Shared and closed-shell O-O interactions in silicates

Bond paths of maximum electron density spanning O-O edges shared between equivalent or quasiequivalent MOn (n > 4) coordination polyhedra are not uncommon electron density features displayed by silicates. On the basis of the positive values for the local electronic energy density, H(rc), at the bond critical points, rc, they qualify as weak "closed-shell" interactions. As observed for M-O bonded interactions (M = first and second row metal atoms), the electron density, rho(rc), and the Laplacian of the electron density increase in a regular way as the separation between the O atoms, R(O-O), decreases. A simple model, based on R(O-O) and the distances of the Si atoms from the midpoint between adjacent pairs of O atoms, partitions the O-O bond paths in the high-pressure silica polymorph coesite into two largely disjoint domains, one with and one without bond paths. The occurrence of O-O bond paths shared in common between equivalent coordination polyhedra suggests that they may be grounded in some cases on factors other than bonded interactions, particularly since they are often displayed by inert procrystal representations of the electron density. In these cases, it can be argued that the accumulation of the electron density along the paths has its origin, at least in part, in the superposition of the peripheral electron density distributions of the metal M atoms occupying the edge-sharing polyhedra. On the other hand, the accumulation of electron density along the paths may stabilize a structure by shielding the adjacent M atoms in the edge-sharing polyhedra. For closed-shell Li-O, Na-O, and Mg-O interactions, H(rc) is positive and increases as the value of rho(rc) increases, unlike the "shared" Be-O, B-O, C-O, Al-O, Si-O, P-O, and S-O interactions, where H(rc) is negative and decreases as rho(rc) increases. The H(rc) values for the weak closed-shell O-O interactions also increase as rho(rc) increases, as observed for the closed-shell M-O interactions. On the basis of the bond critical point properties and the negative H(rc) value, the O-O interaction comprising the O2 molecule in silica III qualifies as a shared interaction.     

Complexation behavior of Schiff base toward transition metal ions

Complexes of Iron, Cobalt, Nickel and Zinc ions with the Schiff base derived from p-dimethylaminobenzaldehyde and o-aminobenzoic acid were synthesized and investigated by several techniques using elemental analyse (C,H,N), molar conductance measurements, infrared and electronic spectra. The elemental analysis data suggest the stoichiometry to be 1:1 [M:L] ratio formation. The molar conductance measurements reveal the presence of non-electrolytic nature complexes. Infrared spectral data agreed with the coordination to the central metal ions through both the nitrogen atom of the azomethine and oxygen atom of the carboxyl group of the 2-aminobenzoic acid moiety. The electronic spectral data suggest the existence of octahedral geometry for Fe(III) complex, square planar geometry for Co(II) and Ni(II) complexes and tetrahedral geometry for Zn(II) complex.     

The combination of transition metal ions and hydrogen-bonding interactions

This feature article presents an overview of the types of hydrogen bonding interactions involving metal complexes and their functional effects. It shows with recent examples why hydrogen bonds have become a crucial functional and structural element in modern inorganic chemistry. The relevance of this combination in tackling current chemistry challenges such as energy production and the development of new materials and more effective catalysts, sensors and medicines is illustrated.     

From subtle to substantial: role of metal ions on pi-pi interactions

Quantum chemistry calculations reveal that the subtle pi-pi interactions, usually in the range 2-4 kcal/mol, will become substantially significant, from 6 to 17 kcal/mol, in the presence of metal ion. The metal ions have higher affinity toward a pi-pi dimer compared to a single pi-moiety. Considering the widespread occurrence of cation-pi-pi motifs in chemistry and biology, as evident from the database analysis, we propose that the two key noncovalent forces, which govern the macromolecular structure, cation-pi and pi-pi, work in concert.     

Progress toward artificial metalloenzymes: New ligands for transition metal ions and neutral molecules

New nickel catalysts have been developed for the oxidation of alkenes to epoxides, alcohols, aldehydes and ketones. Mechanistic studies indicate that the oxidation reactions are very sensitive to the nature of the catalyst; only certain ligands including salen and the macrocycles cyclam and dioxocyclam render Ni(II) effective as a catalyst. A Ni(III) or Ni(IV)-oxo species has been postulated as the catalytically active oxidant which leads to oxygen atom transfer to alkenes in a stepwise process. Both iodosylbenzene and hypochlorite have been used as terminal oxidants; both systems give high yields of epoxidation of alkenes and varying amounts of C=C bond cleavage products. In order to reach an ultimate goal of hydrocarbon oxidation within a molecular recognition system, new molecular receptors for organic substrates have been investigated. The receptors are constructed from two subunits of cholic acid and display amphophilic character — a hydrophobic exterior and a hydrophilic interior. Conformational properties in the presence of polar guests in CDCl₃ are described.     

Electrophoretic studies of polygalacturonate oligomers and their interactions with metal ions

Polygalacturonic acid, a linear homopolysaccharide, was investigated by capillary electrophoresis (CE) using linear polyacrylamide-coated capillaries and laser-induced fluorescence (LIF) detection. A successful separation of its fluorescently labeled oligomers was achieved through sieving in polyacrylamide entangled matrices. The reaction conditions for the derivatization of polygalacturonic acid were optimized. In studying the interactions between polygalacturonic acid and various metal ions, the end-label, free-solution electrophoretic (ELFSE) technique, developed earlier in our laboratory (Sudor, J., Novotny, M. V., Anal. Chem. 1995, 67, 4205-4209) was found preferable to the sieving method. ELFSE is fast and convenient in that no polymer solutions are needed for the separation. The investigation showed that for the moderately large oligomers, the strongest binding occurred with calcium and cadmium ions, while the smallest interaction was observed with magnesium ions.     

Basicity of siloxanes,alkoxysilanes and ethers toward hydrogen bonding

Thermodynamic constants have been determined for hydrogen bonding of phenol to several siloxanes and alkoxysilanes, as well as to Me₃SiSCMe₃ and Me₃SiSSiMe₃. Alkoxysilanes are slightly weaker bases toward phenol than the isostructural ethers. Unstrained siloxanes have low values of △H for hydrogen bonding of 3.5–3.9 kcal/mol and are therefore distincatly weaker bases than either alkoxysilanes or ethers. Siloxanes with small SiOSi bond angles show increased basicity. Me₃SiSCMe₃ (△H 1.9 kcal/mol) and Me₃SiSSiMe₃ (△ 0.8 kcal/mol) are markedly less basic than dialkyl sulfides. The results are discussed using a molecular orbital model.     

Cation-pi interactions of bare and coordinatively saturated metal ions: contrasting structural and energetic characteristics

In the present work, we address an apparent disparity in the structural parameters of the X-ray structures and theoretical models of cation-pi complexes in biological and chemical recognition. Hydrated metal ion (Li+, Na+, K+, Mg2+, Ca2+) complexes with benzene (cation-pi) are considered as model systems to perform quantum mechanical calculations in evaluating the geometrical parameters and interaction energies of these complexes. The computations disclose that there is a variation in the structural parameters as well as in the interaction energies of these complexes with the multiple additions of water molecules. The distance between the cation and the pi-system increases with the addition of water molecules, delineating the influence of solvent or the neighborhood atoms on the structural parameters of cation-pi systems present in crystal structures.     

Synthesis of thiol chelating resins and their adsorption properties toward heavy metal ions

A series of chelating resins, derived from a macroreticular styrene-divinylbenzene (2%) copolymer beads grafted with various poly(ethylene glycols) HO? (? CH₂? CH₂? O? )_n? H(n = 0, 4, 9, 13) and containing thiol groups as chelating functions, have been synthesized in a three-step reaction sequence. The structure of the functionalized resins was confirmed by IR spectrophotometry, elemental analysis, and differential scanning calorimetry. The complexation behavior of these thiol resins was investigated towards Hg(II), Cu(II), and Pb(II) ions in aqueous solution by a batch equilibration technique. The influence of pH on adsorption capacity was also examined. The adsorption values for metal ions' intake followed the order Hg(II) > Cu(II) > Pb(II). The affinity of these polymers towards Hg(II) ions was so high that the total mercury level in the liquid decreased from 20 ppm to below 10 ppb after 2 h of treatment. Polymers can be regenerated by washing with a solution of hydrochloric acid (6N) and 10% by weight of an aqueous solution of thiourea. © 1994 John Wiley & Sons, Inc.     

Selectivity of glassy type mixed zirconium-titanium phosphate toward rare earth metal ions

The selectivity of glass type mixed Zr+Ti phosphates with various Ti contents has been investigated toward rare earth metal ions. The k_d values were determined by radiotracer technique both in HCl and HNO₃ solutions of various concentrations. At room temperature and at pH=4 the following selectivity orders were found: Yb>TmEu>Sm>La (HCl) and Eu>La>Sm>Yb>Tm (HNO₃).     

Preparation of hydrazinodeoxycellulose and carboxyalkyl hydrazinodeoxycelluloses and their adsorption behavior toward heavy metal ions

Nitrogen-containing cellulose derivatives hydrazinodeoxycellulose (HDC) and carboxyalkyl hydrazinodeoxycelluloses (α- and β-CAHDCs) were prepared from 6-chlorodeoxycellulose (CDC). Their adsorption of divalent transition metal ions was determined from dilute aqueous solutions and compared with that of aminoalkyl celluloses (AmACs) reported previously. HDC scarcely adsorbs metal ions in the pH range of 1–2, whereas α- and β-CAHDCs adsorb metal ions in this pH range. However, the adsorption of metal ions on HDC increases rapidly with increasing pH and HDC more effectively adsorbs metal ions than α- and β-CAHDCs in weakly acidic conditions. The ability to adsorb Cu²⁺ ions was in the order of AmAC (carbon number in the diamine moiety m = 2) > HDC > α-CAHDC > β-CAHDC in the weakly acidic region. These adsorbents selectively adsorb Cu²⁺ ions from the solutions containing other metal ions such as Mn²⁺, Co²⁺, and Ni²⁺, and the Irving–Williams series is obeyed in these adsorbent/metal ion systems. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35 : 3359–3363, 1997     

Thermodynamic interactions of the alkaline earth metal ions with citric acid

By using the isothermal titration calorimetry (ITC) technique, thermodynamic parameters have been determined for reactions of the Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺ ions with the citrate anion. The measurements were run in the Cacodylate, Pipes and Mes buffer solutions of a pH of 6, at 298.15 K, as well as in the Tricine, Tapso, and Tris–HCl buffer solutions of a pH of 8. Further, based on the results of potentiometric titration, the conditional stability constants were determined for the citrate complexes at both pH values. The effect of the reaction environment and the metal ion identity on the interaction energy with the citrate ligand and the stability of the resulting compounds have been discussed.     

The laser-induced blue state of bacteriorhodopsin: mechanistic and color regulatory roles of protein-protein interactions, protein-lipid interactions, and metal ions

In this paper we characterize the mechanistic roles of the crystalline purple membrane (PM) lattice, the earliest bacteriorhodopsin (BR) photocycle intermediates, and divalent cations in the conversion of PM to laser-induced blue membrane (LIBM; lambda(max)= 605 nm) upon irradiation with intense 532 nm pulses by contrasting the photoconversion of PM with that of monomeric BR solubilized in reduced Triton X-100 detergent. Monomeric BR forms a previously unreported colorless monomer photoproduct which lacks a chromophore band in the visible region but manifests a new band centered near 360 nm similar to the 360 nm band in LIBM. The 360 nm band in both LIBM and colorless monomer originates from a Schiff base-reduced retinyl chromophore which remains covalently linked to bacterioopsin. Both the PM-->LIBM and monomer-->colorless monomer photoconversions are mediated by similar biphotonic mechanisms, indicating that the photochemistry is localized within single BR monomers and is not influenced by BR-BR interactions. The excessively large two-photon absorptivities (> or =10(6) cm(4) s molecule(-1) photon(-1)) of these photoconversions, the temporal and spectral characteristics of pulses which generate LIBM in high yield, and an action spectrum for the PM-->LIBM photoconversion all indicate that the PM-->LIBM and Mon-->CMon photoconversions are both mediated by a sequential biphotonic mechanism in which is the intermediate which absorbs the second photon. The purple-->blue color change results from subsequent conformational perturbations of the PM lattice which induce the removal of Ca(2+) and Mg(2+) ions from the PM surface.     

The interactions of metal ions with nonsteroidal anti-inflammatory drugs (oxicams)

Much work has been focused on interactions of metal ions with nonsteroidal anti-inflammatory drugs (oxicams). Numerous attempts to synthesize several metal complexes have been published. This review highlights the synthesis and properties of the synthesized metal complexes. Different physico-chemical methods (IR, UV–Vis, measurement, thermal analysis, and NMR spectroscopy) as well as the bioactivity of the metal compounds are mentioned.     

A thiazole-containing tripodal ligand: synthesis, characterization, and interactions with metal ions and matrix metalloproteinases

A new tripodal ligand, tris[2-(((2-thiazolyl)methylidene)amino)ethyl]amine (Tatren), has been synthesized and characterized by NMR, IR, and UV-visible absorbance spectroscopy and elemental analysis. Tatren forms stable complexes with transition metal ions (Zn(2+), 1; Mn(2+), 2; Co(2+), 3) and the alkaline earth metal ions (Ca(2+), 4; Mg(2+), 5). Single-crystal X-ray structures of 1, 2, and 5 revealed six-coordinate chelate complexes with formula [M(Tatren)](ClO(4))(2) in which the metal centers are coordinated by three thiazolyl N atoms and three acyclic imine N atoms. Crystals of 1, 2, and 5 are monoclinic, P2(1)/c space group. Crystals of 4 are triclinic, P space group. The Ca(2+) complex is eight-coordinate with all N atoms of Tatren and one water molecule coordinated to the metal ion. Spectrophotometric titrations show that formation constants for the chelates of metal ions are >1 in methanol. Free Tatren inhibits the catalytic domain of matrix metalloproteinase-13 (MMP-13, collagenase-3) with K(i) = 3.5 +/- 0.6 microM. Molecular mechanics-based docking calculations suggest that one leg of Tatren coordinates to the catalytic Zn(2+) in MMPs-2, -9, and -13 with significant hydrogen bonding to backbone amide groups. High-level DFT calculations suggest that, in the absence of nonbonded interactions between Tatren and the enzyme, the most stable first coordination sphere of the catalytic Zn(2+) is achieved with three imidazolyl groups from His residues and two imine N atoms from one leg of Tatren. While complexes (1-3) do not inhibit MMP-13 to a significant extent, 4 does (K(i) = 30 +/- 10 microM). Hence, this study shows that tripodal chelating ligands of this class and their Ca(2+) complexes have potential as active-site inhibitors for MMPs.     

Relativity,gold, closed-shell interactions,and CsAu.NH3

The chemical properties of gold are strongly influenced by relativistic effects. One example is the large electronegativity of Au, which qualitatively explains the stability of (solid or liquid) cesium auride, Cs(+)Au(-), and other systems with Au(-) ions. An especially impressive compound is CsAu.NH(3), the structure and bonding of which are discussed. Future possibilities for finding further aurides are outlined.     

Examination of Ortho effects in the collisionally activated dissociation of closed-shell aromatic ions

The collisionally activated dissociation of a variety of isomeric disubstituted aromatic ions formed by ion–molecule reactions were examined in order to characterize ortho effects in closed-shell systems. Closed-shell ions of methoxyacetophenone, hydroxyacetophenone, methoxyphenol, anisaldehyde and hydroxybenzaldehyde were formed by proton transfer, methyl addition or methyne addition by using dimethyl ether or ethylene oxide as chemical ionization reagents, and then the structures of these adducts were studied by deuterium-labelling methods and by collisionally activated dissociation techniques in a triple quadrupole mass spectrometer or a quadrupole ion trap. Typically, the meta and para isomers have qualitatively similar dissociation spectra which reflect the types of dissociation reactions observed for the corresponding monosubstituted aromatic ions. The predominant dissociation pathways of the [M + H]⁺ and [M + 15]⁺ ions are directed by the electron-withdrawing substituents, whereas the major dissociation pathways of the [M + 13]⁺ ions are related to the electron-releasing substituent. In contrast, the dissociation routes of the corresponding ortho isomers are dramatically different. This is attributed to the opportunity for functional group interactions of the ortho isomers which facilitate alternative pathways.     

Differential interactions of plasmid DNA, RNA and endotoxin with immobilised and free metal ions

Separation of negatively charged molecules, such as plasmid DNA (pDNA), RNA and endotoxin forms a bottleneck for the development of pDNA vaccine production process. The use of affinity interactions of transition metal ions with these molecules may provide an ideal separation methodology. In this study, the binding behaviour of pDNA, RNA and endotoxin to transition metal ions, either in immobilised or free form, was investigated. Transition metal ions: Cu2+, Ni2+, Zn2+, Co2+ and Fe3+, typically employed in the immobilised metal affinity chromatography (IMAC), showed very different binding behaviour depending on the type of metal ions and their existing state, i.e. immobilised or free. In the alkaline cell lysate, pDNA showed no binding to any of the IMAC chemistries tested whereas RNA interacted significantly with Cu2+-iminodiacetic acid (IDA) and Ni2+-IDA but showed no substantial binding to the rest of the IMAC chemistries. pDNA and RNA, however, interacted to varying degrees with free metal ions in the solution. The greatest selectivity in terms of pDNA and RNA separation was achieved with Zn2+ which enabled almost full precipitation of RNA while keeping pDNA soluble. For both immobilised and free metal ions, ionic strength of solution affected the metal ion-nucleic acid interaction significantly. Endotoxin, being more flexible, was able to interact better with the immobilised metal ions than the nucleic acids and showed binding to all the IMAC chemistries. The specific interactions of immobilised and/or free metal ions with pDNA, RNA and endotoxin showed a good potential, by selectively removing RNA and endotoxin at high efficiency, to develop a simplified pDNA purification process with improved process economics.