Achieving optimum hydrogen permeability in PdAg and PdAu alloys

The present work investigates both the diffusivity and permeability of hydrogen (H) in palladium-silver (PdAg) and palladium-gold (PdAu) alloys over a 400-1200 K temperature range for Pd(100-X)M(X), M=Ag or Au and X=0%-48% using density functional theory (DFT) and kinetic Monte Carlo simulations (KMC). DFT has been employed to obtain octahedral (O)-, tetrahedral (T)-, and transition state (TS)- site energetics as a function of local alloy composition for several PdAg and PdAu alloys with compositions in supercells of X=14.18%, 25.93%, 37.07%, and 48.15% with the nearest (NNs) and next nearest neighbors (NNNs) varied over the entire range of compositions. The estimates were then used to obtain a model relating the O, T, and TS energies of a given site with NN(X), NNN(X), and the lattice constant. The first passage approach combined with KMC simulations was used for the H diffusion coefficient predictions. It was found that the diffusion coefficient of H in PdAg alloy decreases with increasing Ag and increases with increasing temperature, matching closely with the experimental results reported in the literature. The calculated permeabilities of H in these novel binary alloys obtained from both diffusivity and solubility predictions were found to have a maximum at approximately 20% Ag and approximately 12% Au, which agree well with experimental predictions. Specifically, the permeability of H in PdAg alloy with approximately 20% Ag at 456 K is three to four times that of pure Pd, while the PdAu alloy at 12% Au is four to five times that of pure Pd at 456 K.     

Capillary liquid chromatography-mass spectrometry for the rapid identification and quantification of almond flavonoids

A rapid negative ion ESI high-performance capillary liquid chromatography-mass spectrometry method was developed to identify and quantify flavonoids (e.g., flavanols, flavonols, flavanones and glycosides). Fifteen standards and two varieties of almond skin extract powder (Carmel and Nonpareil) were used to demonstrate the chromatographic separation, reproducibility and accuracy of the method that employed a 150 mm x 0.3 mm ChromXP 3C18-EP-120 column. All standards eluted in less than 10 min, providing a 9-12x reduction in analysis time compared to existing methods (90-120 min). However, isomers (e.g., catechin/epicatechin and galactosides/glucosides) were not resolved and, therefore, identified and quantified collectively. RSDs for retention time and peak area reproducibility (mass spectrometry data) were <0.5% and <5.0%, respectively. Peak area reproducibility was greatly improved (from a RSD>10%) after the implementation of a low-flow metal needle in the ESI source. Quantitation by mass spectrometry also afforded a % error less than 5% for most compounds.     

Total synthesis of a 28-member stereoisomer library of murisolins

Total syntheses of two 16-member libraries of murisolin isomers are reported. In the first library, fluorous PMB (p-methoxybenzyl) groups encode configurations, and four mixtures of four dihydroxy-tetrahydrofurans are prepared by Shi epoxidation followed (optionally) by Mitsunobu reaction. The mixtures are coupled by Kocienski-Julia reaction with a single hydroxybutenolide followed by hydrogenation. Demixing and detagging provide the 16 pure stereoisomers. In the second synthesis, a single mixture of four fluorous-tagged dihydroxy-tetrahydrofurans is coupled with a four-compound mixture of hydroxybutenolides that bear derivatives of DMB (dimethoxybenzyl) groups with oligoethylene glycol (OEG) units that encode the configurations at C4 and C34. The 16-compound mixture is subjected to hydrogenation, double demixing, and detagging to provide the 16 isomerically pure murisolins. Twelve of these isomers are new, while four match samples from the first library.     

Intramolecularly sensitized precipitons: a model system for application to metal sequestration

We have investigated light-triggered or catalytically activated precipitation agents and have proposed the name "precipiton" for such molecules or molecular fragments. A phase separation is induced when the precipiton isomerizes to a low-solubility form. In this paper we describe the first intramolecularly activated precipitons. The isomerization process is induced by intramolecular triplet energy transfer from a covalently attached metal complex. As expected, intramolecular sensitization leads to a more rapid isomerization than can be achieved by intermolecular sensitization at accessible concentrations. Two isomeric bichromophoric precipiton species, each containing [Ru(bpy)(3)](2+) and 1,2-bis(biphenyl)ethene units covalently linked together by an ether tether, have been synthesized and characterized, and their photochemical properties have been investigated. The rates of photoisomerization of these complexes, [((Z)-1,2-bis(biphenyl)ethene-bpy)Ru(bpy)(2)](PF(6))(2) (2Z) and [((E)-1,2-bis(biphenyl)ethene-bpy)Ru(bpy)(2)](PF(6))(2) (2E), were compared to those of their untethered analogues, (Z)-1,2-bis(biphenyl)ethene-OTBS (1Z) and (E)-1,2-bis(biphenyl)ethene-OTBS (1E), where ruthenium sensitization occurred through an intermolecular pathway. Upon irradiation with visible light (lambda > or = 400 nm) in degassed solution, 2Z/E and 1Z/E obeyed reversible first-order rate kinetics. The intramolecularly sensitized precipiton 2Z isomerized 250 times faster (k(2Z-->2E) = 1.0 x 10(-3) s(-1) with a 51% neutral density filter) than the intermolecular case 1Z (k(1Z-->1E) = 0.80 x 10(-5) s(-1)). For 1E and 2E, the isomerization rates were k(1E-->1Z) = 11.0 x 10(-5) s(-1) and k(2E-->2Z) = 1.6 x 10(-3) s(-1), respectively. The average Z/E mole ratio at the photostationary state was 62/38 for 2Z/E and 93/7 for 1Z/E. The impetus for this study was our desire to evaluate the possibility of using metal-binding precipitons that would precipitate only upon metal-to-precipiton binding and would be inert to visible light in the absence of metals.     

Effects of carbon nanofiber composites on electrode processes involving liquid|liquid ion transfer

Composite electrodes were prepared from chemical vapor deposition grown carbon nanofibers consisting predominantly of ca. 100 nm diameter fibers. A hydrophobic sol–gel matrix based on a methyl-trimethoxysilane precursor was employed and composites formed with carbon nanofiber or carbon nanofiber—carbon particle mixtures (carbon ceramic electrode). Scanning electron microscopy images and electrochemical measurements show that the composite materials exhibit high surface area with some degree of electrolyte solution penetration into the electrode. These electrodes were modified with redox probe solution in 2-nitrophenyloctylether. A second type of composite electrode was prepared by simple pasting of carbon nanofibers and the same solution (carbon paste electrode). For both types of electrodes it is shown that high surface area carbon nanofibers dominate the electrode process and enhance voltammetric currents for the transfer of anions at liquid|liquid phase boundaries presumably by extending the triple-phase boundary. Both anion insertion and cation expulsion processes were observed driven by the electro-oxidation of decamethylferrocene within the organic phase. A stronger current response is observed for the more hydrophobic anions like ClO₄⁻ or PF₆⁻ when compared to that for the more hydrophilic anions like F⁻ and SO₄²⁻. Presented at the 4th Baltic Conference on Electrochemistry, Greifswald, March 13–16, 2005     

Effect of matrix crystal structure on ion abundance of carbohydrates by matrix-assisted laser desorption/ionization Fourier transform ion cyclotron resonance mass spectrometry

Sample preparation techniques for carbohydrate analysis using matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) are explored, with particular emphasis on analyte/matrix co-crystallization procedures. While carbohydrates are known to prefer 2,5-dihydroxybenzoic acid (2,5-DHB) as the matrix of choice, these analytes are quite specific about matrix crystal structure, which in turn is dependent on the rate of drying of analyte/matrix spots on the MALDI target. With N-acetylglucosamine (GlcNAc) and N-acetylneuraminic acid (sialic acid or NeuAc) as test monosaccharides, significant increases in ion abundances are demonstrated with 2,5-DHB/NeuAc spots (>10-fold improvement) and 2,5-DHB/GlcNAc spots ( approximately 5-fold improvement) with active drying. The fine structure of crystals generated in active and passive drying was investigated using powder diffraction. Passively dried samples were shown to consist of an ordered polymorph, crystallizing in the space group P2(1)/a, while the actively dried samples produced a disordered phase crystallizing in the space group Pa. These data provide the wherewithal to engineer a matrix best suited for carbohydrate analyses.     

Thermodynamics of the As(III)-thiol interaction: arsenite and monomethylarsenite complexes with glutathione, dihydrolipoic acid, and other thiol ligands

Colorimetric (near-UV absorption spectroscopy) and calorimetric (isothermal titration calorimetry) methods have been used to quantify the equilibrium and thermodynamics of arsenite and monomethylarsenite (MMA) coordinating to glutathione (GSH) and the dithiols dimercaptosuccinic acid (DMSA), dihydrolipoic acid (DHLA), and dithiothreitol (DTT). We found that both arsenite and MMA form moderately stable complexes (beta = 10(6)-10(7)) with GSH; that arsenite forms a particularly stable 2:3 complex (beta approximately 10(18)) with the biological cofactor DHLA; that MMA has a somewhat higher affinity than arsenite for thiol ligands; and that entropic factors modulate the overall stability of As(III) complexes with thiols, which are favored by the exothermic formation of As(III)-thiolate bonds. The implications of these results for arsenic toxicity are discussed.     

Hydrogen bonding in aromatic formamides

The solid state structures of two p-substituted aromatic formamides have been determined. p-Nitro formamido benzene (1) crystallizes in the monoclinic space group C2/c with a = 10.9859(7), b = 10.0576(7), c = 13.0331(9) Å, β = 97.148(1)^∘ and Z = 8. p-formamido anisole (2) crystallizes in the orthorhombic space group Pna2₁ with a = 10.5598(7), b = 7.6553(5), c = 9.2522(6) Å and Z = 4. Both compounds show hydrogen bonding in the solid state, forming infinite chains via N–H⋅ < eqid1 > ⋅O = C bridges. For the anisole formamide (2), the molecules exhibit in a zig-zag arrangement. The nitro compound (1) exhibits a spiral-like wavy line with a fourfold repeating unit, making it the first formamide having a chiral N–H⋅ < eqid2 > ⋅O bridged chain reminiscent of those found for α-helices in proteins. While the individual spirals of 1 are orientated either clockwise or counterclockwise, their orientation towards each other is random.     

High-affinity interactions of ligands at recombinant Guinea pig 5HT7 receptors

The serotonin 5HT₇ receptor has been implicated in numerous physiological and pathological processes from circadian rhythms [1] to depression and schizophrenia. Clonal cell lines heterologously expressing recombinant receptors offer good models for understanding drug-receptor interactions and development of quantitative structure-activity relationships (QSAR). Comparative Molecular Field Analysis (CoMFA) is an important modern QSAR procedure that relates the steric and electrostatic fields of a set of aligned compounds to affinity. Here, we utilized CoMFA to predict affinity for a number of high-affinity ligands at the recombinant guinea pig 5HT₇ receptor. Using R-lisuride as the template, a final CoMFA model was derived using procedures similar to those of our recent papers [2, 3, 4] The final cross-validated model accounted for >85% of the variance in the compound affinity data, while the final non-cross validated model accounted for >99% of the variance. Model evaluation was done using cross-validation methods with groups of 5 ligands. Twenty cross-validation runs yielded an average predictive r²(q²) of 0.779 ± 0.015 (range: 0.669–0.867). Furthermore, 3D-chemical database search queries derived from the model yielded hit lists of promising agents with high structural similarity to the template. Together, these results suggest a possible basis for high-affinity drug action at 5HT₇ receptors.     

The pH-Dependent Surface Activity of a Hydrocarbon/Fluorocarbon Catanionic Surfactant for Water/Hydrofluoroether Microemulsions: The Characterization of a New Acid/Soap Complex

The pH sensitivity of a hydrocarbon/fluorocarbon catanionic surfactant is investigated. The catanionic surfactant is sensitive to low pH, which results in the precipitation of an acid/soap complex. This complex is identified by IR, NMR and confirmed by x-ray crystallography.