L-Cysteinyl-L-cysteine crystallizes in the orthorhombic space group P2,2,2, with a = 8.629 (7), b = 23.500 (9), c = 5.145 (2) A. The structure, solved by the heavy-atom method, was refined by least-squares calculations to an R of 0-042, from 963 observed reflexions. The molecule contains a cis-peptide unit and a right-handed disulphide bridge with a torsion angle of 94 ° about the -S-S-bond. A comparison of the observed conformation with that predicted by energy calculations indicates that a better agreement should be obtained by using in the energy calculation a softened potential function for the H • • • H interactions.In connexion with a research programme on the structure of cysteine-containing peptides, the cyclic disulphide L-cysteinyl-L-cysteine (I) has been synthesized in our laboratory and studied by X-ray methods. jS-S CH 2~CH 2

Crystals of 5e-androstan-3,17-dione are monoclinic, space group C2, Z=4, a--12.70, b = 6.19, c= 21.34 /~., ,8= 91 ° 16'. The structure was solved by potential-energy calculations coupled with a minimum residual phasing technique. Three-dimensional X-ray data were used in the refinement, which was carried out by least-squares methods. The crystal packing is characterized by molecular layers, normal to the b axis, tied together by van der Waals, dipole-dipole and n-re interactions.

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