W0259

Modeling Tetragonal Lysozyme Crystal Growth Rates. Sridhar Gorti,^* Elizabeth L. Forsythe^† and Marc L. Pusey^*, ^* Biophysics SD46, NASA/MSFC, Huntsville, AL 35812, ^†Universities Space Research Association, 4950 Corporate Dr., Suite 100, Huntsville, AL 35806, USA.

Tetragonal lysozyme 110 face crystal growth rates, measured over 5 orders of magnitude in range, can be described using a model where growth occurs by 2D nucleation on the crystal surface for solution supersaturations of c/c_eq ≤ 7 ± 2. Based upon the model, the step energy per unit length, β_ was estimated to be ~5.3 ± 0.4 x 10^-7 erg/mol-cm, which for a step height of 56 Å corresponds to barrier of ~7 ± 1 k_BT at 300 K. For supersaturations of c/c_eq > 8, the model emphasizing crystal growth by 2D nucleation not only could not predict, but also consistently overestimated, the highest observable crystal growth rates. Kinetic roughening is hypothesized to occur at a cross-over supersaturation of c/c_eq > 8, where crystal growth is postulated to occur by a different process such as adsorption. Under this assumption, all growth rate data indicated that a kinetic roughening transition and subsequent crystal growth by adsorption for all solution conditions, varying in buffer pH, temperature and precipitant concentration, occurs for c/c_eq(T, pH, NaCl) in the range between 5 and 10, with an energy barrier for adsorption estimated to be ~20 k_BT at 300 K. Based upon these and other estimates, we determined the size of the critical surface nucleate, at the crossover supersaturation and higher concentrations, to range from 4 to 10 molecules.