Correlation of structure and function in oligonuclear zinc(II) model phosphatases

A series of pyrazolate-based dizinc(II) complexes has been synthesized and investigated as functional models for phosphoesterases, focusing on correlations between hydrolytic activity and molecular parameters of the bimetallic core. The Zn...Zn distance, the (bridging or nonbridging) position of the Zn-bound hydroxide nucleophile, and individual metal ion coordination numbers are controlled by the topology of the compartmental ligand scaffold. Species distributions of the various dizinc complexes in solution have been determined potentiometrically, and structures in the solid state have been elucidated by X-ray crystallography. The hydrolysis of bis(p-nitrophenyl)phosphate (BNPP) promoted by the dinuclear phosphoesterase model complexes has been investigated in DMSO/buffered water (1:1) at 50 degrees C as a function of complex concentration, substrate concentration, and pH. Coordination of the phosphodiester has been followed by ESI mass spectrometry, and bidentate binding could be verified crystallographically in two cases. Drastic differences in hydrolytic activity are observed and can be attributed to molecular properties. A significant decrease of the pK(a) of zinc-bound water is observed if the resulting hydroxide is involved in a strongly hydrogen-bonded intramolecular O(2)H(3) bridge, which can be even more pronounced than for a bridging hydroxide. Irrespective of the pK(a) of the Zn-bound water, a hydroxide in a bridging position evidently is a relatively poor nucleophile, while a nonbridging hydroxide position is more favorable for hydrolytic activity. Additionally, the metal array has to provide a sufficient number of coordination sites for activating both the substrate and the nucleophile, where phosphate diesters such as BNPP preferentially bind in a bidentate fashion, requiring a third site for water binding. Product inhibition of the active site by the liberated (p-nitrophenyl)phosphate is observed, and the product-inhibited complex could be characterized crystallographically. In that complex, the phosphate monoester is found to cap a rectangular array of four zinc ions composed of two bimetallic entities.     

5‐Amino‐2,4,6‐tribromoisophthalic acid: the MAD triangle for experimental phasing

The title compound, C₈H₄Br₃NO₄, shows an extensive hydrogen‐bond network. In the crystal structure, molecules are linked into chains by COO—H...O bonds, and pairs of chains are connected by additional COO—H...O bonds. This chain bundle shows stacking interactions and weak N—H...O hydrogen bonds with adjacent chain bundles. The three Br atoms present in the molecule form an equilateral triangle. This can be easily identified in the heavy‐atom substructure when this compound is used as a heavy‐atom derivative for experimental phasing of macromolecules. The title compound crystallizes as a nonmerohedral twin.     

Treatment of Silylene–Phosphinidene with Chalcogens Resulted Exclusively in the Formation of Silicon-Bonded Chalcogens

Chalcogen-bonded silicon phosphinidenes LSi(E)−P−^MecAAC (E=S ( 1 ); Se ( 2 ); Te ( 3 ); L=PhC(NtBu)₂; ^MecAAC=C(CH₂)(CMe₂)₂N-2,6-iPr₂C₆H₃)) were synthesized from the reactions of silylene–phosphinidene LSi−P−^MecAAC ( A ) with elemental chalcogens. All the compounds reported herein have been characterized by multinuclear NMR, elemental analyses, LIFDI-MS, and single-crystal X-ray diffraction techniques. Furthermore, the regeneration of silylene–phosphinidene ( A ) was achieved from the reactions of 2 – 3 with L′Al (L′=HC{(CMe)(2,6-iPr₂C₆H₃N)}₂). Theoretical studies on chalcogen-bonded silicon phosphinidenes indicate that the Si−E (E=S, Se, Te) bond can be best represented as charge-separated electron-sharing σ-bonding interaction between [LSi−P−^MecAAC]⁺ and E⁻. The partial double-bond character of Si−E is attributed to significant hyperconjugative donation from the lone pair on E⁻ to the Si−N and Si−P σ*-molecular orbitals.