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In this work, a novel cube-shaped cationic lipid based on the imidazolium salt of polyhedral oligomeric silsesquioxane (POSS) was complexed with double-stranded DNA. Because of the negative spontaneous curvature of the cationic POSS imidazolium lipid, an inverted hexagonal phase resulted above the melting point of POSS crystals. Depending on the competition between the crystallization of POSS molecules and the negative spontaneous curvature of cationic POSS imidazolium lipids, different self-assembled phase morphologies were obtained. A lamellar phase was obtained when the POSS crystallization was relatively slow. When the POSS crystallization was fast, an inverted hexagonal phase was obtained with POSS lamellar crystals grown in the interstitials of DNA cylinders. On the basis of a circular dichroism study, double-stranded DNA adopted the B-form helical conformation in the inverted hexagonal phase, whereas the helical conformation was largely destroyed in the lamellar phase.
ACS Nano 2008 May
PMID:Conformation transformation determined by different self-assembled phases in a DNA complex with cationic polyhedral oligomeric silsesquioxane lipid. 1920 89

Chemists rely on a toolbox of robust chemical transformations for selectively modifying molecules with spatial and functional precision to make them more complex in a controllable and predictable manner. This manuscript describes proof-of-principle experiments for a conceptually analogous strategy involving the selective, stepwise, and spatially controlled modification of inorganic nanostructures. The key concept is orthogonal reactivity: one component of a multicomponent system reacts with a particular reagent under a specific set of conditions while the others do not, even though they are all present together in the same reaction vessel. Using the chemical conversion of metal nanoparticles into intermetallic, sulfide, and phosphide nanoparticles as representative examples, the concept of orthogonal reactivity is defined and demonstrated for a variety of two- and three-component nanoscale systems. First, solution-phase reactivity data are presented and collectively analyzed for the reaction of metal nanoparticles (Ni, Cu, Rh, Pd, Ag, Pt, Au, Sn) with several metal salt and elemental reagents (Bi, Pb, Sb, Sn, S). From these data, several two- and three-component orthogonal systems are identified. Finally, these results are applied to the spatially selective chemical modification of lithographically patterned surfaces and striped template-grown metal nanowires.
ACS Nano 2009 Apr 28
PMID:Orthogonal reactivity of metal and multimetal nanostructures for selective, stepwise, and spatially-controlled solid-state modification. 1939 43

A rapid, solventless method is described for the decoration of carbon nanotubes with metal nanoparticles. The straightforward two-step process utilizes neither reducing agents nor electric current and involves the dry mixing of a precursor metal salt (e.g., a metal acetate) with carbon nanotubes (single- or multi-walled) followed by heating in an inert atmosphere. The procedure is scalable to multigram quantities and generally applicable to various other carbon substrates (e.g., carbon nanofiber, expanded graphite, and carbon black) and many metal salts (e.g., Ag, Au, Co, Ni, and Pd acetates). As a model system, Ag nanoparticle-decorated carbon nanotube samples were prepared under various mixing techniques, metal loading levels, thermal treatment temperatures, and nanotube oxidative acid treatments. These nanohybrids were characterized by a variety of microscopic and spectroscopic techniques. For example, X-ray diffraction and scanning electron microscopy indicated that the average size of the Ag nanoparticles has little to do with the thermal treatment temperature but can be easily controlled by varying the Ag loading. Raman spectroscopy illustrated both the metal-nanotube electronic interactions and the surface enhancement effect from the Ag nanoparticle attachment. High-resolution transmission electron microscopy captured the in situ salt-to-metal conversion events on the nanotube surface. The mechanistic implications from the characterization results are discussed.
ACS Nano 2009 Apr 28
PMID:Rapid, solventless, bulk preparation of metal nanoparticle-decorated carbon nanotubes. 1927 18

Although macromolecular self-assemblies are mostly fabricated from amphiphilic copolymers, here we report a tubular structure self-assembled solely from hydrophilic dextran-derived homopolymers via electrostatic interaction. To obtain tubular structures, we prepared two oppositely charged dextran derivatives by incorporating 2-bromoethylamine (Dex-BH) and chloroacetic acid (Dex-CA) into dextran, and their structures were confirmed using Fourier transform infrared spectroscopy. The two oppositely charged dextran derivatives self-assembled into microsize tubules when mixed in a pH 4.0 buffer solution. The tubular self-assemblies were sensitive to both pH and salt concentrations. Scanning electron microscopy and light microscopy confirmed that the tubules have hollow structures up to 100 microm long with a diameter between 600 nm and 2 microm. The X-ray study did not reveal any ordered molecular organization. This paper explores the mechanism of the tubule self-assembly and suggests a model.
ACS Nano 2009 May 26
PMID:Self-assembly of chemically engineered hydrophilic dextran into microscopic tubules. 1938 59

Beta-peptides (beta-amino acid oligomers) that mimic the amphiphilic, helical, and cationic properties of natural antimicrobial peptides have previously been shown to display antifungal activity against planktonic Candida albicans cells. Beta-peptides offer several advantages over conventional peptides composed of alpha-amino acid residues, including conformational stability, resistance to proteases, and activity at physiological salt concentrations. We examined sequence-activity relationships toward both planktonic C. albicans cells and C. albicans biofilms, and the results suggest a toxicity mechanism involving membrane disruption. A strategy for fluorescently labeling a beta-peptide without diminishing antifungal activity was devised; labeled beta-peptides penetrated the cell membrane and accumulated in the cytoplasm of both planktonic and biofilm-associated cells. The labeled beta-peptide was detected only in metabolically inactive cells, which suggests that beta-peptide entry is correlated with cell death. The presence of a beta-peptide at a concentration near the minimum inhibitory concentration completely prevented planktonic C. albicans cells from forming a biofilm, suggesting that beta-peptides may be useful in preventing fungal colonization and biofilm formation.
ACS Chem Biol 2009 Jul 17
PMID:Effect of sequence and structural properties on 14-helical beta-peptide activity against Candida albicans planktonic cells and biofilms. 1951 70

We have demonstrated that mixed-base PNA oligomers are effective coagulants of citrate ion-coated gold and silver nanoparticles (AuNPs and AgNPs), and PNA-induced particle aggregation can be disrupted by hybridization of PNA with a specific DNA. Using particles' aggregation/dispersion as a measure, we have investigated how PNA and PNA-DNA complexes bind to AuNPs and AgNPs and modulate particles' stability differently relative to their DNA counterparts. We have made the following original discoveries: (1) mix-base PNA oligomers can induce immediate particle aggregation in a concentration- and chain-length-dependent manner; (2) PNA oligomers have a higher affinity to AuNPs and AgNPs than its ssDNA counterpart; (3) PNA-DNA complexes, although having a stable double helix structure similar to dsDNA, can effectively protect the particles from salt induced aggregation, and the protection effect of different nucleic acids are in the order of PNA-DNA complex > ssDNA > dsDNA; (4) all the characteristics are identical for AuNPs and AgNPs; and (5) AgNPs is more sensitive in response to destabilization effect and is proven a more sensitive platform for colorimetric assays. The control of particle aggregation and dispersion by PNA and PNA-DNA complexes has been used to detect a specific DNA sequence with single-base-mismatch resolution. zeta potential measurements have been conducted to reveal how distinct backbone properties of PNA and PNA-DNA complexes relative to their DNA counterparts contribute to the distinct binding characteristics.
ACS Nano 2009 Sep 22
PMID:Control of metal nanoparticles aggregation and dispersion by PNA and PNA-DNA complexes, and its application for colorimetric DNA detection. 1970 41

We investigated the influence of processing conditions, nanocrystal/substrate interactions and solvent evaporation rate on the ordering of strongly interacting nanocrystals by synergistically combining electron microscopy and synchrotron-based small-angle X-ray scattering analysis. Spin-cast PbSe nanocrystal films exhibited submicrometer-sized supracrystals with face-centered cubic symmetry and (001)(s) planes aligned parallel to the substrate. The ordering of drop-cast lead salt nanocrystal films was sensitive to the nature of the substrate and solvent evaporation dynamics. Nanocrystal films drop-cast on rough indium tin oxide substrates were polycrystalline with small grain size and low degree of orientation with respect to the substrate, whereas films drop-cast on flat Si substrates formed highly ordered face-centered cubic supracrystals with close-packed (111)(s) planes parallel to the substrate. The spatial coherence of nanocrystal films drop-cast in the presence of saturated solvent vapor was significantly improved compared to films drop-cast in a dry environment. Solvent vapor annealing was demonstrated as a postdeposition technique to modify the ordering of nanocrystals in the thin film. Octane vapor significantly improved the long-range order and degree of orientation of initially disordered or polycrystalline nanocrystal assemblies. Exposure to 1,2-ethanedithiol vapor caused partial displacement of surface bound oleic acid ligands and drastically degraded the degree of order in the nanocrystal assembly.
ACS Nano 2009 Oct 27
PMID:Structure/processing relationships of highly ordered lead salt nanocrystal superlattices. 1972 1

We report on reversible temperature-triggered swelling transitions in hydrogen-bonded multilayer films of a polycarboxylic acid and stimuli-responsive block copolymer micelles (BCMs). A neutral hydrogen-bonding temperature-responsive diblock copolymer, poly(N-vinylpyrrolidone)-b-poly(N-isopropylacrylamide) (PVPON-b-PNIPAM), was synthesized by macromolecular design via the interchange of xanthates (MADIX). The block copolymer exhibited reversible micellization, forming PNIPAM-core micelles with PVPON coronae in 0.01 M buffer solutions at temperatures higher than 34 degrees C, or in solutions with high salt concentrations (C(NaCl) > 0.4 M) at 20 degrees C. The PVPON-b-PNIPAM BCMs were then assembled with poly(methacrylic acid) (PMAA) at acidic pH and higher temperature using the layer-by-layer (LbL) technique. Within the hydrogen-bonded multilayer, BCMs were stabilized through hydrogen bonding between PVPON and PMAA units and, unlike in solution, did not dissociate into unimers in low-salt solution at T < 34 degrees C. Instead, PVPON-b-PNIPAM BCMs reversibly swelled within film in response to temperature- or salt-concentration variations, reflecting collapse and dissolution of the BCM PNIPAM cores. The capacity of BCM/PMAA films to retain hydrophobic molecules was also dramatically dependent on temperature and/or ionic strength. The characteristic release time of pyrene from a [BCM/PMAA](10) film decreased from 80 to 10 min upon a decrease in temperature from 37 to 20 degrees C. In addition, at 20 degrees C, ionic strength was also capable of controlling the collapse of PNIPAM micellar cores and the subsequent film swelling and pyrene release rate. Incorporation of stimuli-responsive BCM micelles within LbL films opens new opportunities in designing nanoscale films capable of controlling molecular swelling, transport, and diffusion in response to environmental stimuli.
ACS Nano 2009 Nov 24
PMID:Temperature-induced swelling and small molecule release with hydrogen-bonded multilayers of block copolymer micelles. 1979 44

Candida albicans is an opportunistic fungal pathogen capable of life-threatening disseminated infections particularly in immunocompromised patients. Resistance to many clinically used antifungal agents has created a need to identify and develop a new generation of compounds for therapeutic use. A compound screen to identify potential antifungal natural products was undertaken, identifying 12 saponins, some of which have not been previously described. In the Caenorhabditis elegans model, some saponins conferred nematode survival comparable to that of amphotericin B. Of the 12 antifungal saponins identified, two were selected for further analysis. C. albicans isolates were inhibited by these compounds at relatively low concentrations (16 and 32 microg mL(-1)) including isolates resistant to clinically used antifungal agents. C. albicans hyphae and biofilm formation were also disrupted in the presence of these natural products, and studies demonstrate that fungal cells in the presence of saponins are more susceptible to salt-induced osmotic stress. Although saponins are known for their hemolytic activity, no hemolysis of erythrocytes was observed at three times the minimal inhibitory concentration for C. albicans, suggesting the saponins may have a preference for binding to fungal ergosterol when compared to cholesterol. Importantly, when used in combination with photosensitizer compounds, the fungus displayed increased susceptibility to photodynamic inactivation due to the ability of the saponins to increase cell permeability, thereby facilitating penetration of the photosensitizers. The large proportion of compounds identified as antifungal agents containing saponin structural features suggests it may be a suitable chemical scaffold for a new generation of antifungal compounds.
ACS Chem Biol 2010 Mar 19
PMID:Characterization of plant-derived saponin natural products against Candida albicans. 2009 97

S-Nitrosothiols (RSNOs) represent an important class of post-translational modifications that preserve and amplify the actions of nitric oxide and regulate enzyme activity. Several regulatory proteins are now verified targets of cellular S-nitrosation, and the direct detection of S-nitrosated residues in proteins has become essential to better understand RSNO-mediated signaling. Current RSNO detection depends on indirect assays that limit their overall specificity and reliability. Herein, we report the reaction of S-nitrosated cysteine, glutathione, and a mutated C165S alkyl hydroperoxide reductase with the water-soluble phosphine tris(4,6-dimethyl-3-sulfonatophenyl)phosphine trisodium salt hydrate (TXPTS). A combination of NMR and MS techniques reveals that these reactions produce covalent S-alkylphosphonium ion adducts (with S-P(+) connectivity), TXPTS oxide, and a TXPTS-derived aza-ylide. Mechanistically, this reaction may proceed through an S-substituted aza-ylide or the direct displacement of nitroxyl from the RSNO group. This work provides a new means for detecting and quantifying S-nitrosated species in solution and suggests that phosphines may be useful tools for understanding the complex physiological roles of S-nitrosation and its implications in cell signaling and homeostasis.
ACS Chem Biol 2010 Apr 16
PMID:Water-soluble triarylphosphines as biomarkers for protein S-nitrosation. 2014 2


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