventional protein crystallization techniques, that for each protein there was a set of mineral substrates that promoted nucleation of crystals at lower critical levels of supersaturation then required for spontaneous growth. In at least one case, the growth of lysozyme on the mineral apophyllite, it was shown by lattice analysis and X-ray diffraction that the nucleation and growth of the protein crystal on the mineral was likely to be truly epitaxial.

Ground-based experiments on the minimization of convection during the growth of crystals from solution

Abstract Crystals were grown from solution in the Earths gravitational field with the substantial absence of convection by (a) downward unidirectional growth in an isothermally cooled solution and (b) upward unidirectional growth in a positive vertical thermal gradient. Growth rates were 1.3 to 1.8 times the predictions for convectionless growth and an order of magnitude less than for growth with vigorous convection. Crystals grown in the isothermal configuration were of comparable or superior quality to those grown with substantial convection. The imposition of a 0.5 to 0.8°C/mm gradient greatly stabilized the growth interface so that flat (001) surfaces could be grown on KDP. Crystals grown in a thermal gradient, however, were filled with far more fluid inclusions than crystals grown isothermally.

Feasibility of mapping solution properties during the growth of protein crystals

This paper summarizes the feasibility of using optical techniques for mapping the convection, temperature, and solute concentration in the solution around a growing protein crystal. Convection can be mapped by a variety of techniques which measure either refractive index differences, displacements, velocity, or solute optical absorption. For protein crystal growth, however, ordinary schlieren and interferometric techniques are marginally sensitive and most displacement marking techniques unsuitable; therefore, phase-contrast schlieren, ultraviolet solute absorption, and laser anemometry appear to be the most feasible. Mapping of temperature and concentration by the absorption-interferometric technique appears to be quite feasible for protein solutions because of their low dn/dC. Finally, the monitoring of protein crystal growth rates appears to be feasible by double-exposure holography or birefringence.

Growth of crystals by centrifugation

Abstract : When a solution is rotated in a centrifuge, the solute ions, being denser than the solvent, tend to concentrate toward the outer edge of the cell. If the solution is initially near saturation, then the outer portions of the redistributed solution become supersaturated and crystal growth can occur. Under these conditions, many of the difficulties usually encountered in growing single crystals, such as dirt particles, air bubbles, or spurious seed crystals formed by evaporation at the solution surface, tend to be minimized or eliminated. On the other hand, there is some danger of crystal deformation due to the high force fields involved. Observations of crystal growth by centrifugation can also be used to measure several crystal growth parameters. Measurements of the threshold centrifugal field for crystallization of unseeded solutions can provide improved data on the supersaturation required for homogeneous nucleation. Determinations of the equilibrium form of the crystal at high g values can provide hitherto unobtainable measurements of crystal surface energy. Preliminary experiments have demonstrated the essential feasibility of these techniques, but numerous experimental difficulties must be overcome before usable crystals can be produced or accurate measurements made. (Author)

Free convection about a rectangular prismatic crystal growing from a solution

Abstract Sodium chlorate crystals growing from unstirred aqueous solutions were observed by schlieren techniques. Growth rate data were best correlated by Sh = 0.48 Ra0.25 with finite interface kinetics and a characteristic length L equal to the crystal height plus the inverse reciprocal sum of the horizontal dimensions. The growth rate was slightly higher when the plume was laminar than when it was irregular. The horizontal growth rate was greater than the vertical growth rate, with the ratio tending to decrease with increasing plume instability. The threshold Grashof number for plume instability was greatly decreased with decreasing height for crystals with the same horizontal dimensions. The frequency of eddy emission was proportional to L−2Ra0.8. The velocity of the eddies approached a constant value with increasing distance above the crystal surface, the terminal value being roughly proportional to Ra 1 2 /L .

Crystal growth and materials processing above 1000 g

Abstract The principal phenomena observed to date during kilogravity crystallization experiments have been rapid sedimentation of precipitates and bubbles and differential molecular sedimentation to form compositional gradients. These experiments indicate that kilogravity fields can be used as a driving force for the rapid growth of inclusion-free crystals, even in systems where growth by conventional techniques is infeasible. Therefore the growth of crystals for high-power optics may become a major application of kilogravity processing. Other potential materials-processing applications include the elimination of microvoids from slip castings, the concentration of sol-gels, and the preparation of materials and precursors with compositional gradients. Although kilogravity experiments involving high temperatures or optical monitoring present serious problems, the necessary apparatus is available or can be developed.

Comparison of theory with experiment in convectionless growth of crystals from solution

Abstract Wilcoxs computer program for predicting the growth rates of crystals in convectionless supersaturated solution has been shown to give good agreement with the best available experimental data. Slight modifications in the original program were made to accommodate the change of diffusion coefficient with temperature.