Carol E. Afshar

Metal ion coordination in cobalt formate dihydrate

The structure of cobalt formate dihydrate, Co(HCO2)2 · 2H2O, was determined using single-crystal X-ray diffraction data. The crystals are monoclinic, space groupP21/c, with unit-cell dimensionsa=8.680(2),b=7.160(2),c=9.272(2) Å,β=97.43(2)°,V=571.4(3) Å3Z=4.Robs=0.038 for 1282 unique reflections withI>3σ(I). The crystal structure is found to be isomorphous with those of other divalent metal formates. This structure is interesting crystallographically because the Patterson map is homometric with respect to the positions of the heavy atoms. The asymmetric unit consists of two independent cobalt atoms on special positions, two formate ions (HCOO−), and two water molecules. The two cobalt atoms are each coordinated to six oxygen atoms in an octahedral arrangement. One of the cobalt octahedra contains only oxygen atoms from six formate ions. The second cobalt ion is surrounded by four water molecules and an oxygen atom from each of two formate ions. The two different octahedra are bridged by one of the formate ions and by hydrogen bonds. This network extends in a three-dimensional polymeric manner throughout the crystal structure. Each of the four oxygen atoms in the two independent formate ions forms a hydrogen bond to water and is coordinated to a metal ion. It is found that the metal ions lie in the plane of the formate carboxyl group to which they are coordinated, while molecules to which the formate ion is hydrogen bonded lie more out of this plane.

Structure of a rearrangement product: 1-methyl-3-(5-amino-1-benzylimidazol-4-yl)-1,2,4-triazole, C13H14N6

The title compound was prepared during the course of an attempt to synthesize 8-amino-3-benzyl-6-methylimidazo[4,5-e][1,2,4]triazepine. The crystals are monoclinic, space groupP21/c,a=6.2375(5),b=9.1070(8),c=22.182(2) Å,β=91.797(7)°,Z=4. The structure was solved by direct methods and refined by least squares toR=0.063 on all 2142 measured reflections. The imidazol-4-yl-triazole system is planar, conjugated, and aromatic. One hydrogen atom of the exocyclic amino group is involved in a bifurcated hydrogen bond, of which one branch is an intramolecular attraction to a triazole ring nitrogen atom, while the other branch is intermolecular. The crystal is held together by two N-H⋯N hydrogen bonds between molecules related by translation along thea axis and byπ-π stacking interactions of the imidazol-4-yl-triazole ring systems and the phenyl rings across centers of inversion.

Crystallization and preliminary X-ray diffraction studies of 5-chlorolevulinate-modified bovine porphobilinogen synthase and the PbII-complexed enzyme

Bovine porphobilinogen synthase (PBGS) is an homo-octameric enzyme with four active sites. Each active site binds two Zn(II) atoms whose ligands differ and two molecules of 5-aminolevulinate whose chemical fates differ. The asymmetric binding of two Zn(II) atoms and two identical substrate molecules by a homodimeric active site is apparently unique. Modification by 5-chiorolevulinate can be used to differentiate the two substrate-binding sites; diffraction-quality crystals of 5-chlorolevulinate-modified PBGS have been obtained. Pb(II) can be used to differentiate the two different Zn(II)-binding sites; diffraction-quality crystals of the Pb(II) complex of PBGS have been obtained. Preliminary diffraction data reveal an I422 space group, in agreement with a general model for the quaternary structure of PBGS.

The Molecular Conformation of Chorismic Acid in the Crystalline State

Abstract Chorismic acid is the branch point intermediate in the biosynthesis of all aromatic amino acids, and is essential in the metabolism of plants, fungi, and bacteria, but not mammals, making it an attractive target for herbicide, fungicide, or bacteriocide design. Chorismate has attracted substantial attention from bioorganic chemists because it spontaneously rearranges to prephenate in a reaction that is catalyzed 10 6 -fold by specific enzymes (chorismate mutases) and up to 10 4 -fold by certain catalytic antibodies. The detailed molecular structure of chorismic acid, previously unknown, has been determined by X-ray crystallography. The free acid, formula C 10 H 10 O 6 , crystallizes with orthorhombic space group symmetry C222 1 with unit cell dimensions a = 4.569 (1), b = 20.437 (7), c = 29.229 (9) A , and V = 2729 (1) A 3 . The solvent of crystallization approximates 1.5 H 2 O and 0.5 CCl 4 per molecule of chorismic acid. Crystals were unstable at room temperature and therefore X-ray diffraction data were measured at low temperature (−125°C). The phase problem was solved by direct methods and the model was refined by a full-matrix least-squares technique to give a final R = 0.092 and R w = 0.10. The structure shows the anticipated positions of single and double bonds with typical bond lengths. The conformation of chorismic acid in this crystal structure is trans diequatorial with respect to extracyclic groups C(3) and C(4); this is also reported to be the predominant conformer in solution. In this conformation the carbon atoms which become bonded in the enzymatic rearrangement to prephenate are quite distant from one another. This may explain why it was possible to obtain crystals which, although unstable at room temperature, could be studied at lower temperatures.

CRYSTALLIZATION AND PRELIMINARY X-RAY DIFFRACTION STUDIES OF E. COLI PORPHOBILINOGEN SYNTHASE AND ITS HEAVY-ATOM DERIVATIVES

Porphobilinogen synthase (PBGS) catalyzes the condensation of two identical substrate molecules, 5-aminolevulinic acid (ALA), in an asymmetric manner to form porphobilinogen. E. coli PBGS is an homooctameric enzyme. The number of active sites is not clear, but each subunit binds one ZnII ion and one MgII ion. Diffraction-quality crystals of native E. coli PBGS have been obtained, and unit-cell dimensions (a = 130.8, c = 144.0 A) are reported. These crystals diffract to about 3.0 A resolution.

4-(Methylnitrosamino)-1-(3-pyridyl)-1-butanone, a Nicotine-derived Carcinogenic Nitrosaminoketone (NNK): Three-dimensional Structure

The three-dimensional structure of the carcinogenic nicotine-derived nitrosaminoketone, 4-(methyl-nitrosamino)-l-(3-pyridyl)-l-butanone, has been determined by X-ray crystallographic techniques. The molecule is essentially planar except for the methylnitrosamine group which is oriented at a dihedral angle of 68.7° to the pyridine ring. Molecules pack by way of —H⋯O interactions that involve the —NNO group.

Structural Studies of Pahs and Their Metabolites

Abstract The carcinogenicity of benzo[a]pyrene (BaP) derivatives varies with the substituents they contain. The effects of these substituents on the geometric structures of the PAHs have been examined by X-ray diffraction studies. Data presented include bond lengths and angles. deviations from planarity in the bay region, and non-bonded H˙˙˙˙H distances. Structures in the sequence BaP, 11-methylBaP and dibenzo[a, l]pyrene illustrate the increased nonplanarity of the PAHs as the carcinogenicity increases. For comparison, the geometry of 7,12-dimethylbenz[a]anthracene (DMBA) is also discussed. The structures of a diol epoxide of 5,6-dimethylchrysene and the product of the reaction of the diol epoxide of 5-methylchrysene with methanol are also presented. These show how the bay-region methyl group affects the structure, and presumably also the reactivity of the diol epoxide.