MODELING THE GROWTH-RATES OF TETRAGONAL LYSOZYME CRYSTALS

Although the faceted growth of tetragonal lysozyme crystals is known t o occur by 2D nucleation and dislocation-led growth, the measured grow th rates do not follow model predictions based on these mechanisms. On e possible reason for this deviation is that these models ignore the h ighly aggregated state of lysozyme in supersaturated solutions. In thi s study a growth mechanism for tetragonal lysozyme crystals involving aggregation reactions leading to the formation of the growth unit, mas s transport of the growth unit to the crystal interface and faceted cr ystal growth by growth unit addition, is proposed. The distribution of aggregates in lysozyme nutrient solutions were determined from the eq uilibrium aggregation reactions and comparisons were made with growth rates calculated from the model based on the proposed mechanism and th e measured growth rate data. The results indicated than an octamer cor responding to the tetragonal crystal unit cell was the most likely gro wth unit for the process. Remarkably good fits were obtained with this model to the measured growth rate data for three sets of pH and salt concentrations, suggesting the validity of the proposed mechanism. The values of the kinetic coefficient for the step velocity was in the ra nge for small molecule crystal growth and the heats of reaction compar ed well with that obtained from lysozyme solubility data. The results presented here suggest that the inorganic and protein crystal growth p rocesses are quite similar in many ways. Lysozyme crystal growth diffe rs primarily due to growth by an aggregate growth unit and in the effe ct of nutrient solution conditions on the protein aggregation process.