Metal chelates in vinyl polymerization. I. Ferric dipivaloylmethide as an initiator

The behavior of the chelate, ferric dipivaloylmethide, Fe(DPM)3, in vinyl polymerization systems was investigated. The polymerization was found to be of free-radical nature. The rate of polymerization was proportional to the square root of the concentration of the chelate. The monomer exponent was close to 1.5 for the Fe(DPM)3-initiated polymerization of styrene and methyl methacrylate. The kinetic and transfer constants and activation energies for these systems have been evaluated. Spectral studies revealed the possibility of a complex formation between the chelate and the monomer. A kinetic scheme for the Fe(DPM)3-initiated polymerization is derived based on this initial complex formation.

Metal chelates in vinyl polymerization. II. Mechanism of initiation by Fe(DPM)3

The nature of the interaction between the unsaturated monomer and the chelate, Fe(DPM)3, is studied in detail. The interaction is found to occur only in solution. The stoichiometry of interaction and the equilibrium constant are evaluated. With the help of spectral evidence, attempts are made to point out the specific sites of interaction.

Catalytic vapor-phase oxidation of p-xylene over tin vanadate

Rates of oxidation of p-xylene were measured in the temperature range 320 to 420 °C using tin vanadate as catalyst in an isothermal differential flow reactor. The amounts of p-xylene converted were determined by analyzing the main products (p-tolualdehyde, maleic anhydride, p-toluic acid and traces of terephthalic acid). Negligible amounts of products of complete combustion were formed. The reaction rates obtained for p-xylene followed the relation, Image based on the redox model. The mechanism of the reaction was determined by conducting different sets of experiments and it was found that the reaction followed the parallel-consecutive mechanism, in which p-tolualdehyde and maleic anhydride were formed from the parallel route whereas p-toluic acid was formed from the consecutive route.

Deciphering the Distinct Role for the Metal Coordination Motif in the Catalytic Activity of Mycobacterium smegmatis Topoisomerase I

Mycobacterium smegmatis topoisomerase I (Mstopol) is distinct from typical type IA topoisomerases. The enzyme binds to both single- and double-stranded DNA with high affinity, making specific contacts. The enzyme comprises conserved regions similar to type IA topoisomerases from Escherichia coli and other eubacteria but lacks the typically found zinc fingers in the carboxy-terminal domain. The enzyme can perform DNA cleavage m the absence of Mg2+ but religation needs exogenously added Mg2+. One molecule of Mg2+ tightly bound to the enzyme has no role in DNA cleavage but is needed only for the religation reaction. The toprim. (topoisomerase-primase) domain in MstopoI comprising the Mg2+ binding pocket, conserved in both type IA and type II topoisomerases, was subjected to mutagenesis to understand the role of Mg2+, in different steps of the reaction. The residues D108, D110, and E112 of the enzyme, which form the acidic triad in the DXDXE motif, were changed to alanines. D108A mutation resulted in an enzyme that is Mg2+ dependent for DNA cleavage unlike Mstopol and exhibited enhanced DNA cleavage property and reduced religation activity. The mutant was toxic for cell growth, most likely due to the imbalance in cleavage-religation equilibrium. In contrast, the E112A mutant behaved like wild-type enzyme, cleaving DNA in a Mg2+-independent fashion, albeit to a reduced extent. Intra- and intermolecular religation assays indicated specific roles for D108 and E112 residues during the reaction. Together, these results indicate that the D108 residue has a major role during cleavage and religation, while E112 is important for enhancing the efficiency of cleavage. Thus, although architecturally and mechanistically similar to topoisomerase I from E. coli, the metal coordination pattern of the mycobacterial enzyme is distinct, opening up avenues to exploit the enzyme to develop inhibitors.

Increased Efficacies of an Individual Catalytic Site in Clustered Multivalent Dendritic Catalysts

In the studies reported so far on dendrimer-mediated catalysis, the efficacies of the catalytic units were studied and compared primarily across the generations. In order to identify the efficacy of an individual catalytic unit with respect to the number of such units present within a given generation, a series of catalysts were prepared within a generation. Dendrimers incorporated with phosphinemetal complexes were chosen for the study and as many as 11 catalysts within three generations were synthesized. The C-C bond-forming reactions, namely, the Heck and the Suzuki coupling reactions, were then selected to study the catalytic efficiencies of the series of partially and fully phosphine-metal complex functionalized dendrimers. The efficacies of the formation of cinnamate and biphenyl. catalyzed by the dendritic catalysts, were compared. The comparative analyses show that an individual catalytic site is far more effective in its catalytic activity when presented in multiple numbers, i.e., in a multivalent dendritic system, than as a single unit within the same generation, i.e., in a monovalent dendritic system. The study identifies the beneficial effects of the multivalent presentation of the catalytic moieties, both within and across the dendrimer generations.

Mycobacterium tuberculosis FtsZ requires at least one arginine residue at the C-terminal end for polymerization in vitro

We examined whether C-terminal residues of soluble recombinant FtsZ of Mycobacterium tuberculosis (MtFtsZ) have any role in MtFtsZ polymerization in vitro. MtFtsZ-delta C1, which lacks C-terminal extreme Arg residue (underlined in the C-terminal extreme stretch of 13 residues, DDDDVDVPPFMRR), but retaining the penultimate Arg residue (DDDDVDVPPFMR), polymerizes like full-length MtFtsZ in vitro. However, MtFtsZ-delta C2 that lacks both the Arg residues at the C-terminus (DDDDVDVPPFM), neither polymerizes at pH 6.5 nor forms even single- or double-stranded filaments at pH 7.7 in the presence of 10 mM CaCl2. Neither replacement of the penultimate Arg residue, in the C-terminal Arg deletion mutant DDDDVDVPPFMR, with Lys or His or Ala or Asp (DDDDVDVPPFMK/H/A/D) enabled polymerization. Although MtFtsZ-delta C2 showed secondary and tertiary structural changes, which might have affected polymerization, GTPase activity of MtFtsZ-delta C2 was comparable to that of MtFtsZ. These data suggest that MtFtsZ requires an Arg residue as the extreme C-terminal residue for polymerization in vitro. The polypeptide segment containing C-terminal 67 residues, whose coordinates were absent from MtFtsZ crystal structure, was modeled on tubulin and MtFtsZ dimers. Possibilities for the influence of the C-terminal Arg residues on the stability of the dimer and thereby on MtFtsZ polymerization have been discussed.

Substrate specificity plays an important role in uncoupling the catalytic and scaffolding activities of rat testis DNA topoisomerase IIalpha

Topoisomerase II (topo II) is a dyadic enzyme found in all eukaryotic cells. Topo II is involved in a number of cellular processes related to DNA metabolism, including DNA replication, recombination and the maintenance of genomic stability. We discovered a correlation between the development of postnatal testis and increased binding of topo IIalpha to the chromatin fraction. We used this observation to characterize DNA-binding specificity and catalytic properties of purified testis topo IIalpha. The results indicate that topo IIalpha binds a substrate containing the preferred site with greater affinity and, consequently, catalyzes the conversion of form I to form IV DNA more efficiently in contrast to substrates lacking such a site. Interestingly, topo IIalpha displayed high-affinity and cooperativity in binding to the scaffold associated region. In contrast to the preferred site, however, high-affinity binding of topo IIalpha to the scaffold-associated region failed to result in enhanced catalytic activity. Intriguingly, competition assays involving scaffold-associated region revealed an additional DNA-binding site within the dyadic topo IIalpha. These results implicate a dual role for topo IIalpha in vivo consistent with the notion that its sequestration to the chromatin might play a role in chromosome condensation and decondensation during spermatogenesis.

An Insight to the Dynamics of Conserved Water Molecular Triad in IMPDH II (Human): Recognition of Cofactor and Substrate to Catalytic Arg 322

Inosine 5' monophosphate dehydrogenase (IMPDH II) is a key enzyme involved in the de novo biosynthesis pathway of purine nucleotides and is also considered to be an excellent target for cancer inhibitor design. The conserve R 322 residue (in human) is thought to play some role in the recognition of inhibitor and cofactor through the catalytic D 364 and N 303. The 15 ns simulation and the water dynamics of the three different PDB structures (1B3O, 1NF7, and 1NFB) of human IMPDH by CHARMM force field have clearly indicated the involvement of three conserved water molecules (W-L, W-M, and W-C) in the recognition of catalytic residues (R 322, D 364, and N 303) to inhibitor and cofactor. Both the guanidine nitrogen atoms (NH1 and NH 2) of the R 322 have anchored the di- and mono-nucleotide (cofactor and inhibitor) binding domains via the conserved W-C and W-L water molecules. Another conserved water molecule W-M seems to bridge the two domains including the R 322 and also the W-C and W-L through seven centers H-bonding coordination. The conserved water molecular triad (W-C - W-M - W-L) in the protein complex may thought to play some important role in the recognition of inhibitor and cofactor to the protein through R 322 residue.

Catalytic cyclopropanation of olefins using copper(I) diphosphinoamines

Catalytic cyclopropanation reactions of olefins with ethyl diazoacetate were carried out using copper(I) diphosphinoamine (PPh2)(2)N(R) (R = Pr-i, H, Ph and -CH2-C6H4-CH=CH2) complexes at 40 degrees C in chloroform. High yields of the cyclopropanes were obtained in all cases. The rate of the reaction was influenced by the nuclearity of the complex and the binding mode of the ligand which was either bridging or chelating. Comparison of isostructural complexes shows that the rate follows the order R = Pr-i > H > Ph, where R is the substituent on the N. However, cyclopropane formation versus dimerization of the carbene, and trans to cis ratios of cyclopropane was similar in all cases. The nearly identical selectivity for different products formed was indicative of a common catalytic intermediate. A labile "copper-olefin" complex which does not involve the phosphine or the counterion is the most likely candidate. The differences in the reaction rates for different complexes are attributed to differences in the concentration of the catalytically active species which are in equilibrium with the catalytically inactive copper-phosphinoamine complex. To test the hypothesis a diphosphinoamine polymer complexed to copper(I) was used as a heterogeneous catalyst. Leaching of copper(I) and deactivation of the catalyst confirmed the proposed mechanism. (C) 2008 Elsevier B. V. All rights reserved.

Hydrocarbon oxidation and three-way catalytic activity on a single step directly coated cordierite monolith: High catalytic activity of Ce0.98Pd0.02O2−δ

Catalytic activity of cordierite honeycomb by a completely new coating method for the oxidation of major hydrocarbons in exhaust gas is reported here. The new coating process consists of (a) dipping and growing γ-Al2O3 on cordierite by combustion of monolith dipped in the aqueous solution of Al(NO3)3 and oxalyldihydrazide (ODH) (or glycine) at 600 °C and active catalyst phase Ce0.98Pd0.02O2−δ on γ-Al2O3-coated cordierite again by combustion of monolith dipped in the aqueous solution of ceric ammonium nitrate, ODH and 1.2 × 10−3 M PdCl2 solution at 500 °C. Weight of active catalyst can be varied from 0.02 wt% to 2 wt% which is sufficient but can be loaded even up to 12 wt% by repeating dip dry combustion. Adhesion of catalyst to cordierite surface is via oxide growth, which is very strong. ‘HC’ oxidation over the monolith catalyst is carried out with a mixture having the composition, 470 ppm of both propene and propane and 870 ppm of both ethylene and acetylene with the varying amount of O2. Three-way catalytic test is done by putting hydrocarbon mixture along with CO (10 000 ppm), NO (2000 ppm) and O2 (15 000 ppm). Below 350 °C full conversion is achieved. In this method, handling of nano-material powder is avoided.

A Unique Nickel System having Versatile Catalytic Activity of Biological Significance

A new dinuclear nickel(II) complex, [Ni-2(LH2)(H2O)(2)(OH)(NO3)](NO3)(3) (1), of an ``end-off'' compartmental ligand 2,6-bis(N-ethylpiperazine-iminomethyl)-4-methyl-phenolato, has been synthesized and structurally characterized. The X-ray single crystal structure analysis shows that the piperazine moieties assume the expected chair conformation and are protonated. The complex 1 exhibits versatile catalytic activities of biological significance, viz. catecholase, phosphatase, and DNA cleavage activities, etc. The catecholase activity of the complex observed is very dependent on the nature of the solvent. In acetonitrile medium, the complex is inactive to exhibit catecholase activity. On the other hand, in methanol, it catalyzes not only the oxidation of 3,5-di-tert-butylcatechol (3,5-DTBC) but also tetrachlorocatechol (TCC), a catechol which is very difficult to oxidize, under aerobic conditions. UV vis spectroscopic investigation shows that TCC oxidation proceeds through the formation of an intermediate. The intermediate has been characterized by an electron spray ionizaton-mass spectrometry study, which suggests a bidentate rather than a monodentate mode of TCC coordination in that intermediate, and this proposition have been verified by density functional theory calculation. The complex also exhibits phosphatase (with substrate p-nitrophenylphosphate) and DNA cleavage activities. The DNA cleavage activity exhibited by complex 1 most probably proceeds through a hydroxyl radical pathway. The bioactivity study suggests the possible applications of complex 1 as a site specific recognition of DNA and/or as an anticancer agent.

A Unique Nickel System having Versatile Catalytic Activity of Biological Significance

A new dinuclear nickel(II) complex, [Ni-2(LH2)(H2O)(2)(OH)(NO3)](NO3)(3) (1), of an "end-off" compartmental ligand 2,6-bis(N-ethylpiperazine-iminomethyl)-4-methyl-phenolato, has been synthesized and structurally characterized. The X-ray single crystal structure analysis shows that the piperazine moieties assume the expected chair conformation and are protonated. The complex 1 exhibits versatile catalytic activities of biological significance, viz. catecholase, phosphatase, and DNA cleavage activities, etc. The catecholase activity of the complex observed is very dependent on the nature of the solvent. In acetonitrile medium, the complex is inactive to exhibit catecholase activity. On the other hand, in methanol, it catalyzes not only the oxidation of 3,5-di-tert-butylcatechol (3,5-DTBC) but also tetrachlorocatechol (TCC), a catechol which is very difficult to oxidize, under aerobic conditions. UV vis spectroscopic investigation shows that TCC oxidation proceeds through the formation of an intermediate. The intermediate has been characterized by an electron spray ionizaton-mass spectrometry study, which suggests a bidentate rather than a monodentate mode of TCC coordination in that intermediate, and this proposition have been verified by density functional theory calculation. The complex also exhibits phosphatase (with substrate p-nitrophenylphosphate) and DNA cleavage activities. The DNA cleavage activity exhibited by complex 1 most probably proceeds through a hydroxyl radical pathway. The bioactivity study suggests the possible applications of complex 1 as a site specific recognition of DNA and/or as an anticancer agent.

Recognition of active and inactive catalytic triads: A template based approach

It Is well established that a sequence template along with the database is a powerful tool for identifying the biological function of proteins. Here, we describe a method for predicting the catalytic nature of certain proteins among the several protein structures deposited in the Protein Data Bank (PDB) For the present study, we considered a catalytic triad template (Ser-His-Asp) found in serine proteases We found that a geometrically optimized active site template can be used as a highly selective tool for differentiating an active protein among several inactive proteins, based on their Ser-His-Asp interactions. For any protein to be proteolytic in nature, the bond angle between Ser O-gamma-Ser H-gamma His N-epsilon 2 in the catalytic triad needs to be between 115 degrees and 140 degrees The hydrogen bond distance between Ser H-gamma His N-epsilon 2 is more flexible in nature and it varies from 2 0 angstrom to 27 angstrom while in the case of His H-delta 1 Asp O-delta 1, it is from 1.6 angstrom to 2.0 angstrom In terms of solvent accessibility, most of the active proteins lie in the range of 10-16 angstrom(2), which enables easy accessibility to the substrate These observations hold good for most catalytic triads and they can be employed to predict proteolytic nature of these catalytic triads (C) 2010 Elsevier B V All rights reserved.

Synthesis and characterization of polyaniline salts with phenoxy acetic acids by emulsion polymerization

Polyaniline salts have been synthesized by chemical oxidative polymerization of aniline in the presence of phenoxy acetic acid and its two derivatives using emulsion method at room temperature and characterized by different techniques such as infrared, H-1 and C-13 NMR, UV-visible spectroscopy, SEM, wide angle X-ray diffractograms and conductivity measurements. These polyaniline salts have the desirable property of high solubility for processibility in solvents such as DNIF, DMSO and a mixture of CHCl3 and acetone and they exhibit fairly good conductivity of similar to 3.0 x 10(-3) S cm(-1). The variations in solubility, conductivity and morphology with the protonating strength of the dopants are examined.