Figure 4.

Quartz capillaries containing crystals grown under microgravity in space (a) and crystals grown on earth (b). All capillaries are 50 mm in length and have an inner diameter of 2 mm. In both cases the starting precipitating solution is located on the left. Both space and earth crystallization experiments were prepared with the same proteins, concentrations, loading protocols, locations and laboratory and environmental conditions. For a canonical counter-diffusion experiment, the protein solution should diffuse from left to right, creating a supersaturation wave that moves across the capillary with increasing width and decreasing amplitude (García-Ruiz et al., 2001 ▶). Thus, the number of crystals should decrease and the size of the crystals should increase along the length of the capillary (left to right). This spatial–temporal gradient of supersaturation is clearly observed in the capillaries of the microgravity-grown crystals (a) while it is not observed in the capillaries with earth-grown crystals. The reason is that capillaries of 2 mm in diameter are too large to reduce convective flow, as expected from simple fluid-dynamics considerations. Typical crystals grown in space are shown under polarized light (c).