RESUMEN
Crystals of beta-crustacyanin, a carotenoid-binding protein from lobster carapace, have been grown under oil from solutions containing sodium potassium phosphate as precipitant. They grow slowly over a period of months to reach maximal dimensions of 0.5 x 0.1 x 0.1 mm, and belong to space group P622 with cell dimensions: a = b = 124.39, c = 188.86 A and gamma = 120 degrees. The crystals diffract to beyond 3 A but are very radiation sensitive, limiting the resolution of usable data. The unit-cell volume suggests that there are two beta-crustacyanin molecules per asymmetric unit.
RESUMEN
The automated microbatch technique developed at Imperial College has been used to establish a phase diagram for crystallization. The concentrations of the protein (carboxypeptidase G2) and precipitant (PEG 4000) were varied, while pH and temperature were kept constant. The diagram consists of an undersaturation and a supersaturation zone, the latter being subdivided into the metastable, nucleation and precipitation zones. In the metastable zone, crystals may grow but nucleation of crystals does not occur. It is the best zone for growth of X-ray diffraction quality crystals because of the slower growth rate and the avoidance of uncontrolled nucleation, which uses up protein in the formation of tiny crystals. Nevertheless, in practice, it is rarely well defined or used because nuclei must be introduced artificially into the system. The new method used here consists of setting crystallization droplets at nucleation conditions and later diluting them to conditions where nucleation has not been observed. Single diffracting crystals of typical dimensions 0.3 x 0.3 x 0.2 mm were routinely obtained in the metastable zone, equivalent to the best (very rarely) obtained crystals in the nucleation zone.