So many protein crystals have been grown under microgravity condition to improve the quality of the crystals. However no one has clear answers why the quality is improved or at least is believed to be improved. To answer this question, we have for the first time measured growth rate vs supersaturation and the slope of spiral hillocks of lysozyme crystals from 3 different solutions with respect to concentration by Michelson Interferometry under microgravity condition in the international space station from August to December 2012.

Growth rates from two different solutions with different purity, 99.99% and 98.5% under microgravity were found to be the same, notwithstanding that growth rate under normal gravity differs a lot at relatively low supersaturation, <80%. It was also surprising to notice the disappearance of “dead zone” in all solutions near the equilibrium condition, where the growth rate becomes zero because of the pinning effect of impurities. When the growth rate under microgravity was compared with the rate in gravity, the rate was found to be similar or even larger than that in gravity. This is the same tendency as obtained from our ex-situ measurements of growth rate in the Foton-M3 recovery satellite. 

The shape of growth hillocks is known as largely influenced by the purity of solution. When the solution is purer, the growth hillocks are more elongated in shape. This is the general tendency in laboratories and thus this morphological change would be a good criterion for the impurity effects on growth. The interferometry clearly demonstrated that the shape of spiral hillocks of crystals growing in impure solution were more elongated as if the crystals grew in a very pure solution.

These two findings, the growth rate measurements and morphology change of hillocks suggest that lack of convection allows self-purification of the growing crystal via absorbing impurities from the surrounding solutions.  That may be solution of the long-standing problem on why protein crystals sometimes grow more perfect under microgravity.

The growth mechanism was discussed based on the growth rate vs supersaturation relationship. The interfacial tension for 2D homogeneous nucleation growth under microgravity was calculated to be ~0.5 mJ/m² and thus the same value as obtained in gravity. However 2D heterogeneous nucleation growth, which is dominant in gravity, did not operate under microgravity. This finding also supports the above-mentioned conclusion from a kinetic point of view. 

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