Interaction of metal ions with nucleic acids and related compounds. II. Studies on Au(III)-nucleic acid system

The nature of interaction of Au(III) with nucleic acids was studied by using methods such as uv and ir spectrophotometry, viscometry, pH titrations, and melting-temperature measurements. Au(III) is found to interact slowly with nucleic acids over a period of several hours. The uv spectra of native calf-thymus DNA 9pH 5.6 acetate buffer containing (0.01M NaCIO4) showed a shift in λ max to high wavelengths and an increase in optical density at 260 nm. There was a fourfold decrease in viscosity (expressed as ηsp/c). The reaction was faster at pH 4.0 and also with denatured DNA (pH 5.6) and whole yeast RNA (pH 5.6). The order of preference of Au(III) (as deduced from the time of completion of reaction) for the nucleic acids in RNA > denatured DNA > DNA. The reaction was found to be completely reversible with respect KCN. Infrared spectra of DNA-Au(III) complexes showed binding to both the phosphate and bases of DNA. The same conclusions were also arrived at by melting-temperature studies of Au(III)-DNA system. pH titrations showed liberation of two hydroxylions at r = 0.12 [r = moles of HAuCl4 added per mole of DNA-(P)] and one hydrogen ion at r = 0.5. The probable binding sites could be N(1)/N(7) of adenine, N(7) and/or C(6)O of guanine, N(3) of cytosine and N(3) of thymine. DNAs differing in their (G = C)-contents [Clostridium perfingens DNA(G = C, 29%), salmon sperm DNA (G + C, 42%) and Micrococcus lysodeikticus DNA(G + C, 29%), salmon sperm DNA (G = C, 72%)] behaved differently toward Au(III). The hyperchromicity observed for DNAs differing in (G + C)-content and cyanide reversal titrations indicate selectivity toward ( A + T)-rich DNA at lw values of r. Chemical analysis and job's continuous variation studies indicated the existence of possible complexes above and below r = 1. The results indicate that Au(III) ions probably bind to hte phosphate group in the initial stages of the reaction, particularly at low values of r, and participation of the base interaction also increases. Cross-linking of the two strands by Au(III) may take place, but a complete collapse of the doulbe helix is not envisaged. It is probable that tilting of the bases or rotaiton of the bases around the glucosidic bond, resulting in a significant distrotion of the double helix, might take place due to binding of Au(III) to DNA.

Influence of Geometrical Factors on the Physico-Chemical Properties of Perovskite and Layered Perovskite Oxides

A brief qualitative comparison is made of perovskite ABO sub 3 and layered perovskite ABO sub 3 and layered perovskite A sub 2 BO sub 4 oxides with special emphasis on the influence of geometrical factors on certain physico-chemical properties. The layered perovskite oxides are distinguished from three-dimensional oxides by a looser packing, frustration in three-dimensional interactions, more internal pressure on B--O bonds for small tolerance factors, and by different values of site-percolation thresholds. Their influence on electronic configurations of metal ions, stabilities and syntheses of compounds is discussed. The influence of increased anisotropy in layered oxides on localisation of charge carriers and in suppressing the onset of long-range ferromagnetic ordering is also discussed.

Modelling the chemical weathering fluxes at the watershed scale in the Tropics (Mule Hole, South India): Relative contribution of the smectite/kaolinite assemblage versus primary minerals

We investigate the chemical weathering processes and fluxes in a small experimental watershed (SEW) through a modelling approach. The study site is the Mule Hole SEW developed on a gneissic basement located in the climatic gradient of the Western Ghats, South India. The model couples a lumped hydrological model simulating the water budget at the watershed scale to the WITCH model estimating the dissolution/precipitation rates of minerals using laboratory kinetic laws. Forcing functions and parameters of the simulation are defined by the field data. The coupled model is calibrated with stream and groundwater compositions through the testing of a large range of smectite solubility and abundance in the soil horizons. We found that, despite the low abundance of smectite in the dominant soil type of the watershed (4 vol.%), their net dissolution provides 75% of the export of dissolved silica, while primary silicate mineral dissolution releases only 15% of this flux. Overall, smectites (modelled as montmorillonites) are not stable under the present day climatic conditions. Furthermore, the dissolution of trace carbonates in the saprolitic horizon provides 50% of the calcium export at the watershed scale. Modelling results show the contrasted behavior of the two main soil types of the watershed: red soils (88% of the surface) are provider of calcium, while black soils (smectite-rich and characterized by a lower drainage) consumes calcium through overall carbonate precipitation. Our model results stress the key role played by minor/accessory minerals and by the thermodynamic properties of smectite minerals, and by the drainage of the weathering profiles on the weathering budget of a tropical watershed. (C) 2010 Elsevier B.V. All rights reserved.

Large low-frequency resistance noise in chemical vapor deposited graphene

We report a detailed investigation of resistance noise in single layer graphene films on Si/SiO2 substrates obtained by chemical vapor deposition (CVD) on copper foils. We find that noise in these systems to be rather large, and when expressed in the form of phenomenological Hooge equation, it corresponds to Hooge parameter as large as 0.1-0.5. We also find the variation in the noise magnitude with the gate voltage (or carrier density) and temperature to be surprisingly weak, which is also unlike the behavior of noise in other forms of graphene, in particular those from exfoliation. (C) 2010 American Institute of Physics. doi:10.1063/1.3493655]

Hyperfine effects in the nuclear magnetic resonance of paramagnetic molecules

Paramagnetic, or open-shell, systems are often encountered in the context of metalloproteins, and they are also an essential part of molecular magnets. Nuclear magnetic resonance (NMR) spectroscopy is a powerful tool for chemical structure elucidation, but for paramagnetic molecules it is substantially more complicated than in the diamagnetic case. Before the present work, the theory of NMR of paramagnetic molecules was limited to spin-1/2 systems and it did not include relativistic corrections to the hyperfine effects. It also was not systematically expandable. --- The theory was first expanded by including hyperfine contributions up to the fourth power in the fine structure constant α. It was then reformulated and its scope widened to allow any spin state in any spatial symmetry. This involved including zero-field splitting effects. In both stages the theory was implemented into a separate analysis program. The different levels of theory were tested by demonstrative density functional calculations on molecules selected to showcase the relative strength of new NMR shielding terms. The theory was also tested in a joint experimental and computational effort to confirm assignment of 11 B signals. The new terms were found to be significant and comparable with the terms in the earlier levels of theory. The leading-order magnetic-field dependence of shielding in paramagnetic systems was formulated. The theory is now systematically expandable, allowing for higher-order field dependence and relativistic contributions. The prevailing experimental view of pseudocontact shift was found to be significantly incomplete, as it only includes specific geometric dependence, which is not present in most of the new terms introduced here. The computational uncertainty in density functional calculations of the Fermi contact hyperfine constant and zero-field splitting tensor sets a limit for quantitative prediction of paramagnetic shielding for now.

Sonochemical synthesis of Ce1-xFexO2-delta (0 <= x <= 0.45) and Ce0.65Fe0.33Pd0.02O2-delta nanocrystallites: oxygen storage material, CO oxidation and water gas shift catalyst

Nanocrystalline Ce1-xFexO2-delta (0 <= x <= 0.45) and Ce0.65Fe0.33Pd0.02O2-delta of similar to 4 nm sizes were synthesized by a sonochemical method using diethyletriamine (DETA) as a complexing agent. Compounds were characterized by powder X-ray diffraction (XRD), X-ray photo-electron spectroscopy (XPS) and transmission electron microscopy (TEM). Ce1-xFexO2-delta (0 <= x <= 0.45) and Ce0.65Fe0.33Pd0.02O2-delta crystallize in fluorite structure where Fe is in +3, Ce is in +4 and Pd is in +2 oxidation state. Due to substitution of smaller Fe3+ ion in CeO2, lattice oxygen is activated and 33% Fe substituted CeO2 i.e. Ce0.67Fe0.33O1.835 reversibly releases 0.31O] up to 600 degrees C which is higher or comparable to the oxygen storage capacity of CeO2-ZrO2 based solid solutions (Catal. Today 2002, 74, 225-234). Due to interaction of redox potentials of Pd2+/0(0.89 V) and Fe3+/2+ (0.77 V) with Ce4+/3+ (1.61 V), Pd ion accelerates the electron transfer from Fe2+ to Ce4+ in Ce0.65Fe0.33Pd0.02O1.815, making it a high oxygen storage material as well as a highly active catalyst for CO oxidation and water gas shift reaction. The activation energy for CO oxidation with Ce0.65Fe0.33Pd0.02O1.815 is found to be as low as 38 kJ mol(-1). Ce0.67Fe0.33O1.835 and Ce0.65Fe0.33Pd0.02O1.815 have also shown high activity for the water gas shift reaction. CO conversion to CO2 is 100% H-2 specific with these catalysts and conversion rate was found to be as high 27.2 mu moles g(-1) s(-1) and the activation energy was found to be 46.4 kJ mol(-1) for Ce0.65Fe0.33Pd0.02O1.815.

Octabromotetraphenylporphyrin and its metal derivatives: Electronic structure and electrochemical properties

The free-base octabromotetraphenylporphyrin (H2OBP) has been prepared by a novel bromination reaction of (meso-tetraphenylporphyrinato)copper(II). The metal [V(IV)O, Co(II), Ni(II), Cu(II), Zn(II), Pd(II), Ag(II), Pt(II)] derivatives exhibit interesting electronic spectral features and electrochemical redox properties. The electron-withdrawing bromine substituents at the pyrrole carbons in H2OBP and M(OBP) derivatives produce remarkable red shifts in the Soret (50 nm) and visible bands (100 nm) of the porphyrin. The low magnitude of protonation constants (pK3 = 2.6 and pK4 = 1.75) and the large red-shifted Soret and visible absorption bands make the octabromoporphyrin unique. The effect of electronegative bromine substituents at the peripheral positions of the porphyrin has been quantitatively analyzed by using the four-orbital approach of Gouterman. A comparison of MO parameters of MOBP derivatives with those of the meso-substituted tetraphenylporphyrin (M(TPP)) and unsubstituted porphine (M(P)) derivatives provides an explanation for the unusual spectral features. The configuration interaction matrix element of the M(OBP) derivatives is found to be the lowest among the known substituted porphyrins, indicating delocalization of ring charge caused by the increase in conjugation of p orbitals of the bromine onto the ring orbitals. The electron-transfer reactivities of the porphyrins have been dramatically altered by the peripheral bromine substituents, producing large anodic shifts in the ring and metal-centered redox potentials. The increase in anodic shift in the reduction potential of M(OBP)s relative to M(TPP)s is found to be large (550 mV) compared to the shift in the oxidation potential (300 mV). These shifts are interpreted in terms of the resonance and inductive interactions of the bromine substituents.

Relation between effective atomic charge and chemical shifts in X-ray absorption spectra of transition-metal compounds

Chemical shifts of K absorption discontinuities, Delta E, of several manganese, iron and cobalt oxides with the metal in the formal oxidation states between +2 and +4, have been measured. These data, together with data in the literature on other compounds of these metals, can be fitted into the expression Delta E=aq+bq2, where q is the effective atomic charge on the metal. Theoretical considerations also support this functional relationship between Delta E and q.

29Si NMR chemical shifts in silicates

29Si chemical shifts in a wide variety of silicates in crystalline, glassy and gel states have been related to a parameter, P, which takes into account the electronegativity and the structural description of the silicate units as well as the ionic potential of the modifier cation. The relation, δ(ppm)=28.4 [1−exp(−P)]−110.5, besides having predictive value, satisfactorily accounts for all the available chemical-shifts data on silicates and shows the right kind of limiting behaviour, with δ approaching the Q0 value at large P.

Temperature and concentration dependence of adsorption properties of methane in NaY: a molecular dynamics study

Molecular dynamics calculations on methane sorbed in NaY (Si/Al = 3.0) employing realistic methane-methane and methane-zeolite intermolecular potential functions at different temperatures (50, 150, 220, and 300 K) and concentrations (2, 4, 6, and 8 molecules/cage) are reported. The thermodynamic results are in agreement with the available experimental data. Guest-guest and guest-host radial distribution functions (rdfs), energy distribution functions, distribution of cage occupancy, center-of-cage-center-of-mass (coc-com) rdfs, velocity autocorrelation functions for com and angular motion and the Fourier transformed power spectra, and diffusion coefficients are presented as a function of temperature and concentration. At 50 K, methane is localized near the adsorption site. Site-site migration and essentially free rotational motion are observed at 150 K. Molecules preferentially occupy the region near the inner surface of the alpha-cage. The vibrational frequencies for the com of methane shift toward higher values with decreasing temperature and increasing adsorbate concentration. The observed frequencies for com motion are 36, 53, and 85 cm-1 and for rotational motion at 50 K, 95 and 150 cm-1 in agreement with neutron scattering data. The diffusion coefficients show a type I behavior as a function of loading in agreement with NMR measurements. Cage-to-cage diffusion is found to be always mediated by the surface.