Enantioselective synthesis of new oxazolidinylthiazolidines as enzyme inhibitors

The synthesis of new oxazolidinylthiazolidines bicycles, oxygen analogues of bisthiazolidines, also known as metallo-β-lactamase inhibitors is described. The reaction of β-aminoalcohols and 2,5-dihydroxy-1,4-dithiane led to oxazolidinylthiazolidines and/or dithioazabicycles as the main products. The distribution pattern depends mainly on the aminoalcohol substituents. In a one-pot reaction, four new bonds are formed in good yields and with high atom efficiency. When the oxazolidinylthiazolidines are formed, two stereogenic centres are generated with high enantiospecificity. The reaction mechanism is discussed based on crystallographic data and interconversion studies. Two oxazolidinylthiazolidines were evaluated as inhibitors of the potent lactamase NDM-1 and compound 4f displayed competitive inhibition with K_i = 1.6 ± 0.6 μM.     

Modeling nonbilinear total synchronous fluorescence data matrices with a novel adapted partial least squares method

A new residual modeling algorithm for nonbilinear data is presented, namely unfolded partial least squares with interference modeling of non bilinear data by multivariate curve resolution by alternating least squares (U-PLS/IMNB/MCR-ALS). Nonbilinearity represents a challenging data structure problem to achieve analyte quantitation from second-order data in the presence of uncalibrated components. Total synchronous fluorescence spectroscopy (TSFS) generates matrices which constitute a typical example of this kind of data. Although the nonbilinear profile of the interferent can be achieved by modeling TSFS data with unfolded partial least squares with residual bilinearization (U-PLS/RBL), an extremely large number of RBL factors has to be considered. Simulated data show that the new model can conveniently handle the studied analytical problem with better performance than PARAFAC, U-PLS/RBL and MCR-ALS, the latter modeling the unfolded data. Besides, one example involving TSFS real matrices illustrates the ability of the new method to handle experimental data, which consists in the determination of ciprofloxacin in the presence of norfloxacin as interferent in water samples.     

Determination of N-acetylcysteine via its effect on the aggregation of gold nanoparticles

The effect of thiol compounds on the kinetics of the aggregation of gold nanoparticles in the presence of the cationic surfactant cetyltrimethyl ammonium bromide has been studied. It was applied to the determination of N-acetylcysteine using the stopped-flow mixing technique along with light scattering detection. The signal obtained was measured after about 5?s, and gave the analytical information for a calibration graph in the concentration range from 2.9 to 60???mol?L^?1 of N-acetylcysteine, and a detection limit of 0.87???mol?L^?1. The effect of other thiols on the system is also described. The relative standard deviation ranges between 0.6% and 3.5%. The method was applied to the determination of N-acetylcysteine in several pharmaceutical samples with recoveries that range from 97.7% to 101.1%.

Size distribution effect on the magnetization and magneto-resistance in ferromagnetic nanostructured manganites

We present experimental data of magnetization and magneto-resistance of nanostructured La_2/3B_1/3MnO₃ with B=Ca, Sr, which present difference between the coercive field in the magnetization loop with their corresponding maximum value in the magneto-resistance. This difference is described by a model that include, size distribution of magnetic particles, randomly oriented anisotropy axis and electronic transfer between the particles, which is mediated by spin-polarized tunneling process. Also, the model predicts that the maximum magneto-resistance can be, in the magnetic disorder state, two times larger than the experimental value. The model results can be used to estimate the size dispersion of nanoparticles in similar systems.     

Voltammetric Study of the Hydrolysis Product of Bendiocarb at the Glassy Carbon Electrode

 Differential pulse and square wave voltammetric methods are proposed for the determination of the N-methylcarbamate insecticide Bendiocarb based on the oxidation of its hydrolysis product. A single peak (+ 0.7 V (pH 3.5)) is observed at the glassy carbon electrode. Linear relationships between the peak intensity and the concentration were obtained up to 5 μg/ml, with detection limits of 0.77 μg/ml for DPV and 0.44 μg/ml for SWV, respectively. The methods have been applied satisfactorily for the analysis of river water samples spiked with the pesticide by using the extraction with diethyl ether as preconcentration step. Concentration values as low as 0.11 μg/ml were determined with a recovery of 96 ± 3% (SWV) and 91 ± 2% (DPV). Received June 20, 2000. Revision March 6, 2001.     

Targeting tuberculosis through a small focused library of 1,2,3-triazoles

Looking for new active molecules against Mycobacterium tuberculosis, a small focused library of 1,2,3-triazoles was efficiently prepared by click chemistry. Compounds were subsequently tested against different pathogenic and opportunistic mycobacteria including M. avium and M. tuberculosis. Two of them showed MIC at lower μg/mL concentration for M. avium and even below that for M. tuberculosis, being more potent that control drugs.     

Effects of milling-induced disorder on the lattice parameters and magnetic properties of hematite

A ball-milling treatment in air for 30 min is enough to reduce anhydrous bulk hematite (-Fe₂O₃) grains to nanometric sizes. For milling times, t_m, of 15, 30 min, 1 and 2 h, the crystals suffer an anisotropic lattice dilation, which is more pronounced for the smaller average grain sizes attained. Mössbauer and susceptibility results show that the process alters the effective Morin temperatures, T_M. The transition occurs less sharply than for the non-milled material and spreads over a maximum extent of 50 K for t_m=30 min. The susceptibility data indicate that the T_M for the fraction of material undergoes the transition shifts toward values not lower than 235 K. For t_m=10 h the transition is suppressed down to 12 K. Our results do not allow distinguishing bulk from surface regions of the grains. In addition, the disorder brought about by the milling reduces the magnetic response in the weak ferromagnetic state of -Fe₂O₃. The modification of T_M seems to depend mainly on the anisotropic dilation of the unit cell that affects the whole grain and it is related to the nanometric grain sizes achieved.     

Magnetic and Hyperfine Interactions in Systems of Antiferromagnetic Oxide Nanoparticles with Industrial Applications

We report characterizations of systems of antiferromagnetic iron oxide particles of importance in catalysis where Mössbauer spectroscopy has played an essential role. We show that signals contributing to the Mössbauer spectra that arise from the surface or from non-crystalline environments still find difficulty in their assignment in spite of the large amount of research already done on hematite and similar antiferromagnetic systems.     

Phase transformations induced by implantation of12C− ions into α-Fe and AISI 304 and 316 stainless steels studied by CEMS and SEM

Samples of AISI 304 and 316 stainless steels, initially in austenitic (first set) and martensitic states (second set) and -Fe (third set), were implanted with 180 keV¹²C^– to a dose of 10¹⁷ atoms/cm² at room temperature. Surfaces were examined by SEM (scanning electron microscopy) and the crystalline-phase fractions were estimated through CEMS (conversion electron Mössbauer spectroscopy). Different grades of etching were produced by sputtering during the implantations on the stainless steel samples depending on the previous crystallographic states. CEMS data reveal the transformation in the initially martensitic samples and no noticeable modifications as a result of the implantation on -Fe and austenitic samples.     

Proton-electron double resonance: Spectroscopy and imaging in very low magnetic fields

Proton-electron double resonance, also named dynamic nuclear polarization (DNP) is characterized by an enhancement of the proton NMR signal when the electron resonance of a free radical coupled to water protons is saturated. In very low magnetic fields, new EPR transitions are allowed. The complete theory of DNP in nitroxide solutions is recalled, in order to interpret the experimental spectra. Proton-electron double resonance images (PEDRI) were obtained with 7 out of the 10 EPR transitions allowed in low fields. Field cycled PEDRI were obtained in magnetic fields as low as 0.6 G. The optimum field strength for PEDRI is discussed according to the theoretical and experimental results of DNP obtained with nitroxide solutions.