Determination of Silver‐Modified Titanium Phosphate Nanoparticles by Voltammetric and Electrocatalytic Methods

The electrochemical determination of silver‐modified titanium phosphate nanoparticles (Ag‐TiPNPs) was performed using two electrochemical features of this novel kind of nanoparticles. First, a determination using the voltammetric activity of the silver from the Ag‐TiPNPs was carried out. Secondly, the electrocatalytic effect of Ag‐TiPNPs was shown for the first time to the hydrogen evolution reaction (HER) and the determination of these nanoparticles was performed by chronoamperometry using this electrocatalysis. Moreover, it was verified that the catalytic effect was due to the electroreduced silver since the unmodified titanium phosphate nanoparticles (TiPNPs) did not exhibit this effect. Detection limits as low as 0.1 and 0.75 ng µL^?1 of Ag‐TiPNPs were obtained with the voltammetric and chronoamperometric methods, respectively. 8‐channel screen‐printed electrochemical arrays (8xSPCEs) were employed as transducers to carry out these electrochemical studies, due to its low cost and time saving.     

Analysis of grass silage from Northwestern Spain by near-infrared reflectance spectroscopy

Near-infrared reflectance spectroscopy (NIRS) was evaluated for the determination of protein, crude fiber (CF), acid detergent fiber (ADF), and neutral detergent fiber (NDF) in grass silage. Calibration equations were based on analyses of 366 samples of grass silage produced in Northwestern Spain over 4 consecutive years (1992-1995) and validated by analyses of a set of 72 silage samples harvested during 1996. Dried and ground samples were analyzed by chemical and NIRS procedures. The spectral data were analyzed by regression against a range of chemical parameters, using modified partial least-squares (MPLS) multivariate analysis in conjunction with different mathematical treatments of the spectra. For each parameter, the optimum calibration was evaluated on the basis of the coefficient of multiple determination (R2), the coefficient of simple correlation (r2), the standard error of calibration (SEC), the standard error of cross-validation (SECV), and the standard error of validation (SEV). R2 and r2 were >0.90; SEC values were 0.58, 1.04, 1.40, and 1.75; SECV values were 0.64,1.15,1.50, and 2.04; and SEV values were 0.56,1.02, 1.42, and 1.80 for protein, CF, ADF, and NDF, respectively. The ratio of the standard deviation of the reference data to the SEV was >3.0 for each of the 4 parameters, which indicates that the equations can be used in routine analysis.     

Deprotonation sites of acetohydroxamic acid isomers. A theoretical and experimental study

Theoretical (ab initio calculations) and experimental (NMR, spectrophotometric, and potentiometric measurements) investigations of the isomers of acetohydroxamic acid (AHA) and their deprotonation processes have been performed. Calculations with the Gaussian 98 package, refined at the MP2(FC)/AUG-cc-pVDZ level considering the molecule isolated, indicate that the Z(cis) amide is the most stable form of the neutral molecule. This species and the less stable (Z)-imide form undergo deprotonation, giving rise to two stable anions. Upon deprotonation, the E(trans) forms give three stable anions. The ab initio calculations were performed in solution as well, regarding water as a continuous dielectric; on the basis of the relative energies of the most stable anion and neutral forms, calculated with MP2/PCM/AUG-cc-pVDZ, N-deprotonation of the amide (Z or E) structure appeared to be the most likely process in solution. NMR measurements provided evidence for the existence of (Z)- and (E)-isomers of both the neutral and anion forms in solution. Comparisons of the dynamic NMR and NOESY (one-dimensional) results obtained for the neutral species and their anions were consistent with N-deprotonation, which occurred preferentially to O-deprotonation. The (microscopic) acid dissociation constants of the two isomers determined at 25 degrees C from the pH dependence of the relevant chemical shifts, pK(E) = 9.01 and pK(Z) = 9.35, were consistent with the spectrophotometric and potentiometric evaluations (pK(HA) = 9.31).     

Light-induced aminocarbene to imine dyotropic rearrangement in a chromium(0) center: an unprecedented reaction pathway

1,2-Dyotropic rearrangement can be induced by irradiation of properly functionalized Fischer carbenes. This novel reaction takes place by a stepwise mechanism and with double inversion of configuration at the static scaffold. Good yields are obtained with both cyclic and acyclic structures, thus suggesting that this unprecedented transformation can be extended to other organometallic systems.     

Monitoring Glycan–Protein Interactions by NMR Spectroscopic Analysis: A Simple Chemical Tag That Mimics Natural CH–π Interactions

Detection of molecular recognition processes requires robust, specific, and easily implementable sensing methods, especially for screening applications. Here, we propose the difluoroacetamide moiety (an acetamide bioisoster) as a novel tag for detecting by NMR analysis those glycan–protein interactions that involve N‐acetylated sugars. Although difluoroacetamide has been used previously as a substituent in medicinal chemistry, here we employ it as a specific sensor to monitor interactions between GlcNAc‐containing glycans and a model lectin (wheat germ agglutinin). In contrast to the widely employed trifluoroacetamide group, the difluoroacetamide tag contains geminal ¹H and ¹⁹F atoms that allow both ¹H and ¹⁹F NMR methods for easy and robust detection of molecular recognition processes involving GlcNAc‐ (or GalNAc‐) moieties over a range of binding affinities. The CHF₂CONH‐ moiety behaves in a manner that is very similar to that of the natural acetamide fragment in the involved aromatic‐sugar interactions, providing analogous binding energy and conformations, whereas the perfluorinated CF₃CONH‐ analogue differs more significantly.     

Structural NMR and ab initio study of salicylhydroxamic and p-hydroxybenzohydroxamic acids: evidence for an extended aggregation

The acid-base behavior and self-aggregation of salicylhydroxamic (SHA) and p-hydroxybenzohydroxamic acids (PHBHA)have been investigated by UV and 1HNMR spectroscopy, respectively. The acid-base parameters, measured in H2O at 25 degrees C and I=0.1 M, were pK1=7.56, pK2=9.85 for SHA and pK1=8.4, pK2=9.4 for PHBHA. The 1H NMR signals for salicylhydroxamic and p-hydroxybenzohydroxamic acids measured in acetone indicate that both acids self-aggregate according to a mechanism where two monomers produce planar E-E dimers stabilized by horizontal H-bonds. Further dimer aggregation yields sandwich-like tetramer structures stabilized by vertical H-bonds and pi-pi interactions. The p-hydroxybenzohydroxamic tetramers, less stable than those of salicylhydroxamic, contain two water molecules in their structures. The gas-phase structures of salicylhydroxamic acid and its anions were investigated by ab initio calculations using the density functional theory at the B3LYP/AUG-cc-pVDZ level. The SHA most stable gas-phase conformer is the A-Z amide, a structure with all three phenolate (OP), carboxylate (OC), and hydroxamate (OH) oxygen atoms in the cis position. The B-Z amide, with the OP oxygen trans to OC, lies 5.4 kcal above the A-Z amide. The most stable monoanion is the N-deprotonated A-Z amide.     

Hydrogen and copper ion-induced molecular reorganizations in scorpionand-like ligands. A potentiometric, mechanistic, and solid-state study

Two aza scorpionand-like macrocycles (L2 and L3) have been prepared. L2 consists of a tren amine with two of its arms cyclizized with a 2,6-bis(bromomethyl)pyridine. In L3, the remaining pendant arm has been further functionalized with a fluorophoric naphthalene group. X-ray data on the compounds [H(L3)]ClO4.H2O (1) and [H3(L3)](H2PO4)3.H2O (2) as well as solution studies (pH-metry, UV-vis, and fluorescence data) show the movement of the pendant arm as a result of the protonation degree of the macrocycles and of the formation of intramolecular hydrogen bonds. X-ray data on the complexes [Cu(L2)](ClO4)2]2.H2O (3) and [Cu(L3)](ClO4)2 (4) and solution studies on Cu2+ coordination show the implication of the nitrogen of the arm in the binding to the metal ion. Kinetic studies on the decomposition and formation of the Cu2+ complexes provide additional information about the pH-dependent molecular reorganizations. Moreover, the obtained information suggests that the kinetics of the tail on/off process is essentially independent of the lability of the metal center.     

Properties of 1,8-cineole: a thermophysical and theoretical study

This paper reports on an experimental and theoretical study of 1,8-cineole, one of the main components of essential oils in different plants. The pressure-volume-temperature behavior of this fluid was evaluated accurately over wide temperature and pressure ranges and correlated successfully with the empirical TRIDEN equation. From the measured data, the relevant derived coefficients isothermal compressibility, isobaric expansibility, and internal pressure were calculated. The isobaric heat capacities at high pressure were extrapolated from the data measured at atmospheric pressure. The cubic equations of state by Soave, Peng-Robinson, Stryjek-Vera modification of Peng-Robinson, Patel-Teja, Sako-Wu-Prausnitz, and the SAFT and PC-SAFT molecularly based equations of state were used to predict the PVT behavior. The SAFT and PC-SAFT parameters for 1,8-cineole were obtained from correlation of available saturation literature data; the best results were provided by Sako-Wu-Prausnitz and PC-SAFT equations of state, whereas the classical ones were shown to be inadequate. The molecular structure was studied by quantum computations at the B3LYP/6-311++g(d) level and classical molecular dynamics simulations in the NPT ensemble with the OPLS-AA forcefield. On the basis of both macroscopic and microscopic studies, a complex fluid structure was inferred.     

Progress in the methodological strategies for the detection in real samples of desmosine and isodesmosine, two biological markers of elastin degradation

Desmosines are crosslinking amino acids unique to mature elastin in humans. Owing to this unicity, they have been discussed as potentially attractive indicators of connective tissue disorders whose clinical manifestations are mostly the result of elastin degradation. This review covers advances in immunochemical, chromatographic, and electrophoretic procedures applied in the last 25 years to detect and quantitate these crosslinksin a variety of biological samples. Recent applications of CE with LIF detection (CE-LIF) for investigating the content of desmosines in different fluids will also be discussed.