Methanol oxidation on carbon-supported platinum-tin electrodes in sulfuric acid

A study is made of the electrooxidation of methanol in sulfuric acid on carbon-supported electrodes containing platinum-tin bimetal catalysts that are prepared by an in situ potentiometric-characterization route. The catalysts are investigated by employing chemical analyses, X-ray diffraction (XRD), X-ray absorption-near-edge spectroscopy (XANES) and X-ray photoelectron spectroscopy (XPS) data in conjunction with electrochemical measurements. From the electrochemical data, it is inferred that while an electrode with (3:1) Pt-Sn/C catalyst involves a two-electron rate-limiting step akin to platinum-on-carbon electrodes, it is shifted to a one-electron mechanism on electrodes with (3:2)Pt-Sn/C, (3:3)Pt-Sn/C, and (3:4)Pt-Sn/C catalysts. The study suggests that the tin content in the platinum-tin bimetal catalyst produces: (i) a charge transfer from tin to platinum; (ii) an increase in the coverage of adsorbed methanolic residues with increase in the tin content, as indicated by the shift in rest potential of the electrodes towards the reversible value for oxidation of methanol (0.043 V versus SHE), and (iii) a decrease in the overall content of higher valent platinum sites in the catalyst.

Methanol oxidation on carbon-supported Pt---Sn electrodes in silicotungstic acid

Electro-oxidation of methanol was studied on carbon-supported Pt-Sn/C electrodes in silcotungstic acid (SiWA) at various concentrations. The porous-carbon electrodes employing Pt-Sn/C catalyst have been characterized using chemical analyses, X-ray powder diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) in conjunction with electrochemistry. The presence of Pt-Sn and Pt3Sn alloys along with Pt and SnO2 phases in the catalyst were identified by XRD. XPS analysis showed a lower amount of PtO species in the Pt-Sn/C catalyst with respect to the corresponding Pt/C sample. From the steady-state galvanostatic polarization data on Pt-Sn/C electrodes in SiWA, it is inferred that a one-electron process is the rate determining step. The performance of the electrodes in 0.084 M SiWA was better than in 2.5 M H2SO4 under similar conditions up to load currents of about 100 mA cm-2 indicating the promoting behaviour of the electrolyte. At currents larger than 100 mA cm-2, the performance of the electrodes in 0.084 SiWA was poorer than that in 2.5M H2SO4 mainly due to the dominance of mass polarization in the former owing to the large size of Keggin units associated with the structure of SiWA. This aspect was supported by cyclic voltammetry and ac impedance studies on Pt-Sn/C electrodes. Simulation of the electrochemical impedance response for the oxidation of methanol in SiWA was carried out using the equivalent electrical circuit model.

Performance of a pipelined ring algorithm for Fast Fourier Transform on transputer arrays

In this paper a pipelined ring algorithm is presented for efficient computation of one and two dimensional Fast Fourier Transform (FFT) on a message passing multiprocessor. The algorithm has been implemented on a transputer based system and experiments reveal that the algorithm is very efficient. A model for analysing the performance of the algorithm is developed from its computation-communication characteristics. Expressions for execution time, speedup and efficiency are obtained and these expressions are validated with experimental results obtained on a four transputer system. The analytical model is then used to estimate the performance of the algorithm for different number of processors, and for different sizes of the input data.

Rapid purification of tRNA(Lys) from rat liver

Fast protein liquid chromatography (FPLC) system using Mono Q (HR 5/5) anion-exchange column chromatography followed by highly cross-linked urea-polyacrylamide gel electrophoresis (urea-PAGE) was used for the purification of lysine-specific tRNA (tRNA(Lys)) from rat liver. Crude tRNA from rat liver was fractionated with a linear gradient of NaCl (0.3-0.8 M) in triethanolamine-HCl buffer, pH 4.5, and the activity of tRNA(Lys) was found to elute between 0.51 and 0.57 M NaCl. Using this concentration range of NaCl, tRNA(Lys) was refractionated on the same column with a shallow gradient, where a single peak of tRNA(Lys) activity was obtained. tRNA(Lys)-rich fractions recovered from the second run were electrophoretically separated on 16% polyacrylamide-7 M urea gel into one major band and three minor bands. The major band showed a specific activity of 997 pmols/A260 U for tRNALys with a 43-fold purification and approximately 17% recovery. The minor bands displayed negligible or no activity for lysine. tRNA(Lys) obtained by this method was found to be homogeneous by competitive aminoacylation. The advantages of FPLC followed by urea-PAGE in the purification of an amino acid-specific tRNA over conventional column chromatography are discussed.

Synthon polymorphism and pseudopolymorphism in co-crystals. The 4,4 `-bipyridine-4-hydroxybenzoic acid structural landscape

Co-crystals of 4,4'-bipyridine and 4-hydroxybenzoic acid (1 : 2) show synthon polymorphism with the former being more stable. A 2 : 1 co-crystal is pseudopolymorphic within the same structural landscape with the structural roles of the two bipyridine N-atoms being distinct, as evidenced by mimicry by 4-phenylpyridine.

Regulation of diacylglycerol kinase in rat brain membranes by docosahexaenoic acid

The effect of docosahexaenoic acid (DHA) on the diacylglycerol kinase (DG kinase) activity in rat brain membranes was investigated. DHA at 500 mu M concentration, stimulated the enzyme activity by about 2 fold. This effect was concentration-and time-dependent and was observed after very short periods of incubation (one min). DHA stimulation of DG kinase was observed only with rat brain membranes, and not with rat brain cytosol or rat liver membranes. Treating the rat brain membranes with phospholipase A(2) which released free fatty acids including DHA, significantly stimulated the DG kinase activity. It is concluded that DHA through its stimulatory effect on DG kinase may regulate the signalling events in growth-related situations in the brain such as synaptogenesis.

Asymmetric Diels-Alder Reactions of Chiral Acrylates of Cholic Acid Derivatives

New steroid-based chiral auxiliaries 6, 9, and 12 have been synthesized from readily available cholic acid. These new chiral auxiliaries place the reactive and the shielding sites in a 1,5 relationship to each other. Diels-Alder reaction of cyclopentadiene with corresponding acrylate esters (7, 10, and 13) have been examined. Acrylates 7 and 10 yielded cycloadducts with 29-88% diastereomeric excess with excellent endo selectivity in the presence of an excess of Lewis acids such as AlCl3, BF3.OEt(2), FeCl3, SnCl4, TiCl4, and ZnCl2. Treatment of acrylate 7 with cyclopentadiene in the presence of BF3.OEt(2) at -80 degrees C gave the endo adduct (>99%) with 88% de. Lewis acid catalyzed and uncatalyzed reactions of acrylates 7 and 10 with cyclopentadiene yielded cycloadducts with opposite stereochemistry. The chiral auxiliary was recovered in a nondestructive manner only via iodolactonization. Acrylate ester of alcohol 12 did not show any selectivity in either catalyzed and uncatalyzed reactions with cyclopentadiene. The presence of a flat aromatic surface at C-7 of the steroid was found to be essential to effect high diastereoselection.

High-Throughput Study of the Cu(CH3COO)(2)center dot H2O-5-Nitroisophthalic Acid Heterocyclic Ligand System: Synthesis, Structure, Magnetic, and Heterogeneous Catalytic Studies of Three Copper Nitroisophthalates

A high-throughput screening was employed to identify new compounds in Cu(CH3COO)(2)center dot H2O-NIPA-heterocyclic ligand systems. Of the compounds identified, three compounds, Cu-3{(NO2)-C6H3-(COO)(2)}(3)(C3N6H6)] (1), Cu-2(mu(3)-OH)(H2O){(NO2)-C6H3-(COO)(2)}(CN4H)]center dot-(H2O) (II), and Cu-2(mu(3)-OH)(H2O){(NO2)-C6H3-(COO)(2}-)(CN5H2)]center dot 2(H2O) (III), have been isolated as good quality single crystals by employing conventional hydrothermal methods. Three other compounds, Cu-2{(NO2)-C6H3-(COO)(2)}-(CN4H)(H2O) (IIa), Cu-2{(NO2)-C6H3-(COO)(2)}(CN5H2) (IIIa), and Cu-2{(NO2)-C6H3-(COO)(2)}{(CN5H2)(2)}2H(2)O (IIIb), were identified by a combination of elemental analysis, thermogravimetric analysis (TGA), and IR spectroscopic studies, although their structures are yet to be determined. The single crystalline compounds were also characterized by elemental analysis, TGA, IR, UV vis, magnetic, and catalytic studies. The structures of the compounds have paddle wheel (I) and infinite Cu 0(H) Cu chains (II and HI) connected with NLPA and heterocyclic ligands forming two-(II) and three-dimensional (I and III) structures. The bound and lattice water molecules in 11 and 111 could be reversibly removed/inserted without affecting the structure. In the case of II, the removal of water gives rise to a structural transition, but the dehydrated phase reverts back to the original phase on prolonged exposure to atmospheric conditions. Magnetic studies indicate an overall antiferromagnetism in all of the compounds. Lewis acid catalytic studies indicate that compounds II and HI are active for cyanosilylation of imines.

Inter And Intramolecular Interactions Of Porphyrins With 5,5′-Dithiobis(2-Nitrobenzoic Acid)

meso-Tetraphenylporphyrin and its metal [zinc(II) and copper(II)] derivatives form both inter and intramolecular complexes with 5,5′-dithiobis(2-nitrobenzoic acid) (DTNB). The nature of interaction is predominantly charge transfer (CT) in origin, with the porphyrin functioning as a II-donor and DTNB as an acceptor. Among the covalently linked intramolecular systems, the magnitude of CT interaction varies with the position (of one of the aryl groups of the porphyrin) to which DTNB is attached as ortho meta > para. Steady-state and time-resolved fluorescence studies revealed electron transfer to be the dominant pathway for the fluorescence quenching in these systems. Steady-state photolysis experiments probed using EPR and optical absorption studies have shown that electron transfer (from the excited singlet state of the porphyrin) to DTNB results in the formation of thiyl radical and production of free thiolate anion. It is found that the products of electrochemical reduction of covalently linked porphyrin-DTNB systems are different from those observed for the photochemical studies.

Design, synthesis, and evaluation of bile acid-based molecular tweezers

A family of bile acid-based molecular tweezers (7-9) has been constructed readily from simple precursors. Binding experiments with various electron deficient aromatic compounds showed that tweezer 8 binds trinitrofluorenone 10e with an association constant of 220 M(-1) in CDCl3. Single-crystal X-ray analysis of compound 8 shows aromatic-aromatic interactions producing a two-dimensional lattice of pyrene units. Tweezer 8 was immobilized on Merrifield resin, and binding studies have shown that these data compare well with those of the solution state studies.

Fast algorithm for data exchange in reconfigurable tree structures

This paper presents a fast algorithm for data exchange in a network of processors organized as a reconfigurable tree structure. For a given data exchange table, the algorithm generates a sequence of tree configurations in which the data exchanges are to be executed. A significant feature of the algorithm is that each exchange is executed in a tree configuration in which the source and destination nodes are adjacent to each other. It has been proved in a theorem that for every pair of nodes in the reconfigurable tree structure, there always exists two and only two configurations in which these two nodes are adjacent to each other. The algorithm utilizes this fact and determines the solution so as to optimize both the number of configurations required and the time to perform the data exchanges. Analysis of the algorithm shows that it has linear time complexity, and provides a large reduction in run-time as compared to a previously proposed algorithm. This is well-confirmed from the experimental results obtained by executing a large number of randomly-generated data exchange tables. Another significant feature of the algorithm is that the bit-size of the routing information code is always two bits, irrespective of the number of nodes in the tree. This not only increases the speed of the algorithm but also results in simpler hardware inside each node.

Structure of N-tert-butyloxycarbonyl-[alpha]-aminoisobutyryl-DL-pipecolyl-[alpha]-aminoisobutyric acid methyl ester

C20H35N3O6 (Boc-Aib-DL-Pip-Aib-OMe, Boc = tert-butyloxycarbonyl, Aib = alpha-aminoisobutyric acid, Pip = pipecolic acid, OMe = methoxy), M(r) = 413.5, monoclinic, P2(1)/c, a = 18.055 (3), b = 15.048 (3), c = 17.173 (3) angstrom, beta = 91.7 (1)-degrees, V = 4663.8 (9) angstrom3, Z = 8, D(m) = 1.16, D(x) = 1.178 Mg m-3, lambda(Mo Kalpha) = 0.71069 angstrom, mu = 0.081 mm-1, F(000) = 1792, T = 297 K. The final R value for 4925 [I greater-than-or-equal-to 3sigma(I)] reflections is 0.065 (wR = 0.067). The peptide backbone of the two independent molecules in the asymmetric unit is folded at the -Aib-Pip- sequence to form a type-I (I') beta-bend stabilized by a 1 <-- 4 intramolecular N-H...O=C hydrogen bond between the Aib(3) peptide N-H and Boc urethane C=O groups.

Identification of the active-site peptide of 2,3-dihydroxybenzoic acid decarboxylase from Aspergillus oryzae

The non-oxidative decarboxylation of aromatic acids is a poorly understood reaction. The transformation of 2,3-dihydroxybenzoic acid to catechol in the fungal metabolism of indole is a prototype of such a reaction. 2,3-Dihydroxybenzoic acid decarboxylase (EC 4.1.1.46) which catalyzes this reaction was purified to homogeneity from anthranilate induced cultures of Aspergillus oryzae using affinity chromatography. The enzyme did not require cofactors like NAD(+), PLP, TPP or metal ions for its activity. There was no spectral evidence for the presence of enzyme bound cofactors. The preparation, which was adjudged homogeneous by the criteria of SDS-PAGE, sedimentation analysis and N-terminal analysis, was characterized for its physicochemical and kinetic parameters. The enzyme was inactivated by group-specific modifiers like diethyl pyrocarbonate (DEPC) and N-ethylmaleimide (NEM). The kinetics of inactivation by DEPC suggested the presence of a single class of essential histidine residues, the second order rate constant of inactivation for which was 12.5 M(-1) min(-1). A single class of cysteine residues was modified by NEM with a second order rate constant of 33 M(-1) min(-1). Substrate analogues protected the enzyme against inactivation by both DEPC and NEM, suggesting the Location of the essential histidine and cysteine to be at the active site of the enzyme. The incorporation of radiolabelled NEM in a differential labelling experiment was 0.73 mol per mol subunit confirming the presence of a single essential cysteine per active-site. Differentially labelled enzyme was enzymatically cleaved and the peptide bearing the label was purified and sequenced. The active-site peptide LLGLAETCK and the N-terminal sequence MLGKIALEEAFALPRFEEKT did not bear any similarity to sequences reported in the Swiss-Prot Protein Sequence Databank, a reflection probably of the unique primary structure of this novel enzyme. The sequences reported in this study will appear in the Swiss-Prot Protein Sequence Databank under the accession number P80402.