Evaluation of Colloids and Activation Agents for Determination of Melamine Using UV-SERS

UV-SERS measurements offer a great potential for environmental or food (detection of food contaminats) analytics. Here, the UV-SERS enhancement potential of various kinds of metal colloids, such as Pd, Pt, Au, Ag, Au-Ag core-shell, and Ag-Au core-shell with different shapes and sizes, were studied using melamine as a test molecule. The influence of different activation (KF, KCl, KBr, K(2)SO(4)) agents onto the SERS activity of the nanomaterials was investigated, showing that the combination of a particular nanoparticle with a special activation agent is extremely crucial for the observed SERS enhancement. In particular, the size dependence of spherical nanoparticles of one particular metal on the activator has been exploited. By doing so, it could be shown that the SERS enhancement increases or decreases for increasing or decreasing size of a nanoparticle, respectively. Overall, the presented results demonstrate the necessity to adjust the nanoparticle size and the activation agent for different experiments in order to achieve the best possible UV-SERS results.     

Synthesis of a new chiral cyclic aminal derived from rac-1,2-propanediamine

The cyclic aminal 4,9-dimethyl-1,3,6,8-tetraazatricyclo[4.4.1.1^3,8]dodecane 4c was synthesized by the reaction of commercial rac-1,2-propanediamine with paraformaldehyde in an aqueous solution. ¹H NMR analysis clearly revealed that the compound is chiral and racemic with an axis of chirality. To our knowledge, this is the first example of an azaadamantane derivative having axial chirality. This aminal was used in a Mannich type reaction with p-chlorophenol yielding 2,2′-[(4-methylimidazolidine-1,3-diyl)dimethanediyl]bis(4-chlorophenol) 7 as a racemic mixture. The crystal structure of 7 was determined by single X-ray diffraction analysis.     

HSAB-driven chemoselective N-alkylation of pyrimidine bases and their 4-methoxy- or 4-acetylamino-derivatives

The lithium salts of the conjugated bases of 4-methoxy- and 4-acetylamino-2(1H)-pyrimidinones 1-3 undergo highly chemoselective N¹-methylation or ethylation when treated with methyl- or ethylsulfate (hard electrophiles) in dry dioxane, while the use of DMF as solvent results in competitive O²-alkylation. Potassium salts of the same bases in DMF undergo prevalent O²-attack. Under the same conditions, a similar but less chemoselective behaviour is observed in alkylation of thymine and uracil, where some N³-attack occurs. This can be rationalised in terms of the HSAB principle.     

On the application of accelerated molecular dynamics to liquid water simulations

Our group recently proposed a robust bias potential function that can be used in an efficient all-atom accelerated molecular dynamics (MD) approach to simulate the transition of high energy barriers without any advance knowledge of the potential-energy landscape. The main idea is to modify the potential-energy surface by adding a bias, or boost, potential in regions close to the local minima, such that all transitions rates are increased. By applying the accelerated MD simulation method to liquid water, we observed that this new simulation technique accelerates the molecular motion without losing its microscopic structure and equilibrium properties. Our results showed that the application of a small boost energy on the potential-energy surface significantly reduces the statistical inefficiency of the simulation while keeping all the other calculated properties unchanged. On the other hand, although aggressive acceleration of the dynamics simulation increases the self-diffusion coefficient of water molecules greatly and dramatically reduces the correlation time of the simulation, configurations representative of the true structure of liquid water are poorly sampled. Our results also showed the strength and robustness of this simulation technique, which confirm this approach as a very useful and promising tool to extend the time scale of the all-atom simulations of biological system with explicit solvent models. However, we should keep in mind that there is a compromise between the strength of the boost applied in the simulation and the reproduction of the ensemble average properties.     

Cupreines and cupreidines: an emerging class of bifunctional cinchona organocatalysts

In the steadily expanding field of organocatalysis, cinchona alkaloids play a prominent role. Until the late 1990s, bifunctional catalysts based on this scaffold relied exclusively on the C9-hydroxy group as the hydrogen-bond donor. Recently, new cinchona catalysts have been developed that feature a phenolic OH group in the C6' position-a structural feature that allows a diverse set of reactions to be catalyzed in a highly stereoselective fashion. This Minireview describes the scope and modes of action of this new class of asymmetric bifunctional organocatalysts.     

Quasilinear parabolic P.D.E.s with discontinuous hysteresis

This work deals with an initial- and boundary-value problem for a quasilinear parabolic equation that includes a possibly discontinuous hysteresis operator, :

Novel access to 1-substituted-benzimidazoles via benzotriazole-mediated synthesis

A one-pot good-yielding synthesis of 1-(alcoxymethyl)-1H-benzimidazoles and 1-((1H-benzimidazol-1-yl)methyl)-1H-benzotriazole from N¹,N²-bis((1H-benzotriazol-1-yl)methyl)benzene-1,2-diamine (3) and alcohols is described. The synthesis of 3 from macrocyclic aminal 6H,13H-5:12,7:14-dimethanedibenzo-[d,i][1,3,6,8]tetraazecine (DMDBTA, 1) and benzotriazole is also described. Both these methods are simple, isolation of the products from the reaction mixtures is easy, and the yields are good.     

Fast determination of ethambutol in pharmaceutical formulations using capillary electrophoresis with capacitively coupled contactless conductivity detection

A method for the determination of ethambutol (EMB), a first‐line drug against tuberculosis, based on CE with capacitively coupled contactless conductivity detection is proposed. The separation of EMB and its main product of degradation were achieved in less than 3 min with a resolution of 2.0 using a BGE composed of 50 mmol/L histidine and 30 mmol/L MES, pH 6.30. By raising the pH to 8.03, the analysis time was reduced to 1.0 min, but with a significant loss of resolution (0.7). Using the best separation conditions, linearity of 0.9976 (R², five data points), sensitivity of 1.26×10^?4 V min μmol^?1 L, and LOD and quantification of 23.5 and 78.3 μmol/L, respectively, were obtained. Recoveries at four levels of concentration ranged from 95 to 102% and the concentration range studied ranged from 100 to 500 μmol/L. The results obtained for the determination of EMB in pharmaceutical formulations were compared with those obtained by using CE with photometric detection.     

Thermal characterization of lignocellulosic residue from different sugarcanes

Bagasse samples from four different sugarcane were directly collected as the residues of milling in a processing plant. The samples were dried at 105 °C, compressed to small granules and then their TG/DTA and DSC curves in synthetic air were recorded. Similar thermogravimetric curves were obtained for the different samples and they exhibited four mass loss steps. However, the analysis of the exotherm DSC peaks showed that the oxidation of the organic matter resulted different enthalpy values (ΔH/kJ g⁻¹).