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Crystalline acetylene at 141 K is cubic, a = 6.091 (3) /~, space group Pa3, Z = 4. The intensities of all 164 independent reflexions up to sin 0/2 = 0.80 A -1 were measured accurately on a Nonius four-circle diffractometer. For higher sin 0/2, very few significant reflexion intensities exist due to the strong thermal motion. A valence analysis, with multipole deformation terms and restricted radial functions centred on the atoms, was performed for the 164 independent reflexions by least-squares refinement on L Both ~ and SCF scattering factors were considered. H was constrained to C. The radial functions corresponding to the SCF scattering factors were modified by introduction of an isotropic extinction parameter and an additional isotropic temperature factor for H in the refinement.Rw(I; 0 = 0.0176 and Rw(I;SCF ) = 0.0170, both with 11 independent parameters. Third and fourth cumulant terms describing the non-linearity of the librational motion had no significant effect. Filtered and unfiltered maps including multipole deformations only are based on the 138 reflexions with I _> 0. For the (modified) SCF atomic scattering factors the agreement with theory is better than for the ~ scattering factors. In the present analysis static deformations and thermal smearing could not be separated and no reliable value could be found for the scale factor. The situation would be more favourable for compounds with smaller thermal motion for which both X-ray and neutron data are available.

Bovine pancreatic prophospholipase A2, the precursor of the lipolytic enzyme phospholipase A2, crystallizes in space group P3j21 with cell dimensions a = b = 46.95, c --102.0 A. The structure was determined at 3.0/~ resolution using rotation and translation functions with transaminated phospholipase A 2 as the model structure. The rotation-function calculations appeared to be sensitive to the resolution range selected. The results of the translation function were very well defined. Positioning of the model molecule according to the rotation-and translation-function results yielded an R factor of 0.44 for 2697 reflections between 7.1 and 3.0 A. This could be decreased to 0.40 by a rigid-body R-factor search. Subsequent restrained refinement gave R = 0.27. The position of the calcium ion, which was excluded from the structure factor calculations, shows up as one of the highest features in difference Fourier syntheses. From the difference maps it also appears that the ten N-terminal residues of prophospholipase A 2 are disordered. Disorder is also observed in the loop consisting of residues 62 to 73. This is quite in contrast to the situation in phospholipase A 2 where this loop is well defined. The observed disorder may account for the large difference in activity of phospholipase and prophospholipase with respect to aggregated substrates like micelles.

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