EPR investigations of Cu(II) complexes with 5′-CMP and 5′-GMP

Electron paramagnetic resonance investigation of the Cu(II) complexes with guanosine-5′-monophosphate (5′-GMP) and cytidine-5′-monophosphate (5′-CMP) shows the affinity of the Cu(II) ion to interact with the base and the phosphate group. The different modes of the coordination of the metal ion at the nucleotide and the water molecules lead to octahedral species, distorted by dynamical Jahn-Teller effect (g₀ = 2.106) for the Cu(II)-5′-CMP complex and rhombically distorted (g₁ = 2.358, g₂ = 2.126, g₃ = 2.068) or tetragonally distorted (g_∥^′ = 2.299, g_⊥^′ = 2.126) for the Cu(II)-5′ -GMP complex. The compound with 5′-CMP presents also a more stable in time square-planar species (g_∥ = 2.265, A_∥ = 162 G, g_⊥ = 2.076). The local symmetry changes in aqueous solution by coordination of water molecules.     

Formal hydride transfer mechanism for photoreduction of 3-phenylquinoxalin-2-ones by amines. Association Of 3-phenylquinoxalin-2-one with aliphatic amines

The photophysical and photochemical behavior of 1-methyl-3-phenylquinoxalin-2-one (MeNQ) and 3-phenylquinoxalin-2-one (HNQ) in the presence of amines is reported. While HNQ fluorescence shows an auxochromic effect and a bathochromic shift with added amines, explained by association of HNQ with amine in the ground state and emission from both excited species HNQ and [HNQ-amine], both MeNQ and HNQ are photoreduced efficiently on irradiation in the presence of amines, leading to the semireduced quinoxalin-2-ones, MeNQH(-) and HNQH(-), respectively, via an electron-proton-electron transfer, with unit quantum yields at high amine concentrations. The semireduced quinoxalin-2-ones XNQH(-) (X = H, Me) revert almost quantitatively to the parent XNQ in a dark thermal reaction with an activation free energy for MeNQH(-) of 17.4 and 25.9 kcal/mol in acetonitrile and benzene, respectively. Kinetic and spectroscopic (UV and NMR) evidence supports the proposed reaction mechanism for the reversible photoreduction.     

Fluorimetric determination of calcium by ion-pair extraction with cryptand 2.2.1 and eosin

A fluorimetric study on the extraction of calcium into 1,2-dichloroethane as an ion-pair, formed between the cryptand 2.2.1-calcium complex and the eosinate anion, is described. Optimum conditions for extraction are established and a new fluorimetric determination of ultratraces of calcium is proposed. A linear working range from 1.5 ng ml^–1 (detection limit) to 100 ng ml^–1 of calcium and a relative standard deviation of ± 2.9% at the 70 ng ml^–1 level are obtained. The equilibrium constants involved in the extraction process have been calculated and refined by the Letagrop-DISTR program. The proposed method has been tested for the direct determination of calcium in sugars.     

Thermodynamic assessment of smectic A-nematic tricritical points through the equal gibbs energy analysis

A complete thermodynamic analysis on four two-component phase diagrams between liquid crystals belonging to the nCB and nOCB series of compounds, the so-called cyanobiphenyls, has been performed through the Oonk's equal Gibbs energy method. The binary systems dealt with in this paper show as a common feature the existence of a tricritical point at the SmA-to-N phase transition, all of which reported elsewhere. As a singular finding of the work proposed in this paper, the Oonk's method is extended in order to account for second-order phase transitions. Likewise, this extension allows performing calculations of the tricritical temperature.     

Peruvian Food Chain Jenga: Learning Ecosystems with an Interactive Model

A pilot study was conducted on a multimodal educational tool, Peruvian Food Chain Jenga (PFCJ), with 5th‐grade students (N = 54) at a public charter school. The goal was to compare the effectiveness of the multimodal tool to a more traditional presentation of the same materials (food chain) using an experimental/control design. Data collection included a pretest/posttest and a “What I Did/What I Learned” response sheet. Quantitative analysis of pretest/posttest results showed both groups improved from pretest to posttest; however, there was no statistically significant difference between posttest results of experimental and control groups. Qualitative analysis of student open‐ended responses indicated a difference between students who used the PFCJ and students in the control. The most striking difference occurred in how the students perceived the connectedness of species and the awareness of human impact. Our findings suggest that using a model such as PFCJ as a means of teaching and connecting scientific content with practices related to ecosystems is an effective method of engaging students in intelligent discussions about these topics.     

An efficient nonlinear programming strategy for PCA models with incomplete data sets

Processing plants can produce large amounts of data that process engineers use for analysis, monitoring, or control. Principal component analysis (PCA) is well suited to analyze large amounts of (possibly) correlated data, and for reducing the dimensionality of the variable space. Failing online sensors, lost historical data, or missing experiments can lead to data sets that have missing values where the current methods for obtaining the PCA model parameters may give questionable results due to the properties of the estimated parameters. This paper proposes a method based on nonlinear programming (NLP) techniques to obtain the parameters of PCA models in the presence of incomplete data sets. We show the relationship that exists between the nonlinear iterative partial least squares (NIPALS) algorithm and the optimality conditions of the squared residuals minimization problem, and how this leads to the modified NIPALS used for the missing value problem. Moreover, we compare the current NIPALS‐based methods with the proposed NLP with a simulation example and an industrial case study, and show how the latter is better suited when there are large amounts of missing values. The solutions obtained with the NLP and the iterative algorithm (IA) are very similar. However when using the NLP‐based method, the loadings and scores are guaranteed to be orthogonal, and the scores will have zero mean. The latter is emphasized in the industrial case study. Also, with the industrial data used here we are able to show that the models obtained with the NLP were easier to interpret. Moreover, when using the NLP many fewer iterations were required to obtain them. Copyright © 2010 John Wiley & Sons, Ltd.     

Effects of Adsorbed Polyaniline on Redox Processes on As(2)O(3) Surfaces

Polyaniline deposited on As(2)O(3) surface resulted in a new material, which was characterized by infrared spectoscopy, thermogravimetry, differential scanning calorimetry, scanning electron microscopy, X-ray diffraction, and cyclic voltammetry. The mass percentage of polymer deposited on oxide surface is approximately 13%. The scanning electron microscopy images as well as the X-ray diffraction patterns provided conclusive evidence that the oxide surface is coated by the polymer. The cyclic voltammograms of the polyaniline adsorbed on As(2)O(3) surface showed that the adsorbate exerts remarkable effects on redox processes on this oxide. The pure oxide exhibited two oxidation/reduction peaks at 0.25/-0.06 and 0.47/-0.25 V attributed tentatively to the processes As(2)O(3)(s)+6H(+)+6e(-)=2As(s)+3H(2)O and As(s)+3H(+)+3e(-)=AsH(3)(g), respectively. The polyaniline-coated sample exhibited a better-defined voltammogram in which the first oxidation peak of the oxide had its intensity increased about four times. Copyright 2000 Academic Press.     

Gas chromatography and silver-ion high-performance liquid chromatography analysis of conjugated linoleic acid isomers in free fatty acid form using sulphuric acid in methanol as catalyst

This study used GC and silver-ion HPLC to examine the effects of temperature and time on methylation of individual and mixtures of conjugated linoleic acid (CLA) isomers in free fatty acid form using sulphuric acid as catalyst. In the conditions tested (temperatures between 20 and 50 degrees C and times between 10 and 60min) methylation was complete while avoiding isomerization of conjugated dienes and the formation of artefacts that could interfere with chromatographic determinations. An analytical method using solvent extraction of the lipids followed by selective elution of the free fatty acids from aminopropyl bonded phase columns and methylation with H(2)SO(4) in mild conditions was then applied to determine the CLA isomers in free fatty acid form in rumen fluid, and the results were evaluated.     

Determination of chain transfer constant to dodecanethiol in styrene/methyl methacrylate and butyl acrylate/methyl methacrylate copolymerization and effect of chain length on composition and stereochemical configuration of copolymers

Free radical copolymerization of styrene/methyl methacrylate (S/MMA) and butyl acrylate/methyl methacrylate (BA/MMA) in the presence of n-dodecanthiol (DDT) has been studied at 60°C in a 3 mol/L benzene solution using 2,2′-azobis(isobutyronitrile) (AIBN) as initiator. Overall chain transfer constant to DDT has been determined for both copolymerization systems, as a function of monomer feed composition using complete molecular weight distribution and the Mayo method. Overall transfer coefficients have values which are dependent on both monomer feed composition and individual comonomer transfer values. Composition, sequence distribution, and stereoregularity of copolymers obtained are, in our experimental conditions, independent of copolymer molecular weight. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 2913–2925, 1998