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The crystal structure of porphine, the parent compound in the porphyrin series, was determined by x-ray diffraction analysis. The phases of the normalized structure factors were obtained by the probability methods of Hauptman and Karle. The crystals are monoclinic with four molecules per unit cell, and the C2h5—P21/a space group requires no molecular symmetry. Observed small deviations from planarity are not large enough to be significant. Comparison of the crystal structures of several porphyrins indicates that the porphine skeleton is probably planar or nearly planar in the vapor phase, although marked ruffling occurs in some crystals because of crystal-packing forces. The wide variation in molecular configuration found in different crystals emphasizes the easy deformability of the molecule; this property may be important in biological systems.

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The structure of NAD+ bound to each subunit of cytoplasmic malate dehydrogen?se has been determined from an electron density map at 2.5-A resolution. The structures of the bound coenzyme molecules are closely related by the twofold rotational symmetry of the dimeric enzyme with the