Theoretical study of p‐methacryloyl‐aminophenylarsonic acid

Conformations of p‐methacryloylaminophenylarsonic acid (p‐MAPHA) are determined through molecular mechanics and DFT/B3LYP calculations. Solvation effects are studied within the self‐consistent isodensity continuum model (SCI‐PCM). The stationary points were found to correspond to minima as verified by the analysis of vibrational frequencies in the molecule. The molecular optical absorption was obtained by using different solvent environments. The present results are in good agreement with the available experimental data. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002     

Coupling Cloud Point Extraction to Instrumental Detection Systems for Metal Analysis

The purpose of this article is to review and evaluate cloud point extraction of metals and its coupling to different contemporary instrumental methods of analysis. This review covers a selection of the literature published on this topic over the period mainly between 1997 and September 2005 (consisting of 50 publications). The current state of the art for CPE concerning metals and metal chelates is presented with special emphasis on the hyphenation of this interesting extraction/preconcentration approach mediated by surfactants to spectrophotometry, atomic spectroscopy and separation techniques. We present contemporary CPE developments concerning metal speciation and determination and their application to different environmental, clinical, geological and food samples. Strategies for method development as well as future perspectives are also covered.     

Evaluating the significance threshold in robustness testing. A critical discussion on the influence of time in molecular fluorescence spectrometry

Robustness experiments are performed in analytical chemistry to assess the behaviour of an analytical procedure under conditions slightly different from those of the experimental optimum. This work presents a study of robustness applied to a previously proposed fluorescent methodology using two replicated 2(7-4) saturated fractional factorial designs. A comparison is established between three different ways to test the significance of the effects of the variables involved on the response signal. Critical discussions about the most appropriate threshold to be introduced when testing for significance of the factors and the influence of the time necessary to carry out the experiments are included.     

On the convergence of isolated neutral oxygen vacancy and divacancy properties in metal oxides using supercell models

The properties of isolated neutral oxygen vacancies and divacancies of metal oxides of increasing complexity (MgO, CaO, alpha-Al2O3, and ZnO) have been studied by means of density-functional theory within a supercell periodic approach. Vacancy formation energies, vacancy-vacancy interactions, and geometry rearrangements around these point defects have been investigated in detail. The characterization of the electronic structure of these point defects has been established by analysis of the density of states and of the topology of the electron density and of electron localization function. It is found that the chemical character of the oxide determines the properties of the oxygen vacancies. For the covalent ZnO oxide, a more complex scheme arises in which the relaxation around the oxygen vacancy is much larger leading to the formation of Zn4-like almost metallic particles in the crystal. The relationship of these structures with the crystal shear planes is discussed. The present study shows that supercells containing approximately 200-300 atoms provide converged values for the geometric and electronic structure of oxygen vacancies of these metal oxides in the point defect low concentration limit.     

A family of alkynylgold(III) complexes [AuI(mu-{CH2}2PPh2)2Au(III)(C triple bond CR)2] (R = Ph, tBu, Me3Si): facile and reversible comproportionation of gold(I)/gold(III) to digold(II)

The symmetric digold(II)dichloride bis(ylide) complex [Au2Cl2(mu-{CH2}2PPh2)2] reacts with acetylides to form the asymmetric heterovalent gold(I)/gold(III) complexes [AuI(mu-{CH2}2PPh2)2AuIII(CCR)2] [R = Ph, tBu, and SiMe3], the phenyl analogue of which was characterized by X-ray crystallography. These compounds represent the first examples of gold(III) complexes containing two acetylide ligands. [AuI(mu-{CH2}2PPh2)2AuIII(CCPh)2] undergoes a reversible comproportionation reaction upon treatment with [Ag(ClO4)tht] to give the symmetric digold(II) cationic complex [Au2(tht)2(mu-{CH2}2PPh2)2](ClO4)2. If this complex is treated with phenylacetylene in the presence of base, the heterovalent gold(I)/gold(III) complex is re-formed. This reversible interconversion between binuclear gold(I)/gold(III) and digold(II) bis(ylide) complexes is unprecedented.     

Primary Transformation Rate Measurements Through Differential Scanning Calorimetry

The primary crystallization of molten alloy systems at high undercooling is studied by a precise quantitative analysis of the calorimetric signal obtained during the transformation in terms of the reaction rate under isothermal and continuous heating regimes. It is shown that, under specific conditions, namely, stoechiometric primary precipitates, generalized relationships for the crystallization enthalpy and the reaction rate may be obtained.     

Protein secondary structure and stability determined by combining exoproteolysis and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

In the post-genomic era, several projects focused on the massive experimental resolution of the three-dimensional structures of all the proteins of different organisms have been initiated. Simultaneously, significant progress has been made in the ab initio prediction of protein three-dimensional structure. One of the keys to the success of such a prediction is the use of local information (i.e. secondary structure). Here we describe a new limited proteolysis methodology, based on the use of unspecific exoproteases coupled with matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS), to map quickly secondary structure elements of a protein from both ends, the N- and C-termini. We show that the proteolytic patterns (mass spectra series) obtained can be interpreted in the light of the conformation and local stability of the analyzed proteins, a direct correlation being observed between the predicted and the experimentally derived protein secondary structure. Further, this methodology can be easily applied to check rapidly the folding state of a protein and characterize mutational effects on protein conformation and stability. Moreover, given global stability information, this methodology allows one to locate the protein regions of increased or decreased conformational stability. All of this can be done with a small fraction of the amount of protein required by most of the other methods for conformational analysis. Thus limited exoproteolysis, together with MALDI-TOF MS, can be a useful tool to achieve quickly the elucidation of protein structure and stability.     

UV index experimental values during the years 2000 and 2001 from the Spanish broadband UV-B radiometric network

An analysis is made of experimental ultraviolet erythemal solar radiation data measured during the years 2000 and 2001 by the Spanish UV-B radiation evaluation and prediction network. This network consists of 16 Robertson-Berger type pyranometers for evaluating solar erythemal radiation and five Brewer spectroradiometers for evaluating the stratospheric ozone. On the basis of these data the Ultraviolet Index (UVI) was evaluated for the measuring stations that are located either in coastal regions or in the more densely populated regions inland on the Iberian Peninsula. It has been checked that in most cases the maximum irradiance values corresponded to solar noon, although there were exceptions that could be explained by cloudiness. The maximum experimental values of the UVI were around 9 during the summer, though frequently passing this value at the inland measurement stations. The annual accumulated dose of irradiation on a horizontal plane has also been studied, as well as the evolution through the year in units of energy, standard erythemal doses and minimum erythemal doses, according to different phototypes.     

13C-NMR relaxation studies of alkali metal cryptate complexes

The ¹³C spin-lattice relaxation times (T₁'s) of cryptands [2.1.1], [2.2.1] and [2.2.2] as well as those of the corresponding cryptate complexes with Li⁺, Na⁺, and K⁺ in CDCl₃ and CH₃OH:D₂O (90:10) were measured and the results are interpreted in terms of molecular compression and desolvation effects.     

Determination of hopane biomarkers in Arabian crude oils by gas chromatography-tandem mass spectrometry

Gas chromatography-mass spectrometry with the mass spectrometer operated in the tandem mode at a constant accelerating voltage was used to analyse for triterpanes in Arabian crude oils. Specific molecular parent-daughter ion pairs were selected for quantitative measurements owing to their metastable transitions in the first field-free region. More accurate relative distribution ratios of hydrocarbon biomarkers were determined by this sequential gating technique than by the conventional diagnostic daughter ion mass fragmentogram in common GC-MS. The data and spectra obtained from MS-MS operation may serve as unique “fingerprints” for these regional crudes