Study of the behaviour of amino acids in aqueous solution by time-domain NMR and high-resolution NMR

The study of protein hydration by time-domain NMR is complicated by the great number of interactions involved, resulting from the presence of several amino acids and the possible modifications produced by the various structures. Moreover, a good comprehension of the molecular interactions of the simple amino acids in solution is essential to elucidate the mechanism of the biological functions of proteins. Measurements of transverse relaxation rates of the protons of water (R(2) = 1/T(2)) in aqueous solutions of amino acids such as L-glycine, L-asparagine, L-arginine and L-tryptophan were carried out in order to study the effects of chemical exchange and molecular diffusion on the amplitude of R(2). The values of R(2) measured by the Carr-Purcell-Meiboom-Gill (CPMG) sequence were studied while varying the solution pH and the parameters of the CPMG sequence. The dependence of R(2) on pH and tau (inter-pulse delay between the first and the second pulses of the CPMG sequence) is interpreted in terms of chemical exchange between the protons of water and those of the labile amino acid groups. This interpretation was confirmed by the analysis of the proton spectra acquired using a 300 MHz NMR spectrometer.     

Copper corrosion in buffered and non-buffered synthetic seawater: a comparative study

The electrochemical behaviour of copper in neutral buffered and non-buffered synthetic seawater and in pure chloride solutions has been studied by cyclic voltammetry, weight loss measurements, open circuit potential and scanning electron microscopy (SEM). Values of the repassivation potentials of Cu in non-buffered and buffered synthetic seawater, at 50 mV s^–1, were 0.12 and 0.46 V vs. SCE, respectively. The sharpness, heights and location of the different peaks as well as their charges were shown to be influenced by the composition of the solution, buffering conditions, deoxygenation, polarization potential and time. High chloride concentrations lead to higher oxidation charges. The anodic and the cathodic charges were shown to increase as the chloride concentration increases. The open circuit potential transients of copper in non-deoxygenated, non-buffered synthetic seawater indicate pitting from the beginning of the exposure, while in buffered solutions the pitting appeared only after a quite long exposure period, i.e. after 40 days. Corrosion rates of Cu samples after 3 months of immersion were higher in solutions of pure chloride (0.5 M) than in synthetic seawater. After six months the differences were even more noticeable. SEM images have showed a somewhat higher density of pits on copper samples immersed in the chloride solution (0.5 M), in comparison with those in synthetic seawater.     

Elucidation of the pore structure of SBA-2 using Monte Carlo simulation to interpret experimental data for the adsorption of light hydrocarbons

We have measured the adsorption of methane and ethane to high pressure on SBA-2, a structured mesoporous silica composed of spheres connected by narrow channels. The experimental data were analyzed by carrying out Monte Carlo simulations of adsorption in pore structure models of different complexity and then adjusting the parameters of the models to match the Monte Carlo results to the experimental data. We found that a model based on single-sized spherical cavities was inadequate and that it is necessary to explicitly account for the interconnecting channels. Further, we found that despite the basic regularity of the SBA-2 structure, it is necessary to allow for a distribution of the sizes of both the cavities and the channels. These size distributions were obtained by fitting the parameters of the model to the experimental adsorption data, revealing detailed structural information not previously known for this material. The channels were found to be 5-15 A in diameter, while the cavities were 40-50 A in diameter. There is some evidence that the distribution of channel sizes leads to a percolation effect whereby the pore structure is not equally accessible to all adsorptives.     

Efficient chiral discrimination by 77Se NMR

[reaction: see text] Several (77)Se NMR experiments were performed by titrating a sample of selenides with the chiral shift reagent methylbenzylamine (MBA), followed by acquisition of (77)Se NMR spectra. Eventually, we observed the appearance of two anisochronous resonances, with a relatively large separation, from 37 to 56 Hz, corresponding to the formation of the diastereomeric complexes. This methodology avoids derivatization processes, and the studied compound can be easily recovered from the NMR tube.     

Monitoring some phenoxyl-type N-methylcarbamate pesticide residues in fruit juices using high-performance liquid chromatography with peroxyoxalate-chemiluminescence detection

A systematic investigation of optimal conditions for determining three of the most common phenoxyl-type N-methylcarbamate pesticides (carbaryl, carbofuran and propoxur) in fruit juices by HPLC with peroxyoxalate-chemiluminescence detection is described. The required pre-column hydrolysis of pesticides and derivatization of their hydrolytic metabolites with dansyl chloride was simultaneously carried out in a short time thanks to the micellar catalytic effect provided by cetyltrimethyl ammonium bromide micelles on the hydrolysis step. The liquid chromatographic separation of the dansylated phenols was performed on a reversed-phase C18 column with isocratic elution. The analytes were detected by using an integrated derivatization chemiluminescence detection unit based on the bis(2,4,6-trichlorophenyl)oxalate-hydrogen peroxide system. Fruit juice samples containing 4.0-1500 microg/l pesticides were analysed with a precision of ca. 6.5%. After contamination of the fruit juice samples, average recovery > 93% at fortification levels of 10-100 microg/l was obtained.     

A Comparative Study of Naproxen – Beta Cyclodextrin Complexes Prepared by Conventional Methods and Using Supercritical Carbon Dioxide

Naproxen is a poorly soluble anti-inflammatorydrug, the solubility of which canbe enhanced by complexation withbeta-cyclodextrin. Besides that, the inclusioncomplex reduces the incidence of gastrointestinal side effects of the drug. The aim of this work was to compare the physicochemical characteristics of the solid complexes prepared by traditional methods (kneading, freeze-drying and spray-drying) and using a supercritical fluid technology. The unusual solvent properties of carbon dioxide above their critical temperature and pressure were exploited in order to prepare inclusion compounds. Complexes prepared using supercritical fluid technology showed similar properties to those of freeze-drying andspray-drying complexes as proved by DSC, FT-IRand UV.     

Kinetic fluorimetric determination of nanogram amounts of manganese, based on its catalysis of the oxidation of 2-hydroxybenzaldehyde thiosemicarbazone with hydrogen peroxide

A kinetic method is described for the determination of trace amounts of manganese(II), based on its catalytic effect on the oxidation of 2-hydroxybenzaldehyde thiosemicarbazone by hydrogen peroxide. The reaction is followed by measuring the rate of change of fluorescence (lambda(ex) 365 and lambda(em) 440 nm). The calibration is linear over the manganese range 2-9 ng ml with a precision of +/-1%. The proposed method suffers from few interferences.     

Ag(I) complexes with alkylidene-bis(2-aminopyrimidines) as building units for discrete metallomacrocyclic frames. A structural and solution study

Alkylidene-bis(2-aminopyrimidines) (pyr2Cx, x = 2-5) are useful ligands to interact with Ag(I) yielding discrete metallocycles. Crystal structures of the [(pyr2C2)Ag(NO3)]2 and [(H-pyr2C4)Ag(NO3)2]2 have been isolated where each macrocyclic moiety interacts with their surroundings through weak interactions, yielding 3D discrete structures, On the other hand, the solution study shows that the equilibrium constants for the formation of Ag(pyr2Cx)+ complexes are higher than the literature values for Ag(I) complexes with single pyrimidines, although the differences could be explained by invoking the solid-state structures of the Ag(I)-pyr2Cx complexes.     

Determination of mercury in soils and sediments by graphite furnace atomic absorption spectrometry with slurry sampling

A graphite furnace atomic absorption spectrometric procedure for the determination of mercury is presented, in which the samples are suspended in a solution containing hydrofluoric and nitric acids. Silver nitrate (4% m/v) and potassium permanganate (3%) are incorporated, in the order specified, and aliquots are directly introduced into the graphite furnace. A fast heating programme with no conventional pyrolysis step is used. The detection limit for mercury in a 125 mg ml⁻¹ suspension is 0.1 μg g⁻¹. Calibration is performed by using aqueous standards. The reliability of the procedure is proved by analysing certified reference materials.     

Simultaneously multiply-configurable or superposed molecular logic systems composed of ICT (internal charge transfer) chromophores and fluorophores integrated with one- or two-ion receptors

Integrated "ICT chromophore-receptor" systems show ion-induced shifts in their electronic absorption spectra. The wavelength of observation can be used to reversibly configure the system to any of the four logic operations permissible with a single input (YES, NOT, PASS 1, PASS 0), under conditions of ion input and transmittance output. We demonstrate these with dyes integrated into Tsien's calcium receptor, 1-2. Applying multiple ion inputs to 1-2 also allows us to perform two- or three-input OR or NOR operations. The weak fluorescence output of 1 also shows YES or NOT logic depending on how it is configured by excitation and emission wavelengths. Integrated "receptor(1)-ICT chromophore-receptor(2)" systems 3-5 selectively target two ions into the receptor terminals. The ion-induced transmittance output of 3-5 can also be configured via wavelength to illustrate several logic types including, most importantly, XOR. The opposite effects of the two ions on the energy of the chromophore excited state is responsible for this behaviour. INHIBIT and REVERSE IMPLICATION are two of the other logic types seen here. Integration of XOR logic with a preceding OR operation can be arranged by using three ion inputs. The fluorescence output of these systems can be configured via wavelength to display INHIBIT or NOR logic under two-input conditions. The superposition or multiplicity of logic gate configurations is an unusual consequence of the ability to simultaneously observe multiple wavelengths.