Source Of Catalytic Activity Research Articles

Dextran-Coated Iron Oxide Nanoparticles as Biomimetic Catalysts for Localized and pH-Activated Biofilm Disruption.

Biofilms are surface-attached bacterial communities embedded within an extracellular matrix that create localized and protected microenvironments. Acidogenic oral biofilms can demineralize the enamel-apatite on teeth, causing dental caries (tooth decay). Current antimicrobials have low efficacy and do not target the protective matrix and acidic pH within the biofilm. Recently, catalytic nanoparticles were shown to disrupt biofilms but lacked a stabilizing coating required for clinical applications. Here, we report dextran-coated iron oxide nanoparticles termed nanozymes (Dex-NZM) that display strong catalytic (peroxidase-like) activity at acidic pH values, target biofilms with high specificity, and prevent severe caries without impacting surrounding oral tissues in vivo. Nanoparticle formulations were synthesized with dextran coatings (molecular weights from 1.5 to 40 kDa were used), and their catalytic performance and bioactivity were assessed. We found that 10 kDa dextran coating provided maximal catalytic activity, biofilm uptake, and antibiofilm properties. Mechanistic studies indicated that iron oxide cores are the source of catalytic activity, whereas dextran on the nanoparticle surface provided stability without blocking catalysis. Dextran-coating facilitated NZM incorporation into exopolysaccharides (EPS) structure and binding within biofilms, which activated hydrogen peroxide (H2O2) for localized bacterial killing and EPS-matrix breakdown. Surprisingly, dextran coating enhanced selectivity toward biofilms while avoiding binding to gingival cells. Furthermore, Dex-NZM/H2O2 treatment significantly reduced the onset and severity of caries lesions (vs control or either Dex-NZM or H2O2 alone) without adverse effects on gingival tissues or oral microbiota diversity in vivo. Therefore, dextran-coated nanozymes have potential as an alternative treatment to control tooth decay and possibly other biofilm-associated diseases.

In-Situ Platinum Deposition on Nitrogen-Doped Carbon Films as a Source of Catalytic Activity in a Hydrogen Evolution Reaction.

Copolymer-templated nitrogen-doped carbon (CTNC) films deposited on glassy carbon were used as electrodes to study electrochemically driven hydrogen evolution reaction (HER) in 0.5 M H2SO4. The activity of these materials was extremely enhanced when a platinum counter electrode was used instead of a graphite rod and reached the level of commercial Pt/C electrodes. Postreaction scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) measurements of electrode surfaces revealed that incorporation of even extremely low amounts of Pt resulted in this considerable gain of HER activity. High resolution XPS analysis and density functional theory (DFT) calculations confirmed that pyridinic nitrogen atoms act as active sites for Pt coordination and deposition. The Pt can be incorporated in both molecular (Pt(2+)) and metallic (Pt(0)) form. This study shows that great caution must be taken when designing "metal-free" HER catalysts based on N-doped carbons.

Comparison of precious metal doped and impregnated perovskite oxides for TWC application

Four high-surface-area (50–60m2/g) perovskite-based powder catalysts, Pd-doped LaFeO3, Pd-impregnated LaFeO3, Rh-doped CaTiO3, and Rh-impregnated CaTiO3, were characterized by scanning transmission electron microscopy and CO oxidation measurements in fresh and redox-aged (14h at 800°C) states. Both intrinsic catalytic activity and stability were significantly higher in Pd-doped LaFeO3 than in Rh-doped CaTiO3 under lean to stoichiometric reaction conditions. Activities of doped were initially lower than impregnated versions, though aging led to convergence in the catalytic performance for both systems. A mixture of metallic and cationic forms of Pd appear to contribute to activity in the Pd catalysts, whereas metallic Rh particles, which can easily segregate onto the surface of CaTiO3 in Rh-doped CaTiO3 upon aging, are the likely source of catalytic activity in the Rh catalysts.

Efficient synthesis of E-1,2-bis(silyl)ethenes via ruthenium-catalyzed homocoupling of vinylsilanes carried out in ionic liquids

A series of ruthenium complexes (RuCl3×3H2O, [C5H5Ru(CH3CN)3]+[PF6]−, [RuCl2(PPh3)3], [RuHCl(CO)(PPh3)3], [RuHCl(CO)(PCy3)2]) immobilized in [bmim][Tf2N] were tested in homocoupling of vinylsilanes (H2CCHSi(OEt)3, H2CCHSi(OiPr)3, H2CCHSi(OSiMe3)3, H2CCHSiMe(OSiMe3)2, and H2CCHSiPh(OSiMe3)2) performed in a biphasic system. The highest catalytic activity and selectivity of homocoupling product was observed for [RuHCl(CO)(PCy3)2]. The complex [RuHCl(CO)(PCy3)2] immobilized in a variety ionic liquids ([bmim][PF6], [bmim][BF4], [bmim][Tf2N], [bmim][TfO], [bmim][HSO4], [bmim][Cl], [trimim][MeSO4], [NBu3Me][MeSO4], [PBu4][Cl], [bpy][PF6], [bpy][BF4], [bpy][Cl]) exhibits high catalytic activity in homocoupling of H2CCHSiMe2Ph, H2CCHSi(OEt)3 and H2CCHSiMe(OSiMe3)2 and enables high-yield stereoselective synthesis of the corresponding E-1,2-bis(silyl)ethenes. The immobilized complex can be easily recycled up to 12 times in the homocoupling of H2CCHSiMe2Ph and up to 10 times in the homocoupling of H2CCHSiMe(OSiMe3)2 without a significant drop in activity and selectivity. Results of a split test indicate that the source of catalytic activity is the catalyst immobilized in ionic liquid phase.

Internal Enzyme Motions as a Source of Catalytic Activity: Rate-Promoting Vibrations and Hydrogen Tunneling

The standard view of the origin of the catalytic properties of enzymes focuses on the binding energy differences between the ground state and the transition state arising from the arrangement of residues in the active site (i.e., statics). There is an alternative view that suggests that protein motions (i.e., dynamics) might play a role in catalysis. Klinman and co-workers recently published (Kohen, A.; Cannio, R.; Bartolucci, S.; Klinman, J. Nature 1999, 399, 496−499) findings on rate measurements in thermophilic alcohol dehydrogonase (ADH). At lower temperatures (below 30 °C), this enzyme undergoes a transition to a more rigid structure, and it was found that the corresponding apparent activation energy increases and the primary kinetic isotope effect (KIE) increases and becomes temperature-dependent. Explaining these results presents a challenge to theory. We show that a model of the reaction coordinate for the rate-determining step, coupled to an enzymatic environment and a specific strongly coupled a...

The possible involvement of a fatty acyl thioesterase in the termination of the synthesis of fatty acids in a psychrophilic bacterium, Vibrio sp. strain ABE-1

In Vibrio sp. strain ABE-1, very long chain fatty acids with up to 30 carbon atoms were synthesized in vitro in the form of acyl-acyl carrier protein by a fatty acid synthetase that had been prepared from the cytosol fraction precipitated between 55 and 75% saturation with ammonium sulfate. In contrast, fatty acids with 10–18 carbon atoms, which are the usual acyl components in this bacterium, were synthesized in vitro when the unfractionated cytosol fraction was used as the source of catalytic activity. When partially purified fatty acid synthetase was used together with a subfraction that had been prepared from the cytosol fraction precipitated between 0 and 55% saturation with ammonium sulfate, 16-carbon fatty acids were recovered as the dominant free fatty acids, and fatty acids with more than 20 carbon atoms were not synthesized in vitro. Acyl-acyl carrier proteins and acyl-CoAs with 16-carbon fatty acids were preferentially hydrolyzed when this subfraction was used as the source of catalytic activity. These results suggest that (a) fatty acyl thioesterase(s) with high specificity for acyl-acyl carrier proteins with 16-carbon fatty acids regulate(s) acyl chain length. This activity could explain the high levels of 16-carbon fatty acids in this bacterium.Key words: acyl chain length regulation, fatty acyl thioesterase, acyl-ACP, fatty acid synthetase, psychrophilic bacteria, Vibrio sp.

Burial Diagenesis and Specific Catalytic Activity of Illite-Smectite Clays from Vienna Basin, Austria

A sequence of Tertiary shales from the Vienna basin, Austria, contains a series of illite-smectite (I/S) clays that are the source of catalytic activity as measured by the method developed by Goldstein in 1983. The source of this activity is Bronsted acidity resulting from the dissociation of water molecules bonded to exchangeable cations in the interlayers of the smectite and on the external basal surfaces of illite platelets. This specific catalytic activity (SCA) is exchangeable cation dependent and correlates directly with increasing interlayer charge and tetrahedral substitution accompanying the diagenetic formation of thin interlayer illite particles with high-charge external surfaces. As a result, diagenetic transformation of smectite to illite in this series resul s in increased catalytic activity. From consideration of cation exchange capacity and interlayer charge of end-member components of this I/S series, we show that a cation site resulting from tetrahedral substitution of Al3+ for Si4+ has an activity about 40 times greater than a site of octahedral origin. The end-member illite particles of this I/S series have calculated mean thicknesses of about 80A (eight 2:1 mica layers). The linear relationship between SCA values and estimated platelet thickness for three different micas and illites confirms the role of cation sites on external basal surfaces as sources of catalytic activity. The possibility of direct involvement of catalytically active diagenetic illite in natural kerogen pyrolysis, acting simultaneously with a fluid release mechanism and its geopressure development, may provide optimum conditions for hydrocarbon generation and migration during I/S diagenesis. The presence of high-surface-area illite in diagenetically altered reservoir sandstones suggests the possibility for modifying the hydrocarbon constitution of crude oils within buried reservoirs. The effects of exchange cation composition on catalytic properties of I/S clays show that formation water chemistry must be considered in evaluating the effectiveness of clay catalysts in hydrocarbon-forming reactions.

Proteolysis of skeletal muscle adenylate cyclase. Destruction and reconstitution of fluoride and guanylnucleotide sensitivity.

Adenylate cyclase was measured in skeletal muscle plasma membranes incubated with subtilisin. Under specific conditions the protease preferentially inactivated fluoride and guanylnucleotide sensitivity. Following protease treatment, membranes were solubilized with Lubrol 12A9 and subjected to ion-exchange chromatography. Adenylate cyclase was eluted with 200 mM NaCl; the enzyme recovered was completely unresponsive to either NaF or guanylyl imidodiphosphate. Responsiveness to the two ligands was restored by adding a heart fraction in which basal activity had been destroyed by heating at 40 degrees C or by adding a soluble skeletal muscle fraction in which basal activity had been largely destroyed by N-ethylmaleimide. The solubilized subtilisin-treated skeletal muscle preparation may serve as a source of catalytic activity for the study and purification of regulatory factors for adenylate cyclase.