Coordination Sphere Of Cd Research Articles

Solid-State Associative Reactions and the Coordination Compression Mechanism.

Coordination polymers and metal-organic frameworks can be modified by high pressure, according to its effects on the radii of central and ligand atoms. The pressure reduces the ligands' radii, and the coordination number is usually increased. Such transformations of the coordination quite generally conform to the inverse rule of pressure and temperature effects, although the temperature-induced transformations are much less frequently observed. The two-dimensional coordination polymer Cd(APP)2NO3·NO3 [APP = 1,4-bis(3-aminopropyl)piperazine] undergoes a pressure-induced isostructural phase transition triggered by a topochemical reaction, yielding Cd(APP)2(NO3)2. The transition retains the symmetry of both phases, and their structures have been determined by X-ray diffraction for the single crystals compressed in a diamond-anvil cell. The reaction increases the Cd coordination, from 6-fold in phase I to 7-fold in phase II, where the new Cd-O bond involves an additional nitrate anion in the Cd coordination sphere.

Crystal structures and spectral properties of new Cd(II) and Hg(II) complexes of monensic acid with different coordination modes of the ligand

Abstract The single crystal X-ray structures and the spectroscopic properties of complexes of monensic acid (C36H62O11·H2O) with toxic metal ions of Cd(II) and Hg(II) are discussed. The cadmium(II) complex (1) is of composition [Cd(C36H61O11)2(H2O)2] and crystallizes in the monoclinic system (space group P2(1), Z = 2) with a = 12.4090(8), b = 24.7688(16), c = 14.4358(11) Å, β = 91.979(7)°. Two ligand monoanions are bound in a bidentate coordination mode to Cd(II) via the carboxylate and the primary hydroxyl oxygens occupying the equatorial plane of the complex. The axial positions of the inner coordination sphere of Cd(II) are filled by two water molecules additionally engaged in intramolecular hydrogen bonds. The Hg(II) complex (2), [Hg(C36H60O11)(H2O)], crystallizes in the orthorhombic system (space group P2(1)2(1)2(1), Z = 4) with a = 12.7316(2), b = 16.4379(3), c = 18.7184(4) Å. The monensic acid reacts with Hg(II) in a tetradentate coordination manner via both oxygen atoms of the carboxylate function and oxygens of two hydroxyl groups. The twofold negative charge of the ligand is achieved by deprotonation of carboxylic and secondary hydroxyl groups located at the opposite ends of the molecule. Hg(II) is surrounded by five oxygen atoms in a distorted square pyramidal molecular geometry.

Synthesis and spectral studies on mixed ligand complexes of Cd(II) dithiocarbamates with nitrogen donors: single crystal X-ray structure of bis (4-methylpiperidinecarbodithioato-S,S′)(1,10-phenanthroline)cadmium(II)

The current article describes the synthesis and characterization of the following adducts: [Cd(2-mpipdtc)2(1,10-phen)], [Cd(2-mpipdtc)2(bipy)], [Cd(4-mpipdtc)2(1,10-phen)], [Cd(4-mpipdtc)2(bipy)] (where 2-mpipdtc− = 2-methylpiperidinecarbodithioate anion, 4-mpipdtc− = 4-methylpiperidinecarbodithioate anion, 1,10-phen = 1,10-phenanthroline and bipy = 2,2′-bipyridine). A single crystal X-ray structural analysis was carried out for [Cd(4-mpipdtc)2(1,10-phen)]. IR spectra of the complexes show the contribution of the thioureide form to the structures. Reduction in νC–N(thioureide) for the mixed ligand complexes is attributed to the change in coordination number from four to six and the steric effect exerted by 1,10-phenanthroline or 2,2′-bipyridine. Deshielding of the protons adjacent to nitrogen in the 1H NMR spectra is attributed to drift of electrons from the nitrogen of NR2, forcing electron density towards sulfur via the thioureide π-system. Single crystal X-ray structural analysis of [Cd(4-mpipdtc)2(1,10-phen)] showed that the cadmium is in a distorted octahedral environment with a CdS4N2 chromophore. The presence of 1,10-phenanthroline in the coordination sphere of Cd(dtc)2 increases the Cd–S distances and decreases the S–Cd–S angles. VBS analysis supports the determined structure.

Potential influence of Asp in the Ca2+ coordination position 5 of parvalbumin on the calcium-binding affinity: A computational study

Parvalbumins (PV) are calcium-binding proteins, all sharing the common helix–loop–helix (EF-hand) motif. This motif contains a central twelve-residue Ca2+-binding loop with the flanking helices positioned roughly perpendicular to each other. The precise role of these coordination residues has been the subject of intense studies. In this work, we focus on the coordination position 5 in the CD Ca2+-binding site of silver hake parvalbumin isoform B (SHPV-B). The most common residue at site 5 of calcium-binding loop in canonical EF-hands is Asp [B.J. Marsden, G.S. Shaw, B.D. Sykes, Biochem. Cell Biol. 68 (1990) 587–601], but in the CD site of PV, this position is almost always serine (Ser). The substitution of Ser with Asp will add the 5th carboxylate residue in the CD coordination sphere. However, as predicted by the acid pair hypothesis, the Ca2+-binding affinity would be maximized in an EF-hand motif that has four carboxylate ligands paired along the ±x, and ±z-axes [R.E. Reid, R.S. Hodges, J. Theor. Biol. 84 (1980) 401–444]. Molecular dynamics simulations and free energy calculations were employed to investigate the influence of Ser to Asp mutation at position 5 on calcium-binding affinity. We found that the Asp variant exhibited remarkable stability during the entire molecular dynamics simulation, with not only the retention of the Ca2+-binding site, but also increased compactness in the coordination sphere. The S55D fragment also accommodated Ca2+ well. We conclude that the reason why Asp which is the most common residue at site 5 of calcium-binding loop in canonical EF-hands has never been identified at this position experimentally for PVs might be related to its physiological functions.

Function of His185 in Aquifex aeolicus 3-Deoxy- d- manno-octulosonate 8-Phosphate Synthase

Aquifex aeolicus 3-deoxy- d- manno-octulosonate 8-phosphate synthase (KDO8PS) catalyzes the condensation of arabinose 5-phosphate (A5P) and phosphoenolpyruvate (PEP) by favoring the activation of a water molecule coordinated to the active-site metal ion. Cys11, His185, Glu222 and Asp233 are the other metal ligands. Wild-type KDO8PS is purified with Zn 2+ or Fe 2+ in the active site, but maximal activity in vitro is achieved when the endogenous metal is replaced with Cd 2+. The H185G enzyme retains 8% of the wild-type activity. ICP mass spectrometry analysis indicates that loss of His185 decreases the enzyme affinity for Fe 2+, but not for Zn 2+. However, maximal activity is again achieved by substitution of the endogenous metal with Cd 2+. We have determined the X-ray structures of the Cd 2+ H185G enzyme in its substrate-free form, and in complex with PEP, and PEP plus A5P. These structures show a normal amount of Cd 2+ bound, suggesting that coordination by His185 is not essential to retain Cd 2+ in the active site. Nonetheless, there are significant changes in the coordination sphere of Cd 2+ with respect to the wild-type enzyme, as the carboxylate moiety of PEP binds directly to the metal ion and replaces water and His185 as ligands. These observations indicate that the primary function of His185 in A. aeolicus KDO8PS is to orient PEP in the active site of the enzyme in such a way that a water molecule on the sinister ( si) side of PEP can be activated by direct coordination to the metal ion.

Introduction of a fifth carboxylate ligand heightens the affinity of the oncomodulin CD and EF sites for Ca2+.

The acid-pair hypothesis, proposed by Reid and Hodges [(1980) J. Theor. Biol. 84, 401-444], suggests that the affinity of an EF-hand motif for Ca2+ will be maximal with four acidic ligands, paired along the +x, -x and +z, -z axes. Addition of a fifth anionic ligand is predicted to reduce Ca(2+)-binding affinity, as a consequence of increased electrostatic repulsion. Interestingly, for oncomodulin, we observe that introduction of a fifth carboxylate residue at the +z position in the CD coordination sphere or at the -x position in the EF coordination sphere significantly increases the affinity of those sites for Ca2+. The variants resulting from replacement of serine-55 by aspartate (S55D), glycine-98 by aspartate (G98D), and the combined mutations (55/98) have been examined in Ca(2+)- and Mg(2+)-binding studies, titration calorimetry, and differential scanning calorimetry. The KCa for the CD site is reduced from 800 to 67 nM by the S55D mutation, while KCa for the EF site is reduced from 45 to 4 nM by the G98D mutation. Both mutations destabilize the apo form of the protein and increase the thermal stability on the Ca(2+)-bound state. Interestingly, the S55D mutation also increases the affinity of the oncomodulin CD site for Mg2+, decreasing the dissociation constant from > 1 mM to approximately 30 microM. This increase in affinity is reflected in a substantially increased thermal stability of the Mg(2+)-bound form of the protein. In 0.15 M NaCl, 0.025 M Hepes (pH 7.4), and 0.01 M Mg2+, the wild-type protein denatures at 68.5 degrees C. By contrast, under identical conditions, the S55D mutations denatures at 79.0 degrees C. The increased metal ion-binding affinity displayed by the variant proteins may result in part from preferential destabilization of the apo-protein by the additional carboxylate.

ESR detection of pressure-induced structural change around Pb 2+ and Cd 2+ in soda borate glasses

The pressure-induced changes in the coordination structure of Cd 2+ and Pb 2+ were examined by measuring the ESR spectra of γ-ray-induced Cd + and Pb 3+ centers. The s-character of the magnetic electron in the centers increased on densification and was estimated from the analysis of the hyperfine absorption due to a 207Pb 3+ and 111Cd + and 113Cd +. The increase in the s-character means that the strength of both Pb 2+O and Cd 2+O bonds was reduced upon densification and that the coordination number of Cd 2+ and Pb 2+ increased. The coordination sphere of Cd 2+, which responds more sensitively than that of Pb 2+ to the change in chemical environments, was more heavily compressed than that of Pb 2+.