Current-less photoreactivity catalyzed by functionalized AFM tips

A spatially confined photocatalytic oxidation of a thin film of synthetic textile azo dye (Procion Red MX-5B) using TiO2-functionalized AFM probes is described.     

Analytical TEM examinations of CoPt-TiO2 perpendicular magnetic recording media.

For this analytical TEM study, nonmagnetic oxygen-rich boundaries were introduced into Co-Pt-alloy perpendicular recording media by cosputtering Co and Pt with TiO2. Increasing the TiO2 content resulted in changes to the microstructure and elemental distribution within grains and boundaries in these films. EFTEM imaging was used to generate composition maps spanning many tens of grains, thereby giving an overall depiction of the changes in elemental distribution occurring with increasing TiO2 content. Comparing EFTEM with spectrum-imaging maps created by high-resolution STEM with EDXS and EELS enabled both corroboration of EFTEM results and quantification of the chemical composition within individual grain boundary areas. The difficulty of interpreting data from EDXS for these extremely thin films is discussed. Increasing the TiO2 content of the media was found to create more uniformly wide Ti- and O-rich grain boundaries as well as Ti- and O-rich regions within grains.     

New stabilising groups for lateral lithiation of ortho-cresol derivatives

1-(2-Methoxyethoxy)-2-methylbenzene and 1-(2-dimethylaminoethoxy)-2-methylbenzene have been lithiated using sec-BuLi under a variety of conditions and the laterally lithiated species trapped with electrophiles.     

Detection of metal ions using ion-channel sensor based on self-assembled monolayer of thioctic acid

Gold electrodes were chemically modified with thioctic acid monolayer designed to mimic biological ion-channel membranes. The technique was then used in the determination of alkali, alkaline earth, thallium(I), and lanthanum metal cations as analytes. Cyclic voltammograms (CV) of [Fe(CN)6]³⁻ an electroactive marker, were measured in the presence of the various types of analyte cations. In the absence of the analyte cation, electrostatic repulsion between the marker anions and the carboxylate groups of the receptor monolayer hindered the approach of the marker anion to the electrode surface and hence hindered its reduction. The modified electrodes responded well to the metal cations except the alkali metal cations. The sensors could detect the trivalent cation La³⁺ at concentrations as low as 10⁻⁸ M. The response of the sensor to the metal cations increase in the order alkali metal3+ can be discriminated in the ratio 1:100. This makes it possible to determine the trivalent ion in a sample matrix containing monovalent and divalent cations. Thallium(I) ion showed marked deviation in its response as compared to monovalent ions of the alkali metals. The ion-channel sensor based on self-assembled monolayer of thioctic acid therefore offers a potential alternative technique for the selective determination of metal ions.     

Addressing the metabolic activation potential of new leads in drug discovery: a case study using ion trap mass spectrometry and tritium labeling techniques

Metabolic activation of drug candidates to electrophilic reactive metabolites that can covalently modify cellular macromolecules may result in acute and/or idiosyncratic immune system-mediated toxicities in humans. This presents a significant potential liability for the future development of these compounds as safe therapeutic agents. We present here an example of an approach where sites of metabolic activation within a new drug candidate series were rapidly identified using online liquid chromatography/multi-stage mass spectrometry on an ion trap mass spectrometer. This was accomplished by trapping the reactive intermediates formed upon incubation of compounds with rat and human liver microsomes as their corresponding glutathione conjugates and mass spectral characterization of these thiol adducts. Based on the structures of the GSH adducts identified, potential sites and mechanisms of bioactivation within the chemical structure were proposed. These metabolism studies were interfaced with iterative structural modifications of the chemical series in order to block these bioactivation sites within the molecule. This strategy led to a significant reduction in the propensity of the compounds to undergo metabolic activation as evidenced by reductions in the irreversible binding of radioactivity to liver microsomal material upon incubation of tritium-labeled compounds with this in vitro system. With the efficiency and throughput achievable with such an approach, it appears feasible to identify and address the metabolic activation potential of new drug leads during routine metabolite identification studies in an early drug discovery setting.     

Cell surface labeling of Escherichia coli via copper(I)-catalyzed [3+2] cycloaddition

Labeling of the cell surface of Escherichia coli was accomplished by expression of a recombinant outer membrane protein, OmpC, in the presence of the unnatural amino acid azidohomoalanine, which acts as a methionine surrogate. The surface-exposed azide moieties of whole cells were biotinylated via Cu(1)-catalyzed [3+2] azide-alkyne cycloaddition. The specificity of labeling of both wild-type OmpC and a mutant containing additional methionine sites for azidohomoalanine incorporation was confirmed by Western blotting. Flow cytometry was performed to examine the specificity of the labeling. Cells that express the mutant form of OmpC in the presence of azidohomoalanine, which were biotinylated and stained with fluorescent avidin, exhibit a mean fluorescence 10-fold higher than the background. Incorporation of an unnatural amino acid can thus be determined on a single-cell basis.     

On the High Viscosity of Aqueous Solution of Lysozyme Induced by Some Organic Solvents

A remarkably high viscosity has been induced in protein aqueous solutions by the addition of certain structurally related organic solvents. The effect has been observed for lysozyme aqueous solutions containing tetramethylurea (TMU), dimethylsulfoxide (DMSO), dimethylformamide, and hexamethylphosphortriamide. The effect has also been induced in ferrocytochrome c aqueous solutions by TMU. Critical concentrations for both the protein and organic solvent were verified for the onset of the viscosity increase. A common feature of the solvents which were able to induce the effect is a dipolar moiety (C=O, S → O and P → O) and a nonpolar region represented by the methyl groups. The resulting fluids show an extremely restricted flow and a typical non-Newtonian, pseudoplastic behavior. Use was made of¹H nuclear magnetic resonance (NMR) and Raman spectroscopy to characterize protein structural modifications and of¹³C NMR to investigate changes in relaxation times and chemical shifts in the solvent/water solutions. A systematic rheological characterization of the systems was undertaken for some of the solvents, and unusual patterns of viscous effects were identified for the solvent/water systems both with and without protein. The process was found to be at least partially reversible, as concluded from the recovered original solution rheological characteristics and the original protein¹H NMR spectrum, after eliminating the organic solvent by ultrafiltration. The whole process was characterized as consisting of two mutually independent stages. The first involves an extensive conformational transition of the polypeptide backbone, from a predominantly α-helical to increased random coiled and β-sheet structures, with the occurrence of nonorthodox protein secondary structures at regions above the solvent critical point. The second stage consists of short-lived interchain contacts leading to an entanglement of the macromolecular system as a whole. A microphase reversion in the organic solvent/water mixture, supported by¹³C NMR and rheological results, is proposed as the driving force causing the observed behavior.     

A selective electrolytic sensor for nitrite based on a modified platinum electrode

The electrolytic sensor described is based on the oxidation of nitrite at a platinum electrode modified with chemisorbed iodine and coated with a thin layer of quaternized poly(4-vinylpyridine), qPVP. The sealed sensor uses an anion-exchange membrane to separate Donnan transport of nitrite across the membrane and controlled potential electrolysis at the Pt/qPVP indicator electrode. The sensor has a linear response to nitrate concentration in aqueous samples over the range 4 × 10^?6?2 × 10^?3 M nitrite. The detection limit is 2 × 10^?6 M nitrite. The sensor is free of interference by nitrate, dissolved oxygen, cations, and many neutral species. Anions that are electroactive at 0.7 V vs. Ag/ AgCl would interfere, but they are uncommon in most samples. Initial tests with lake water samples suggest that this sensor is unaffected by this matrix. The system was also evaluated for monitoring nitrite levels in spiked meat extracts.     

Stable silica gel-bound crown ethers. Selective separation of metal ions and a potential for separations of amine enantiomers

Silica gel-bound crown ethers and aza macrocycles have been synthesized with the attaching arm connected to the carbon framework of the macrocycles. The interactions of these bound macrocycles with cations are almost identical to those involving the analogous free macrocycles. This has allowed for predictable cation separation, concentration, and removal processes to be performed on a small scale. Quantum mechanical calculations and NMR measurements indicate that similarly bound chiral macrocycles will be capable of use in separating chiral organic amines.Dedicated to the memory of Professor James J. Christensen who died on 5 September 1987.     

A stereoelectronic effect on turn formation due to proline substitution in elastin-mimetic polypeptides

Stereoelectronic effects have been identified as contributing factors to the conformational stability of collagen-mimetic peptide sequences. To assess the relevance of these factors within other protein structural contexts, three polypeptide sequences were prepared in which the sequences were derived from the canonical repeat unit (Val-Pro-Gly-Val-Gly) of the protein material elastin. These elastin-mimetic polypeptides, elastin-1, elastin-2, and elastin-3, incorporate (2S)-proline, (2S,4S)-4-fluoroproline, and (2S,4R)-4-fluoroproline, respectively, at the second position of the elastin repeat. Calorimetric and spectroscopic investigations of these three polypeptides indicate that the incorporation of the substituted proline residues had a dramatic effect upon the self-assembly of the corresponding elastin peptide. The presence of (2S,4R)-4-fluoroproline in elastin-3 lowered the temperature of the phase transition and increased the type II beta-turn population with respect to the parent polypeptide, while the presence of (2S,4S)-4-fluoroproline in elastin-2 had the opposite effect. These results suggest that stereoelectronic effects could either enhance or hinder the self-assembly of elastin-mimetic polypeptides, depending on the influence of the proline analogue on the energetics of the beta-turn conformation that develops within the pentapeptide structural repeats above the phase transition. Density functional theory (DFT) was employed to model three possible turn types (betaI-, betaII-, and inverse gamma-turns) derived from model peptide segments (MeCO-Xaa-Gly-NHMe) (Xaa = Pro, 4S-F-Pro, or 4R-F-Pro) corresponding to the turn-forming residues of the elastin repeat unit (Val-Pro-Gly-Val-Gly). The results of the these calculations suggested a similar outcome to the experimental data for the elastin-mimetic polypeptides, in that type II beta-turn structures were stabilized for peptide segments containing (2S,4R)-fluoroproline and destabilized for segments containing (2S,4S)-fluoroproline relative to the canonical proline residue.