Coordination polymers and isomerism; a study using silver(I) and a ∏-stacked ligand

The ligand 2,5-bis(2-pyridylmethylsulfanylmethyl)pyrazine (L) was prepared by the base coupling of 2-(sulfanylmethyl)pyridine and 2,5-bis(chloromethyl)pyrazine. This new ligand was treated with AgClO₄ in a 1 1 metal-to-ligand ratio and with AgNO₃in a 2 1 metal-to-ligand ratio to give coordination polymers. The crystal structures of {[Ag(L)]ClO₄}∞ ( 1) and {[Ag₂(L)](NO₃)₂}∞ ( 2) were determined. The Ag(I) ions in the one-dimensional polymeric chains of 1 adopted square-pyramidal geometries with the pyridine and pyrazine N donors coordinated in an extremely bent fashion. The structure of 2 revealed two isomeric polymer chains in the one crystal forming a single supramolecular array. The isomeric polymers differed in the donor atoms about the Ag(I) ions and in the arrangement of adjacent ligands along the chain. A feature of both structures was that L adopted a three-layer ∏-stacked arrangement

Flexible Ligands and Structural Diversity: Isomerism in Cd(NO₃)₂ Coordination Polymers

The ligands 1,4-bis(2-pyridylmethylsulfanylmethyl)benzene (L1) and 2,5-bis(2-pyridylmethylsulfanylmethyl)pyrazine (L2) were treated with Cd(NO3)2·4H2O in metal-to-ligand ratios of 1:1 and 2:1, respectively; L2 was also treated with CdCl2·2.5H2O in a 2:1 ratio. All products were found to be coordination polymers. The crystal structures of {[Cd(L1)(NO3)2]·CH2Cl2}∞ (1a), {[Cd(L1)(NO3)2]·4/3CH3CN}∞ (1b), {[Cd2(L2)(NO3)4]·2CH3CN}∞ (2·2CH3CN), and {[Cd2(L2)Cl4]·2CH2Cl2}∞ (3·2CH2Cl2) were determined. Compounds 1a and 1b were found to be conformational supramolecular isomers. The structure of 1b displayed topological isomerism with two isomeric polymer chains, 1b(1) and 1b(2), in the one crystal forming a single supramolecular array. The structure of 2·2CH3CN showed Cd2(L2) units linked together by nitrates bridging between the Cd(II) centers in a mode previously not seen in Cd(II) compounds. The overall structure of 3·2CH2Cl2 was found to be similar to that of 2·2CH3CN despite the presence of different anions and solvent molecules. Powder X-ray diffraction was used to investigate the nature of bulk preparations of compounds 1-3.</p

The Crystal Structure of Streptococcus pyogenes Uridine Phosphorylase Reveals a Distinct Subfamily of Nucleoside Phosphorylases

Uridine phosphorylase (UP), a key enzyme in the pyrimidine salvage pathway catalyzes the reversible phosphorolysis of uridine or 2'-deoxyuridine to uracil and ribose 1-phosphate or 2'-deoxyribose 1-phosphate. This enzyme belongs to the nucleoside phosphorylase I superfamily whose members show diverse specificity for nucleoside substrates. Phylogenetic analysis shows Streptococcus pyogenes uridine phosphorylase (SpUP) is found in a distinct branch of the pyrimidine subfamily of nucleoside phosphorylases. To further characterize SpUP, we determined the crystal structure in complex with the products, ribose I-phosphate and uracil, at 1.8 angstrom resolution. Like Escherichia coli UP (EcUP), the biological unit of SpUP is a hexamer with an alpha/beta monomeric fold. A novel feature of the active site is the presence of His169, which structurally aligns with Arg168 of the EcUP structure. A second active site residue, Lys162, is not present in previously determined UP structures and interacts with O2 of uracil. Biochemical studies of wild-type SpUTP showed that its substrate specificity is similar to that of EcUP, while EcUP is similar to 7-fold more efficient than SpUP. Biochemical studies of SpUP mutants showed that mutations of His 169 reduced activity, while mutation of Lys162 abolished all activity, suggesting that the negative charge in the transition state resides mostly on uracil O2. This is in contrast to EcUP for which transition state stabilization occurs mostly at O4