The three-dimensional structures of bovine lens leucine aminopeptidase (blLAP) complexed with L-leucinal and of the unliganded enzyme have been determined at crystallographic resolutions of 1.9 and 1.6 A, respectively. Leucinal binds as a hydrated gem-diol to the active site of b1LAP), resembling the presumed gem-diolated intermediate in the catalytic pathway. One hydroxyl group bridges the two active site metal ions, and the other OH group is coordinated to Zn1. The high-resolution structure of the unliganded enzyme reveals one metal-bound water ligand, which is bridging both zinc ions. Together, these structures support a mechanism in which the bridging water ligand is the attacking hydroxide ion nucleophile. The gem-diolate intermediate is probably stabilized by four coordinating bonds to the dizinc center and by interaction with Lys-262 and Arg-336. In the mechanism, Lys-262 polarizes the peptide carbonyl group, which is also coordinated to Zn1. The Arg-336 side chain interacts with the substrate and the gem-diolate intermediate via water molecules. Near Arg-336 in the b1LAP-leucinal structure, an unusually short hydrogen bond is found between two active site water molecules.

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A three-dimensional crystal structure analysis of hydrogen cyanide tetramer has led to a location of all atoms with average standard deviations of 0.0025 A for carbon and nitrogen atoms and 0.04 A for hydrogens. The diaminomaleonitrile structure is proved. In its crystal setting the molecule has no symmetry. The two amino groups have different configurations, one being planar and one tetrahedral while both being involved in intermolecular N-H • • • 1W hydrogen bonds. An analysis is made of the anisotropic nature of the atomic thermal vibrations. Molecular orbital calculations have been made for this molecule and for tetracyanoethylene leading to z bond orders for all bonds. Order/length relationships for the carbon-carbon bonds are discussed in some detail.

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