Electrochemically switchable hydrogen-bonded molecular shuttles

A series of [2]rotaxanes containing succinamide and naphthalimide hydrogen-bonding stations for a benzylic amide macrocycle is described. Electrochemical reduction and oxidation of the naphthalimide group alters its ability to form hydrogen bonds to the macrocycle to such a degree that redox processes can be used to switch the relative macrocycle-binding affinities of the two stations in a rotaxane by over 8 orders of magnitude. The structure of the neutral [2]rotaxane in solution is established by (1)H NMR spectroscopy and shows that the macrocycle exhibits remarkable positional integrity for the succinamide station in a variety of solvents. Cyclic voltammetry experiments allow the simultaneous stimulation and observation of a redox-induced dynamic process in the rotaxane which is both reversible and cyclable. Model compounds in which various conformational and co-conformational changes are prohibited demonstrate unequivocally that the redox response is the result of shuttling of the macrocycle between the two stations. At room temperature in tetrahydrofuran the electrochemically induced movement of the macrocycle between the two stations takes approximately 50 micros.     

Evaluation of the use of consensus values in proficiency testing programmes

Proficiency testing (PT) is an essential tool used by laboratory accreditation bodies to assess the competency of laboratories. Because of limited resources of PT providers or for other reasons, the assigned reference value used in the calculation of z-score values has usually been derived from some sort of consensus value obtained by central tendency estimators such as the arithmetic mean or robust mean. However, if the assigned reference value deviates significantly from the ‘true value’ of the analyte in the test material, laboratories’ performance will be evaluated incorrectly. This paper evaluates the use of consensus values in proficiency testing programmes using the Monte Carlo simulation technique. The results indicated that the deviation of the assigned value from the true value could be as large as 40%, depending on the parameters of the proficiency testing programmes under investigation such as sample homogeneity, number of participant laboratories, concentration level, method precision and laboratory bias. To study how these parameters affect the degree of discrepancy between the consensus value and the true value, a fractional factorial design was also applied. The findings indicate that the number of participating laboratories and the distribution of laboratory bias were the prime two factors affecting the deviation of the consensus value from the true value.     

Manganese/Bicarbonate-catalyzed epoxidation of lipophilic alkenes with hydrogen peroxide in ionic liquids

[reaction: see text] Effective epoxidation of lipophilic alkenes using hydrogen peroxide was accomplished with the manganese sulfate/bicarbonate catalytic system in an ionic liquid at room temperature.     

Langmuir — Blodgett deposition of lipid films on hydrogel as a basis for biosensor development

Lipid membranes composed of phosphatidyl choline and cholesterol were interfaced to polyacrylamine hydrogen by Langmuir—Blodgett thin-film deposition. The extent of lipid adsorption to the gel surface was critically dependent on the hydration of the polymer as determined by contact angle measurements. Some electrochemical transducers incorporating the deposited membrane/gel structure responded positively to phloretin and valinomycin. Limitations to the construction of this device are discussed.     

ESTIMATION OF ENERGY DISTRIBUTION AND REDISTRIBUTION AMONG TWO PHOTOSYSTEMS USING PARALLEL MEASUREMENTS OF FLUORESCENCE LIFETIMES AND TRANSIENTS AT 77 K

Abstract— A single-sample method for estimating energy distribution and redistribution among the two photosystems using fluorescence lifetimes and transients at 77 K is presented. In this method,α(the fraction of photons absorbed by photosystem I, PSI) is F_1(α)/(F_1(α)+ (τ_{F 1(M)}/τ_{F 2(M)}).F_2(M)) where, F_1(α) is the fluorescence intensity from PSI excited by photons initially absorbed by the latter, τ_{F 1(M)} and τ_{F 2(M)} are the maximum lifetimes of fluorescence from chlorophyll- a in PSI (1) and II (2), and, F_2(M) is the maximum fluorescence intensity from PSII (P level). Analysis of the intensities and lifetimes of wavelength resolved fluorescence of thylakoids (pH 7.0), with and without cations, leads to the following conclusions: The addition of 10 m M Na⁺ to cation-depleted thylakoids (pH 7.0) increases α by ˜ 10%, while the subsequent addition of 10 m M Mg²⁺ leads to three principal concomitant changes (in the order of importance): a 50% decrease in PSII to PSI energy transfer, a 20% increase in other radiation-less losses, and a 10% decrease in α.     

A strategy for the solution-phase parallel synthesis of N-(pyrrolidinylmethyl)hydroxamic acids

Both five- and six-membered iminocyclitols have proven to be useful transition-state analogue inhibitors of glycosidases. They also mimic the transition-state sugar moiety of the nucleoside phosphate sugar in glycosyltransferase-catalyzed reactions. Described here is the development of a general strategy toward the parallel synthesis of a five-membered iminocyclitol linked to a hydroxamic acid group designed to mimic the transition state of GDP-fucose complexed with Mn(II) in fucosyltransferase reactions. The iminocyclitol 8 containing a protected hydroxylamine unit was prepared from D-mannitol. The hydroxamic acid moiety was introduced via the reaction of 8 with various acid chlorides. The strategy is generally applicable to the construction of libraries for identification of glycosyltransferase inhibitors.     

Isolation and purification of the bioactive carotenoid zeaxanthin from the microalga Microcystis aeruginosa by high-speed counter-current chromatography

High-speed counter-current chromatography was successfully applied for the first time to the isolation and purification of the bioactive carotenoid zeaxanthin from the cyanobacterium Microcystis aeruginosa. The crude zeaxanthin was obtained by extraction with organic solvents after the microalgal sample had been saponified. Preparative high-speed counter-current chromatography with a two-phase solvent system composed of n-hexane-ethyl acetate-ethanol-water (8:2:7:3, v/v/v/v) was successfully performed yielding zeaxanthin at 96.2% purity from 150 mg of the crude extract in a one-step separation. The recovery of zeaxanthin was 91.4%. This was also the first report that zeaxanthin was successfully separated and purified from microalgae.     

Carbohydrate-Based Antibiotics: A New Approach to Tackling the Problem of Resistance

Recent interest in the problem of antibiotic resistance has led to the identification of new targets and strategies for antibiotic discovery. Among these efforts, the development of small molecules as antibiotics to target carbohydrate receptors or carbohydrate-modifying enzymes represents a new direction. This review covers recent work in this regard and discusses the impact of each strategy on the development of drug resistance. Particularly interesting targets include unique cell-surface carbohydrates, the transglycosylase involved in peptidoglycan biosynthesis, and bacterial RNA. With a greater understanding of the genome of different bacteria as well as advances in functional genomics and proteomics, we can expect the discovery of a variety of targets for the development of novel antibiotics.     

On-Line Monitoring of the Photolysis of Benzyl Acetate and 3,5-Dimethoxybenzyl Acetate by Membrane Introduction Mass Spectrometry

Membrane introduction mass spectrometry (MIMS) allows on-line monitoring of the products of photolysis (254 nm) of benzyl acetate in aqueous methanol and 3,5-dimethoxybenzyl acetate in water. The reaction mixture is continuously exposed to a silicone membrane through which analyte molecules permeate into a triple quadrupole mass spectrometer for qualitative and quantitative analysis. Ionization is achieved by either isobutane or ammonia chemical ionization, and ions characteristic of the reactant ester and its products are monitored simultaneously and continuously. Three products, benzyl methyl ether, ethylbenzene, and bibenzyl are observed in the benzyl acetate photolysis. Two products, 3,5-dimethoxybenzyl alcohol and 3,5-dimethoxyethylbenzene, are formed in the photolysis of 3,5-dimethoxybenzyl acetate. Quantitation is achieved through calibration using external standard solutions and, in the case of benzyl methyl ether, tandem mass spectrometry is used to verify product identification. During the photolysis of benzyl acetate, benzyl methyl ether and ethylbenzene are produced at onset with similar efficiencies. For the 3,5-dimethoxy ester photolysis, performed in aqueous solution, the efficiency of formation of the polar product 3,5-dimethoxybenzyl alcohol is about 300 times greater than that of the nonpolar product 3,5-dimethoxyethylbenzene. The results show that the relative reaction rates are dependent on the solvent and on the photon intensity and are consistent with earlier off-line experiments by Pincock et al. which showed that the photolysis proceeds through both ion and radical pair intermediates. To the best of our knowledge, the work reported here describes the first analysis of the photochemistry of an aralkyl ester in water and the first use of on-line mass spectrometry in a mechanistic study.     

Hydration of superoxide studied by molecular dynamics simulation

Molecular dynamics was used to study the hydration of superoxide (O). The Helmholtz free energy of hydration of O was estimated by the thermodynamic integration method. The diffusion of O and the water structure around O were also studied. Two water models were used in the calculations and the results were compared to experiments.