Higher growth rate of protein crystals in space than on the Earth

Comparative growth rate measurement of 51 lysozyme seeded crystals at various supersaturations under microgravity and in normal gravity were successfully conducted by ex-situ method in a Russian Foton-M3 recovery satellite. Growth rate of crystals both in space and on the Earth were calculated by measuring the thickness of each growth zone that was caused by the change of gravity level. It is surprising to find that growth rate under microgravity was the same as that in normal gravity or, unexpectedly, even larger by more than 20 %. The experiment was conducted in the satellite in 12 days under microgravity after 6 days transportation on the ground from Nijmegen, the Netherlands, to Baikonur, Russia. 51 growth cells were kept at 20celcius after the protein solutions and the seeds were sealed. The trace of movement of 2D macro-steps on the (110) face was visualized after the flight by laser confocal microscopy. These steps were buried inside the crystals, 0-140 mu m deep from the surface and so they were grown in space. The observed elongated sharp 2D steps implies that the effect of impurity was considerably reduced in space condition, compared with the rounded shape of 2D steps formed with more impurities in normal gravity. The increase in growth rate in space could be concluded due to the reduction of impurities on the surface of crystals. Several space grown crystals were examined by synchrotron X-ray topography at KEK in Tsukuba after optical observations were completed. The change of lattice perfection due to the change from normal G to mu G was investigated using local rocking curve analysis (peak position, intensity, full width at half maximum) as well as X-ray topography. Clear increase of diffraction and thus better perfection were observed when the gravity changed from normal G to mu G condition. However, it should be noted that perfection of the space grown crystal reached the value of the best portion of the seed crystal that was carefully prepared on the Earth. Based on these data, increased growth rate in space is conclude to be due to the reduction of impurity effects at the surface of the crystals. Better perfection might additionally increase the growth rate.