Abstract. Bovine proinsulin has been crystallized under a variety of conditions at both neutral and acid pH. Microtechniques were employed with sample weights of about 200 jig and volumes of 5-20 1AL The crystalline preparations all differ from each other morphologically.X-ray photographs of one form, tetragonal bipyramids grown at pH 3 with added ammonium sulphate solution, established the space group P41212 (or its enlantiomorph P43212). The cell dimensions are a = 50.8 i 0.2 A, c = 148.0 + 0.4 A. The asymmetric unit in this form is a dimer of proinsulin which is also the dominant species in solution at this pH.Introduction. Following the studies of Steiner arid his co-w%-orkersl2 who established that insulin is synthesized via a single chain precursor protein, proinsulin has been isolated and characterized from bovine,3-6 porcine,' cod,8 anglerfish,9 and human10"1 tissues. The connecting peptide segment of proinsulin which links the carboxyl-terminal group of the insulin B chain to the amino-terminal group of the insulin A chain is cleaved in vivo by proteolysis. The lengths and sequences of the C-peptide vary markedly between species. In bovine proinsulin, where the connecting peptide is 30 amino acid residues long," the proinsulin molecule with a single 81 residue chain has a molecular weight of 868.5.

Synthetic crystals of tachyhydrite are trigonal, diffraction aspect R**, with hexagonal parameters a = 10.136 (1), c = 17.318 (2) Å, U = 1540.9 (3) Å3, Z = 3, Dc = 1.673 Mg m−3. Indexed powder data are given.

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