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

The normal growth rates <i>R</i> and apparent step velocities (lateral growth rates of a spiral hillock) <i>V</i> 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 <i>V</i> under microgravity was confirmed compared to step velocities <i>V</i>_step on the ground, while a decrease in <i>R</i> was also confirmed compared to that in the purified solution under microgravity as expected. Because of exact measurement of growth rates, kinetic analyses of <i>R</i> were conducted as a function of supersaturation, σ (σ ≡ ln­(<i>C</i>/<i>C</i>_e), where <i>C</i> is the concentration; <i>C</i>_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, <i>R</i> 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 <i>V</i>_p and <i>V</i>_i, where <i>V</i>_p is the apparent step velocity in the purified solution, and <i>V</i>_i is that in the impure solution. The difference between <i>V</i>_p and <i>V</i>_i was smaller than the difference in step velocities on the ground, <i>V</i>_step,p and <i>V</i>_step,i, where <i>V</i>_step,p is the step velocity in the purified solution, and <i>V</i>_step,i is the step velocity in the impure solution