Synthesis and catalytic properties of cobalt- and molybdenum-containing mesoporous MCM-41 molecular sieves

Mesoporous MCM-41 type silicas containing molybdenum and cobalt have been prepared with pore sizes in the range 30-38 Angstrom and 54-59 Angstrom. Catalytic properties of these materials have been examined with respect to the oxidation of cyclooctene and aniline.

Role of Water in the Enzymatic Catalysis: Study of ATP plus AMP -> 2ADP Conversion by Adenylate Kinase

The catalytic conversion ATP + AMP -> 2ADP by the enzyme adenylate kinase (ADK) involves the binding of one ATP. molecule to the LID domain and one AMP molecule to the NMP domain. The latter is followed by a. phosphate transfer and then the release of two ADP molecules. We have computed a novel two-dimensional configurational free energy surface (2DCFES), with one reaction coordinate each for the LID and the NMP domain motions, while considering explicit water interactions. Our computed 2DCFES clearly reveals the existence of a stable half-open half-closed (HOHC) intermediate stale of the enzyme. Cycling of the enzyme through the HOHC state reduces the conformational free energy barrier for. the reaction by about 20 kJ/mol. We find that the stability of the HOHC state (missed in all earlier studies with implicit solvent model) is largely because of the increase of specific interactions of the polar amino acid side chains with water, particularly with the arginine and the histidine residues. Free energy surface of the LID domain is rather rugged, which can conveniently slow down LID's conformational motion, thus facilitating a new substrate capture after the product release in the catalytic cycle.

Ammonia Channeling in Plasmodium falciparum GMP Synthetase: Investigation by NMR Spectroscopy and Biochemical Assays

GMP synthetase, a class I amidotransferase, catalyzes the last step of the purine biosynthetic pathway, where ammonia from glutamine is incorporated into xanthosine 5'-monophospate to yield guanosine 5'-monnophosphate as the main product. Combined biochemical, structural, and computational studies of glutamine amidotransferases have revealed the existence of physically separate active sites connected by molecular tunnels that efficiently transfer ammonia from the glutaminase site to the synthetase site. Here, we have investigated aspects of ammonia channeling in P. falciparum GMP synthetase using biochemical assays in conjunction with N-15-edited proton NMR spectroscopy. Our results suggest that (1) ammonia released from glutamine is not equilibrated with the external medium (2) saturating concentrations of glutamine do not obliterate the incorporation of external ammonia into GMP, and (3) ammonia in the external medium can access the thioester intermediate when the ATPPase domain is bound to substrates. Further, mutation of Cys-102 to alanine confirmed its identity as the catalytic residue in the glutaminase domain, and ammonia-dependent assays on the mutant indicated glutamine to be a partial uncompetitive inhibitor of the enzyme.

Enhanced sensitivity for the differential pulse polarographic determination of molybdenum(VI) based on adsorption catalytic current

A differential pulse polarographic (DPP) method based on the adsorption catalytic current in a medium containing chlorate and 8-hydroxyquinoline (oxine) is suggested for the determination of molybdenum(VI). Experimental conditions such as pH and the composition of supporting electrolyte have been optimized to get a linear calibration graph at trace levels of Mo(VI). The sensitivity for molybdenum can be considerably enhanced by this method. The influence of possible interferences on the catalytic current has been investigated. The sensitivity of the method is compared with those obtained for other DPP methods for molybdenum. A detection limit of 1.0 x 10(-8) mol/L has been found.

Cloning, expression, purification, crystallization and preliminary X-ray studies of the mannose-binding lectin domain of MSMEG_3662 from Mycobacterium smegmatis

The mannose-binding lectin domain of MSMEG_3662 from Mycobacterium smegmatis has been cloned, expressed, purified and crystallized and the crystals have been characterized using X-ray diffraction. The Matthews coefficient suggests the possibility of two lectin domains in the triclinic cell. The amino-acid sequence of the domain indicates structural similarity to well characterized beta-prism II fold lectins.

Synthesis and esterolytic chemistry of some (dialkylamino) pyridine-functionalized micellar aggregates. Evidence of catalytic turnover

Four new (dialkylamino)pyridine-functionalized surfactants have been synthesized. Micelles were generated either from the surfactant alone in aqueous buffer (pH 8.5 or 9.0) or by comicellization in 1 x 10(-3)-1 x 10(-4) M aqueous micellar cetyltrimethylammonium bromide (CTABr) solution at pH 8.5 or 9.0. Such aggregates were used to cleave p-nitrophenyl alkanoates or p-nitrophenyl diphenylphosphate. The nucleophilic reagents and the second-order ''catalytic'' rate constants toward esterolysis of the substrate p-nitrophenyl octanoate (at 25 degrees C, pH 9.0) were [cat.] = 1 x 10(-4) M, [CTABr] = 1 x 10(-3) M, and k(cat.) = 440.13 M(-1) s(-1) for 1b, [cat.] = 5 x 10(-4) M, [CTABr] = 5 x 10(-4) M, and k(cat.) = 30.8 M(-1) s(-1) for 1c, [cat.] = 5 x 10(-4) M, [CTABr] = 5 x 10(-3) M, and k(cat.) = 183.64 M(-1) s(-1) for 2a, and [cat.] = 3 x 10(-4) M and k(cat.) = 54.1 M(-1) s(-1) for 2b. The catalytic systems, especially 1b/CTABr and 2a/CTABr, also conferred significantly greater reactivity toward the esters derived from alkanoic acids of moderate chain length (C-6-C-10) during hydrolytic cleavages relative to their shorter and longer counterparts. Importantly, the catalytic systems comprising the coaggregates of either neutral 1b and CTABr (1:10) or anionic 2a and CTABr (1:10) conformed to the Michaelis-Menten kinetic scheme and demonstrated turnover behavior in the presence of excess substrate.

Influence of growth below and above T-c on the morphology and domain structure in flux-grown KTP crystals

KTP crystals have been grown below and above the ferroelectric transition temperature by flux method employing both spontaneous and top-seeded solution growth techniques. A slight morphological difference has been observed in these crystals when grown below and above the T-c. Ferroelectric domains are studied in these crystals by selective domain etching. It is seen that the ferroelectric domains in crystals grown spontaneously below T, show a complicated structure. A systematic investigation of the factors influencing domain structure has been carried out. Stress to some extent has been shown to affect the domain structure. Finally, a convenient way of converting the multidomain crystals into monodomain ones is described.

Stereospecific and regioselective catalytic epoxidation of alkenes by a novel ruthenium(II) complex under aerobic conditions

Epoxidation of alkenes by molecular oxygen is effected in high yields by catalysis of RuCl2(biox)(2) using isobutyraldehyde as the co-reductant: the reaction is stereospecific and regioselective.

Ultrafast Microwave-Assisted Route to Surfactant-Free Ultrafine Pt Nanoparticles on Graphene: Synergistic Co-reduction Mechanism and High Catalytic Activity

We demonstrate an ultrafast method for the formation of, graphene supported Pt catalysts by the co-reduction of graphene oxide and Pt salt using ethylene glycol under microwave irradiation conditions. Detailed analysis of the mechanism of formation of the hybrids indicates a synergistic co-reduction mechanism whereby the presence of the Pt ions leads to a faster reduction of GO and the presence of the defect sites on the reduced GO serves as anchor points for the heterogeneous nucleation of Pt. The resulting hybrid consists of ultrafine nanoparticles of Pt uniformly distributed on the reduced GO susbtrate. We have shown that the hybrid exhibits good catalytic activity for methanol oxidation and hydrogen conversion reactions. The mechanism is general and applicable for the synthesis of other multifunctional hybrids based on graphene.

Structures of the catalytic site mutants D99A and H48Q and the calcium-loop mutant D49E of phospholipase A(2)

Crystal structures of the active-site mutants D99A and H48Q and the calcium-loop mutant D49E of bovine phospholipase A(2) have been determined at around 1.9 Angstrom resolution. The D99A mutant is isomorphous to the orthorhombic recombinant enzyme, space group P2(1)2(1)2(1), The H48Q and the calcium-loop mutant D49E are isomorphous to the trigonal recombinant enzyme, space group P3(1)21, The two active-site mutants show no major structural perturbations. The structural water is absent in D99A and, therefore, the hydrogen-bonding scheme is changed. In H48Q, the catalytic water is present and hydrogen bonded to Gln48 N, but the second water found in native His48 is absent. In the calcium-loop mutant D49E, the two water molecules forming the pentagonal bipyramid around calcium are absent and only one O atom of the Glu49 carboxylate group is coordinated to calcium, resulting in only four ligands.

Catalytic oxidative polymerization of 1,1-diphenylethylene at ambient temperature and potential application of peroxide macroinitiator

The Co(II)TPP(Py) complex was used as an efficient dioxygen carrier for the radical polymerization of 1,1-diphenylethylene (DPE), which has a low ceiling temperature, at ambient temperature and low oxygen pressure. The mechanism of polymerization is discussed' on the basis of kinetic data, W-vis, ESR, and H-1 NMR studies. The rate of polymerization (RP) and number-average molecular weights (M) of poly(1,1-diphenylethylene peroxide) (PDPEP) are higher and the polydispersity is lower than in 2,2'-azobis(isobutyronitrile) (AIBN) initiated polymerization. PDPEP was further. used as a macroinitiator for the polymerization of MMA. The polymerization obeys classical kinetics. The K-2 value of the PDPEP has been determined from the slope of R-P(2) VS [M](2)[I], which reveals that it can also be used at higher temperature for the polymerization. An "active" PMMA was also synthesized, containing initiating segments in the polymer backbone.

Characterization and catalytic properties of combustion synthesized Au/CeO2 catalyst

Ceria-supported Au catalyst has been synthesized by the solution combustion method for the first time and characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). Au is dispersed as Au as well as Au3+ states on CeO2 surface of 20-30 nm crystallites. On heating the as-prepared 1% Au/CeO2 in air, the concentration of Au3- ions on CeO2 increases at the expense of Au. Catalytic activities for CO and hydrocarbon oxidation and NO reduction over the as-prepared and the heat-treated 1% Au/CeO2 have been carried out using a temperature-programmed reaction technique in a packed bed tubular reactor. The results are compared with nano-sized Au metal particles dispersed on alpha-Al2O3 substrate prepared by the same method. All the reactions over heat-treated Au/CeO2 occur at lower temperature in comparison with the as-prepared Au/CeO2 and Au/Al2O3. The rate of NO + CO reaction over as-prepared and heat-treated 1% Au/CeO2 are 28.3 and 54.0 mumol g(-1) s(-1) at 250 and 300 degreesC respeceively. Activation energy (E,) values are 106 and 90 kJ mol(-1) for CO + O-2 reaction respectively over as-prepared and heat-treated 1% Au/CeO2 respectively.

Mechanistic investigations on the efficient catalytic decomposition of peroxynitrite by ebselen analogues

In this study, ebselen and its analogues are shown to be catalysts for the decomposition of peroxynitrite (PN). This study suggests that the PN-scavenging ability of selenenyl amides can be enhanced by a suitable substitution at the phenyl ring in ebselen. Detailed mechanistic studies on the reactivity of ebselen and its analogues towards PN reveal that these compounds react directly with PN to generate highly unstable selenoxides that undergo a rapid hydrolysis to produce the corresponding seleninic acids. The selenoxides interact with nitrite more effectively than the corresponding seleninic acids to produce nitrate with the regeneration of the selenenyl amides. Therefore, the amount of nitrate formed in the reactions mainly depends on the stability of the selenoxides. Interestingly, substitution of an oxazoline moiety on the phenyl ring stabilizes the selenoxide, and therefore, enhances the isomerization of PN to nitrate.

Effect of convection on domain growth during demixing transitions in fluids

Current analytical work on the effect of convection on the late stages of spinodal decomposition in liquids is briefly described. The morphology formed during the spinodal decomposition process depends on the relative composition of the two species. Droplet spinodal decomposition occurs when the concentration of one of the species is small. Convective transport has a significant effect on the scaling laws in the late-stage coarsening of droplets in translational or shear flows. In addition, convective transport could result in an attractive interaction between non-Brownian droplets which could lead to coalescence. The effect of convective transport for the growth of random interfaces in a near-symmetric quench was analysed using an area distribution function, which gives the distribution of surface area of the interface in curvature space. It was found that the curvature of the interface decreases proportional to time t in the late stages of spinodal decomposition, and the surface area also decreases proportional to t.