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 NanoStep 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, σ (σ ≡ ln( C / C e ), where C is the concentration; C e is the solubility), using a spiral growth model and a two-dimensional (2D) nucleation growth model. For both models over a wide range of σ, 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 i 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,p and V step,i, where V step,p is the step velocity in the purified solution, and V step,i is the step velocity in the impure solution.