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Abstract. C2H2N2S , tetragonal, P4~212 or its enantiomorph; a = 5.324, c= 12-643 A. Cell dimensions indicate a stacking of the molecules along the c axis. No intermolecular contacts seem to occur in the crystalline state, in agreement with the relatively low melting point and observed diffuse scattering of X-rays at room temperature. The calculated density assuming 4 molecules/ cell is Q= 1-60 g cm -3. It was difficult to measure the density because of the limited amount and size of the material provided; it was, however, possible to observe crystals in gravitational equilibrium in CCI4 (0--1.6 gcm -3) before they dissolved.Introduction. A small sample of crystals, of which three were suitable for diffraction work, was kindly provided by Dr L. Nygaard. The crystals were prismatic with dimensions 0.2 x 0.2 × 0.3 mm. As the crystals were very hygroscopic they were sealed in glass capillaries under dry nitrogen. It was decided to use this compound as a test for a low-temperature device built by the author, because of its low melting point (303 K) and high volatility at room temperature.Preliminary precession photographs showed the sym-

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SHORT COMMUNICATIONS 231 effect on the starting point of crystallization, which occurred 40 times faster. In addition, the mutation may have increased the crystal growth rate. This point was difficult to establish, however, because the growing surfaces of the numerous smaller mutant crystals were larger than those of the single wild-type crystals, which may well explain the observed rate difference of a factor of about three in this series of experiments (Figs. 3a,b).The gas-diffusion progress in the crystallization drops (Fig. 3c) indicated that the mutant enzyme succeeds in nuclei formation shortly after the PEG-10000 concentration crosses the solubility and crystal stability limits of about 5.3%, requiring almost no supersaturation. In contrast, the wild-type enzyme requires a high and enduring supersaturation for nucleation. Wild-type nuclei form 60 h after the drop crosses the solubility and crystal stability limit, which is 25 h after the drop has equilibrated at a high level of supersaturation (Fig. 3c). Most surprising is the uniform timing in all eight successful crystallization trials of the series (Figs. 3a,b).In conclusion, we find that the addition of contact C' accelerated the crystal nucleation process, shortening the crystallization time by a factor of 40. The engineered contact may have increased the crystal growth rate. It did not affect protein solubility, crystal stability and crystalline order. The experiments demonstrate that contact engineering can have a strong influence on crystallization, nurturing the hope that it can be used for improving the crystalline order in the future. References AbstractXylose isomerase from Bacillus coagulans has been crystallized in two different crystal forms. One crystal form is in space group P2~2~2, cell dimensions a = 462, b = 165, c = 82 A. The other is in space group I422, cell dimensions a b= 113, c= 153~,.