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Magnetic linear response properties for achiral and chiral carbon nanotubes were investigated with first-principles electronic structure methods. We have computed the magnetic shielding inside and outside the tubes originating from electronic current densities induced by the application of an external magnetic field. This electronic response of the nanotubes was analyzed for external magnetic fields both parallel and perpendicular to the tube axis. The magnetic screening would be experienced by guest molecules inside the tubes, measurable by NMR spectroscopy on isotopically labeled samples. Special attention is given to chiral nanotubes, in which longitudinal fields induce a non-zero longitudinal current density and thus tangential magnetic fields outside the tubes.
ACS Nano 2008 Apr
PMID:Electronic response properties of carbon nanotubes in magnetic fields. 1920 96

The synthesis and characterization of colloidal mesoporous silica (CMS) functionalized with vinyl-, benzyl-, phenyl-, cyano-, mercapto-, aminopropyl- or dihydroimidazole moieties is reported. Uniform mesoporous particles ranging in size from 40 to 150 nm are generated in a co-condensation process of tetraethylorthosilicate (TEOS) and organotriethoxysilanes (RTES) in alkaline aqueous media containing triethanolamine (TEA) in combination with cetyltrimethylammonium chloride (CTACl) serving as a structure-directing agent. The materials are obtained as colloidal suspensions featuring long-term stability after template removal by ion exchange with an ethanolic solution of ammonium nitrate or HCl. The spherical particles exhibit a wormlike pore system with defined pore sizes and high surface areas. Samples are analyzed by a number of techniques including TEM, SEM, DLS, TGA, Raman, and cross-polarized (29)Si-MAS NMR spectroscopy, as well as nitrogen sorption measurements. We demonstrate that co-condensation and grafting methods result in similar changes in the nitrogen adsorption behavior, indicating a successful internal lining of the pores with functional groups through both procedures.
ACS Nano 2008 Apr
PMID:Colloidal suspensions of functionalized mesoporous silica nanoparticles. 1920 12

Ttrioctylphosphine oxide (TOPO) stabilized CdSe/ZnS quantum dots (QD) were modified with 6-ferrocenyl-1-hexanethiol (FcHT) or 11-ferrocenyl-1-undecanethiol (FcUT) via ligand exchange. The presence of ferrocenyl thiol ligands on the surface of the QDs was proven by diffusion ordered NMR spectroscopy. Upon replacement of the initial TOPO ligand with ferrocene derivatives the emission of the QDs decreased. Phase transfer of ferrocene-modified QDs from organic solvents into water was achieved by complexation reactions with beta-cyclodextrin (beta-CD). The QDs coated with ferrocene thiols are soluble in nonpolar solvents and are transferred into the aqueous phase upon formation of host-guest complexes between the ferrocene units and the cavity of beta-CD. The reversibility of the phase transfer was probed by the addition of naphthalene and adamantane derivatives to the aqueous phase containing QD-[Fc-CD] adduct.
ACS Nano 2009 Mar 24
PMID:Reversible phase transfer of (CdSe/ZnS) quantum dots between organic and aqueous solutions. 1923 90

This report presents a detailed investigation on the structural stability of the Au(25)(SG)(18) nanoclusters (where, -SG represents glutathionate) with a focus on the oxidation resistance and thermal stability of the Au-S bonds in the cluster. Two types of Au-S binding modes were previously indentified in the crystal structure of phenylethylthiolate-capped Au(25) clusters, and the Au(25)(SG)(18) nanoclusters have been confirmed to adopt the same structure as Au(25)(SCH(2)CH(2)Ph)(18) in our previous works. Herein, NMR in combination with optical spectroscopy revealed some distinct differences in both antioxidation and thermal stability of the two Au-S binding modes in the Au(25)(SG)(18) nanocluster, that is, the mode I thiolate ligands (total 12 ligands) exhibit a much higher stability than the mode II ligands (total 6 ligands). Upon the basis of this major observation as well as an abundant [Au(25)S(12)](-) species detected in laser desorption ionization (LDI) mass spectrometry analysis of Au(25)(SG)(18) clusters, a metastable species similar to Au(25)(SG)(12) (i.e., a loss of six type II ligands of -SG) may exist in the processes of oxidation and thermal treatment. Overall, this work provides a deep insight into the intriguing structure of thiolate-capped Au(25) clusters, which will benefit future studies pertaining to the potential applications of such nanoclusters.
ACS Nano 2009 Jul 28
PMID:Stability of the Two Au-S Binding Modes in Au(25)(SG)(18) Nanoclusters Probed by NMR and Optical Spectroscopy. 1954 95

Malonylation of an acyl carrier protein (ACP) by malonyl Coenzyme A-ACP transacylase (MCAT) is fundamental to bacterial fatty acid biosynthesis. Here, we report the structure of the Steptomyces coelicolor (Sc) fatty acid synthase (FAS) ACP and studies of its binding to MCAT. The carrier protein adopts an alpha-helical bundle structure common to other known carrier proteins. The Sc FAS ACP shows close structural homology with other fatty acid ACPs and less similarity with Sc actinorhodin (act) polyketide synthase (PKS) ACP where the orientation of helix I differs. NMR experiments were used to map the binding of ACP to MCAT. This data suggests that Sc FAS ACP interacts with MCAT through the negatively charged helix II of ACP, consistent with proposed models for ACP recognition by other FAS enzymes. Differential roles for residues at the interface are demonstrated using site-directed mutagenesis and in vitro assays. MCAT has been suggested, moreover, to participate in bacterial polyketide synthesis in vivo. We demonstrate that the affinity of the polyketide synthase ACP for MCAT is lower than that of the FAS ACP. Mutagenesis of homologous helix II residues on the polyketide synthase ACP suggests that the PKS ACP may bind to MCAT in a different manner than the FAS counterpart.
ACS Chem Biol 2009 Aug 21
PMID:Structure and malonyl CoA-ACP transacylase binding of streptomyces coelicolor fatty acid synthase acyl carrier protein. 1955 75

The backbone and side chain resonance assignments of human ACS (approximately 22 KD), apoptosis-associated speck-like protein containing a caspase recruitment domain and a pyrin domain, have been determined by triple-resonance NMR techniques.
Biomol NMR Assign 2007 Jul
PMID:1H, 15N and 13C backbone and side chain chemical shifts of human ASC (apoptosis-associated speck-like protein containing a CARD domain). 1963 48

HIV-1 requires a -1 translational frameshift to properly synthesize the viral enzymes required for replication. The frameshift mechanism is dependent upon two RNA elements, a seven-nucleotide slippery sequence (UUUUUUA) and a downstream RNA structure. Frameshifting occurs with a frequency of approximately 5%, and increasing or decreasing this frequency may result in a decrease in viral replication. Here, we report the results of a high-throughput screen designed to find small molecules that bind to the HIV-1 frameshift site RNA. Out of 34,500 compounds screened, 202 were identified as positive hits. We show that one of these compounds, doxorubicin, binds the HIV-1 RNA with low micromolar affinity (K(d) = 2.8 microM). This binding was confirmed and localized to the RNA using NMR. Further analysis revealed that this compound increased the RNA stability by approximately 5 degrees C and decreased translational frameshifting by 28% (+/-14%), as measured in vitro.
ACS Chem Biol 2009 Oct 16
PMID:Selection and characterization of small molecules that bind the HIV-1 frameshift site RNA. 1967 41

Mechanistic details of mammalian metabolism in vivo and dynamic metabolic changes in intact organisms are difficult to monitor because of the lack of spatial, chemical, or temporal resolution when applying traditional analytical tools. These limitations can be addressed by sensitivity enhancement technology for fast in vivo NMR assays of enzymatic fluxes in tissues of interest. We apply this methodology to characterize organ-specific short chain fatty acid metabolism and the changes of carnitine and coenzyme A pools in ischemia reperfusion. This is achieved by assaying acetyl-CoA synthetase and acetyl-carnitine transferase catalyzed transformations in vivo. The fast and predominant flux of acetate and propionate signal into acyl-carnitine pools shows the efficient buffering of free CoA levels. Sizeable acetyl-carnitine formation from exogenous acetate is even found in liver, where acetyl-CoA synthetase and acetyl-carnitine transferase activities have been assumed sequestered in different compartments. In vivo assays of altered acetate metabolism were applied to characterize pathological changes of acetate metabolism upon ischemia. Coenzyme pools in ischemic skeletal muscle are reduced in vivo even 1 h after disturbing muscle perfusion. Impaired mitochondrial metabolism and slow restoration of free CoA are corroborated by assays employing fumarate to show persistently reduced tricarboxylic acid (TCA) cycle activity upon ischemia. In the same animal model, anaerobic metabolism of pyruvate and tissue perfusion normalize faster than mitochondrial bioenergetics.
 ...
PMID:Tissue-specific short chain fatty acid metabolism and slow metabolic recovery after ischemia from hyperpolarized NMR in vivo. 1986 11

While some organisms, including humans, eliminate oxidized purines to get rid of excess nitrogen, for many others the recovery of the purine ring nitrogen is vital. In the so-called ureide pathway, nitrogen is released as ammonia from allantoate through a series of reactions starting with allantoate amidohydrolase (AAH), a manganese-dependent enzyme found in plants and bacteria. We report NMR evidence that the true product of the AAH reaction is S-ureidoglycine, a nonstandard alpha-amino acid that spontaneously releases ammonia in vitro. Using gene proximity and logical genome analysis, we identified a candidate gene (ylbA) for S-ureidoglycine metabolism. The proteins encoded by Escherichia coli and Arabidopsis thaliana genes catalyze the manganese-dependent release of ammonia through hydrolysis of S-ureidoglycine. Hydrolysis then inverts the configuration and yields S-ureidoglycolate. S-Ureidoglycine aminohydrolase (UGHY) is cytosolic in bacteria, whereas in plants it is localized, like allantoate amidohydrolase, in the endoplasmic reticulum. These findings strengthen the basis for the known sensitivity of the ureide pathway to Mn availability and suggest a further rationale for the active transport of Mn in the endoplasmic reticulum of plant cells.
ACS Chem Biol 2010 Feb 19
PMID:Chemical basis of nitrogen recovery through the ureide pathway: formation and hydrolysis of S-ureidoglycine in plants and bacteria. 2003 85

In this paper, we report the functionalization of the surface of multiwalled carbon nanotubes (MWNTs) with Au dendrimer encapsulated nanoparticles (DENs). The results show that, when pristine MWNTs having hydrophobic surfaces are exposed to DENs, the dendrimers aggregate on the MWNT surface. However, when the MWNTs are oxidized in acid prior to exposure to DENs, well-dispersed submonolayer coverages of Au nanoparticles are observed on the MWNT surface. This suggests that acid-induced debundling of the nanotubes is an essential prerequisite for attachment of nearly monodisperse DENs. Electron microscopy and NMR spectroscopy confirm that the structures of the DENs and dendrimers are retained after immobilization on the surface of acid-functionalized MWNTs.
ACS Nano 2010 Feb 23
PMID:Gold dendrimer encapsulated nanoparticles as labeling agents for multiwalled carbon nanotubes. 2011 60


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