Synthesis of 2-(p-R-benzoylmethylene)-3-(p-R-phenyl)-1H-quinoxalines

The reaction of 1,2-diaminobenzenes with substituted 1,2-dibenzoyl-1,2-dibromoethanes constitutes a convenient synthetic route to the hitherto 2-(p-R-benzoylmethylene)-3-(p-R-phenyl)-1H-quinoxalines. Structures of all products were elucidated by ir, ¹H and ¹³C-nmr, mass spectra data. X-Ray crystallography data confirm assigned structures.     

Comparison of 17beta-estradiol structures from x-ray diffraction and solution NMR

The NMR-derived structure of estrogen (17beta-estradiol, E2), the drug of choice for postmenopausal women, was compared with a recent literature crystal x-ray structure of Fab-bound E2. 1H and 13C NMR spectra of E2 were acquired in DMSO-d6. Assignments were obtained from an analysis of DQF-COSY, TOCSY, HETCOR, HMQC and HMBC 2D NMR spectra. The 1H and 13C NMR assignments are the first reported for E2 in DMSO-d6. Two solution structures, S1 and S2, were obtained with molecular modeling using NOE constraints. S1 overlaps with the crystal structure for all rings. S2 shows prominent differences in the C-ring (C9--C11--C12--C13) segment, which deviates from a chair conformation, and excellent overlap in the A-, B- and D-rings of E2. The C-ring in S2 adopts a boat conformation as opposed to a chair conformation in the x-ray and S1 structures. The S2 structure is about 6 degrees more twisted than the bound x-ray and S1 models. The S1, S2 and x-ray structures had ring bowing values of 10.1 +/- 0.3, 11 +/- 1 and 10.37 degrees , respectively. Of the 100 solution conformers generated, 83 had S1 conformation and 17 had S2 conformation, with average internal energies of 112 +/- 2 and 141 +/- 2 kcal mol(-1), respectively. The 100 S1- and S2- derived conformers showed a r.m.s.d. of 0.72 A for all atoms. The x-ray, S1 and S2 C18--O17 distances were 2.93, 2.92 +/- 0.01 and 2.93 +/- 0.01 A, respectively, and the O3--O17 distances were 11.06, 11.18 +/- 0.12, and 10.89 +/- 0.05 A, respectively.     

Calculation of Kinetic Parameters for Crystallization Processes

In this work, kinetic data of crystallization processes have been determined by measurement of the intensities of reflection of X-ray diffraction spectra and modeled using the Avrami-Eroféev and Jander expressions. We have created a simple Microsoft Excel spreadsheet that allows students to calculate the kinetic data. Students will be able to calculate the kinetic parameters of any crystallization process, for example, hydrothermal crystallization of catalytic materials like zeolites. The possibility of using the spreadsheet with different models or expressions and discriminating among them is also validated by comparing the model results with experimental data (differential thermal analyses, DTA) from papers available in the recent literature.     

Study of the catalytic behavior of montmorillonite/iron(III) and Mn(III) cationic porphyrins

Two samples of montmorillonite (one of Brazilian origin, BNC1 clay, and the other STX-1, supplied by the Clay Mineral Society Repository (University of Missouri, USA) were allowed to react with biomimetic metalloporphyrins of Fe(III) and Mn(III) in cationic form. The compounds were characterized by several techniques, showing that the metalloporphyrins molecules were adsorbed at the surface of the clay platelet crystals. The catalytic activities of the intercalated complexes for the oxidation of alkane were dependent upon the concentration of the porphyrin immobilized in the clay and factors such as the metal ion species in the porphyrins, choice of solvent, and concentration of the iodosylbenzene oxidant. Good selectivity to cyclohexanol instead of cyclohexanone was observed for the catalytic oxidation of cyclohexane by iodosylbenzene.     

Substituent effects in eight-electron electrocyclic reactions

The main features of transition structures associated with eight-electron electrocyclic reactions have been studied with Density Functional Theory. It is found that conrotatory electrocyclization reaction of (3Z,5Z)-octa-1,3,5,7-tetraenes takes place via M?bius aromatic transition structures of helical conformation. The reaction is completely periselective. In general, transition structures having outward substituents are preferred with respect to the inward transition structures, irrespective of the pi-donor or pi-acceptor character of the substituent. In contrast with four-electron thermal conrotatory electrocyclic reactions, there is no satisfactory correlation between the difference in energy of activation between inward and outward substituents and the Taft resonance sigma(R) parameter.     

Conformational mobility of thianthrene-5-oxide

[reaction: see text] Data on the apparent dipole moment of thianthrene-5-oxide (1) and (1)H NMR spectra in different solvents support the conformational mobility of 1, which flaps between two limit boat conformations with the sulfinyl group in pseudoequatorial and pseudoaxial positions, respectively. The conformational equilibrium of 1 occurs too fast for the (1)H NMR (500 MHz) time-scale even at -130 degrees C, and the equilibrium constant has not been determined. The apparent dipole moments of 1 in n-hexane and 1,4-dioxane and the (1)H NMR spectra of 1 and the model compounds cis- and trans-thianthrene-5,10-dioxides (2) and thianthrene (5) in different solvents and at various temperatures confirm that the relative position of the conformational equilibrium of 1 is solvent-dependent, and more polar solvents favor the conformation with the sulfoxide group in the pseudoaxial position (1(')(ax)). Variable-temperature (1)H NMR spectra have established the interconversion barrier of trans-2 and confirmed that the conformational equilibrium of cis-2 is strongly displaced toward the conformation with both sulfinyl groups in the pseudoequatorial position. The (1)H NMR data support the transannular interaction of the functional groups in 1 and trans-2.     

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.     

Nutritional parameters of commercially available milk samples by FTIR and chemometric techniques

A chemometric study on the prediction of the main nutritional aspects of milk has been carried out by using fourier transform infrared spectroscopy (FTIR) attenuated total reflectance (ATR) measurements of commercially available milk samples of different types. Whole, semi and skimmed milks, enriched or not with calcium, vitamins or modified by alteration of lipid or sugar composition were considered. After evaluating different strategies for data acquisition and ATR cleaning between samples, hierarchical cluster analysis (HCA) was carried out for classification of samples in order to choose the calibration set. The prediction capabilities of partial least squares (PLS) data treatment were evaluated in order to obtain information about total fat, total protein, total carbohydrates (CH), calories and calcium. On using the mean square error of cross-validation and prediction as control variables, a critical evaluation were made about the calibration set to be used, the spectral range to be considered and the data treatment (PLS-1 or PLS-2) to be performed. By selecting a calibration set of 33 samples the properties of 48 samples were predicted with relative precision of triplicates of 0.062, 0.040 and 0.039% w/v for total fat, protein and carbohydrates, and 0.66 kcal/100 ml for calories, and 2.1 mg of Ca/100 ml. The mean difference (d_x−y) between predicted and actual values and standard deviation of mean differences (s_x−y), were of 0.06 (0.38), 0.03 (0.18) and −0.15 (0.41), being s_x−y values between brackets, for total fat, proteins and carbohydrates, 0.06 (3.8) kcal/100 ml for calories and −4.5 (9) mg/100 ml for calcium.The sensitivity and selectivity of the methodology developed were evaluated on terms of the net analyte signal. Selectivity factors ranging from 2 to 7.6% have been calculated for the five parameters considered.     

Mass distribution in a dynamic sample zone inside a flow injection manifold: modelling integrated conductimetric profiles

A mathematical model for fitting the experimental ICM (integrated conductimetric method) curves developed by the authors in a previous work, is presented for the first time in this study. The proposed model fits the experimental curves with great precision and allows to predict physical dispersion for single-line flow injection system. The correlation of the model’s parameters with typical reactionless FIA peak parameters is also assessed. The IDQ coefficient—a novel dispersion estimator previously reported by the authors—can also be predicted when operational FIA variables are changed. Experimental and modelled profiles are compared as a function of the system’s variables, showing an excellent agreement.     

Peptide backbone folding induced by the C(alpha)-tetrasubstituted cyclic alpha-amino acids 4-amino-1,2-dithiolane-4-carboxylic acid (Adt) and 1-aminocyclopentane-1-carboxylic acid (Ac5c). A joint computational and experimental study

The conformational study of a new group of synthetic peptides containing 4-amino-1,2-dithiolane-4-carboxylic acid (Adt), a cysteine-related achiral residue, has been carried out through a joint application of computational and experimental methodologies. Molecular Dynamics simulations clearly suggest the tendency of this molecule to adopt a gamma-turn conformation in vacuum and help in analyzing the complex and crucial conformational behaviour of the dithiolane ring which appears to preferentially adopt a C(S)-like structure. Electronic structure calculations carried out in solution using the Density Functional Theory also indicate the preservation of the gamma-like folding in apolar solvents and the helix-like one in more polar solvents. A comparison with the achiral 1-aminocycloalkane-1-carboxylic acid (Ac5c) has been carried out using the same computational tools. NMR and IR data on dipeptide derivatives containing the Adt or Ac5c residue show that in chloroform solution all the models prefer a gamma-turn structure, centered at the cyclic residue, stabilized by an intramolecular H-bond, whereas in a more polar solvent, i.e. dimethyl sulfoxide, this folding is not maintained. The experimental conformational studies, extended to N-Boc protected tripeptides, clearly indicate the remarkable tendency of both the five-membered C(alpha)-tetrasubstituted cyclic amino acids Adt and Ac5c to induce the gamma-turn structure also in models able to adopt the beta-bend conformation.