Slurry sampling and high-resolution continuum source flame atomic absorption spectrometry using secondary lines for the determination of Ca and Mg in dairy products

The present paper proposes a simple and fast analytical procedure for the sequential multi-element determination of Ca and Mg in dairy products employing sampling slurry and high resolution-continuum source flame atomic absorption spectrometry (HR-CS FAAS). Considering the high concentration of these species in these matrices, the analytical measurements were carried out at the secondary lines of 239.856 and 202.852 for Ca and Mg, respectively. The experimental conditions established for the preparation of the slurries during the optimization step were: 2.0 mol L^{− 1} hydrochloric acid, sonication time of 20 min and sample mass of 1.0 g for a slurry volume of 25 mL. Experiments demonstrated that the analytical curves can be established using the external calibration technique employing aqueous standards. The method allows the determination of Ca and Mg with limits of quantification of 0.038 and 0.016 mg g^{− 1}, respectively. The precision was evaluated under reproducibility and repeatability conditions and expressed as relative standard deviation. The results varied from 2.7 to 2.9% (all tests with n = 10) and using a yogurt sample containing Ca and Mg concentrations of 1.40 and 0.13 mg g^{− 1}, respectively.The accuracy was confirmed by the analysis of a certified reference material of non-fat milk powder furnished by the National Institute of Standard and Technology. The proposed method was applied for the determination of Ca and Mg in yogurt, cow milk and milk powder samples. The samples were also analyzed after complete acid digestion and Ca and Mg determination by HR-CS FAAS. No statistical difference was observed between the results obtained by both of the procedures performed.     

Biheteroaromatic diphosphine oxides-catalyzed stereoselective direct aldol reactions

A highly stereoselective direct aldol condensation of ketones to aromatic aldehydes was realized; the trichlorosilyl enolether generated in situ in the presence of tetrachlorosilane is activated by catalytic amounts of an enantiomerically pure biheteroaromatic phosphine oxide to react with aldehydes, coordinated as well as activated by the chiral cationic hypervalent silicon species. This Lewis acid-mediated Lewis base-catalyzed transformation allowed, starting from two carbonyl compounds, to directly synthesize β-hydroxy ketones generally with high anti stereoselectivity and up to 93% ee for the anti isomer.     

Alcohols immobilization onto 2-chlorotritylchloride resin under microwave irradiation

The immobilization of alcohols onto 2-chlorotritylchloride resin using microwave irradiation was studied. Three different Fmoc-aminoalcohols were tested: the phenol-like Fmoc-tyramine, the primary alcohol Fmoc-ethanolamine, and the secondary alcohol Fmoc-4-hydroxypiperidine. Several reaction conditions were evaluated: different bases, reaction times, temperatures, and concentrations. Microwave immobilization resulted is effective in binding to the resin all three types of alcohols with loadings which were superior or comparable to the ‘classical’ methods in shorter time and without employing toxic and racemizing reagents. This method resulted also useful for the immobilization, through the hydroxyl group, of FmocTyrOAll, FmocSerOAll, and FmocThrOAll, important building blocks for the synthesis of cyclic peptides.     

Electrokinetic techniques applied to electrochemical DNA biosensors

Electrokinetic techniques are contact-free methods currently used in many applications, where precise handling of biological entities, such as cells, bacteria or nucleic acids, is needed. These techniques are based on the effect of electric fields on molecules suspended in a fluid, and the corresponding induced motion, which can be tuned according to some known physical laws and observed behaviours. Increasing interest on the application of such strategies in order to improve the detection of DNA strands has appeared during the recent decades. Classical electrode-based DNA electrochemical biosensors with combined electrokinetic techniques present the advantage of being able to improve the working electrode's bioactive part during their fabrication and also the hybridization yield during the sensor detection phase. This can be achieved by selectively manipulating, driving and directing the molecules towards the electrodes increasing the speed and yield of the floating DNA strands attached to them. On the other hand, this technique can be also used in order to make biosensors reusable, or reconfigurable, by simply inverting its working principle and pulling DNA strands away from the electrodes. Finally, the combination of these techniques with nanostructures, such as nanopores or nanochannels, has recently boosted the appearance of new types of electrochemical sensors that exploit the time-varying position of DNA strands in order to continuously scan these molecules and to detect their properties. This review gives an insight into the main forces involved in DNA electrokinetics and discusses the state of the art and uses of these techniques in recent years.     

Diffusion in macromolecular crowded media: Monte Carlo simulation of obstructed diffusion vs. FRAP experiments

The diffusion of tracer particles in 3D macromolecular crowded media has been studied using two methodologies, simulation and experimental, with the aim of comparing their results. First, the diffusion of a tracer in an obstructed 3D lattice with mobile and big size obstacles has been analyzed through a Monte Carlo (MC) simulation procedure. Secondly, fluorescence recovery after photobleaching (FRAP) experiments have been carried out to study the diffusion of a model protein (alpha-chymotrypsin) in in vitro crowded solution where two type of Dextran molecules are used as crowder agents. To facilitate the comparison, the relative size between the tracer and the crowder is the same in both studies. The results indicate a qualitative agreement between the diffusional behaviors observed in the two studies. The dependence of the anomalous diffusion exponent and the limiting diffusion coefficient on the obstacle size and excluded volume shows, in both cases, a similar tendency. The introduction of a reduced mobility parameter in the simulation model accounting for the short-range tracer?Cobstacle interactions allows obtaining a quantitative agreement between the limiting diffusion coefficient values yielded by both procedures. The simulation?Cexperiment quantitative agreement for the anomalous diffusion exponent requires further improvements. As far as we know, this is the first reported work where both techniques are used in parallel to study the diffusion in macromolecular crowded media.     

Hydrodynamics from statistical mechanics: combined dynamical-NEMD and conditional sampling to relax an interface between two immiscible liquids

We present a method to study hydrodynamic phenomena from atomistic simulations. In statistical mechanics, these fields are computed as the ensemble average over the time dependent probability density function corresponding to the time evolution of an initial conditional probability density function consistent with some initial conditions. These initial conditions typically consist in constraints on some macroscopic fields, e.g. the density field. We show how these processes can be studied by combining the dynamical approach to non-equilibrium molecular dynamics with the restrained simulation approach. As an illustration of our method, we study the relaxation to the equilibrium of an interface between two immiscible liquids. We show that, at a variance with the local time average method, the standard atomistic approach used in this field, our method is able to produce (macroscopic) fields satisfying the symmetry conditions of the problem.     

New insights into diffusion in 3D crowded media by Monte Carlo simulations: effect of size, mobility and spatial distribution of obstacles

Particle diffusion in crowded media was studied through Monte Carlo simulations in 3D obstructed lattices. Three particular aspects affecting the diffusion, not extensively treated in a three-dimensional geometry, were analysed: the relative particle-obstacle size, the relative particle-obstacle mobility and the way of having the obstacles distributed in the simulation space (randomly or uniformly). The results are interpreted in terms of the parameters that characterize the time dependence of the diffusion coefficient: the anomalous diffusion exponent (α), the crossover time from anomalous to normal diffusion regimes (τ) and the long time diffusion coefficient (D*). Simulation results indicate that there are a more anomalous diffusion (smaller α) and a lower long time diffusion coefficient (D*) when obstacle concentration increases, and that, for a given total excluded volume and immobile obstacles, the anomalous diffusion effect is less important for bigger size obstacles. However, for the case of mobile obstacles, this size effect is inverted yielding values that are in qualitatively good agreement with in vitro experiments of protein diffusion in crowded media. These results underline that the pattern of the spatial partitioning of the obstacle excluded volume is a factor to be considered together with the value of the excluded volume itself.     

C1-symmetric carbohydrate diphosphite ligands for asymmetric Pd-allylic alkylation reactions. Study of the key Pd-allyl intermediates

A series of C(1)-symmetrical 1,3-diphosphite ligands with a furanoside backbone have been applied in the Pd-catalysed asymmetric allylic alkylation of mono- and disubstituted linear substrates. These diphosphite ligands were designed by selective modification of the successful diphosphite ligand L1a with the 6-deoxy-1,2-O-isopropylidene-glucofuranose backbone in order to study the effect of the ligand structure on the catalytic performance. The effect of the solvent, the substrate/metal ratio and ligand/metal ratio were also investigated. The results in the Pd-allylic alkylation of rac-1,3-diphenyl-3-acetoxyprop-1-ene showed that the ligand structure and the reaction conditions had a considerable effect on enantioselectivity and on the kinetics of the reaction producing the kinetic resolution of the substrate. The alkylated product 2 was therefore obtained in 95% ee at 53% conversion and the enantiopure substrate 1 was recovered in 99.9% ee. Furthermore, the effect of the ligand structure and solvent were also observed in the Pd-allylic alkylation of monosubstituted 1-phenyl-3-acetoxyprop-1-ene. The use of a pro-chiral nucleophile was also explored in this reaction leading to excellent regioselectivities but moderate enantioselectivities. Finally, in order to determine how the ligand structure affected the chiral pocket of the Pd-π-allyl intermediates, the complexes [Pd(η(3)-C(15)H(13))(L)]PF(6), where L = L1a, L5-L8a, were synthesised and characterised by NMR spectroscopy.