First Direct Observation of Impurity Effects on the Growth Rate of Tetragonal Lysozyme Crystals under Microgravity as Measured by Interferometry

The normal growth rates R and apparent step velocities (lateral growth rates of a spiral hillock) V of tetragonal hen egg-white lysozyme (HEWL) crystals were for the first time measured by Michelson interferometry in the international space station (as part of the Nano Step project) using commercialized HEWL samples containing 1.5% impurities. A significant increase in V under microgravity was confirmed compared to step velocities V-step on the ground, while a decrease in R was also confirmed compared to that in the purified solution under microgravity as expected. Because of exact measurement of growth rates, kinetic analyses of R were conducted as a function of supersaturation, sigma (sigma ln(C/C-e), where C is the concentration; C-e is the solubility), using a spiral growth model and a two-dimensional (2D) -3.5 nucleation growth model. For both models over a wide range of sigma, R in the impure solution was significantly lower than that in the purified solution. The degree of the suppression of impurity effects was also evaluated using the difference in V-p and V-i, where V-p is the apparent step velocity in the purified solution, and V-1, is that in the impure solution. The difference between V-p and V-i was smaller than the difference in step velocities on the ground, V-step,V-p and V-step,V-p where Vs tem is the step velocity in the purified solution, and V-step,V-1 is the step velocity in the impure solution.