Problem of hidden variables

The problem of hidden variables in quantum mechanics is formalized as follows. A general or contextual (noncontextual) hidden-variables theory is defined as a mappingf: Q×M C (f: QC) whereQ is the set of projection operators in the appropriate (quantum) Hilbert space,M is the set of maximal Boolean subalgebras ofQ andC is a (classical) Boolean algebra. It is shown that contextual (noncontextual) hidden-variables always exist (do not exist).     

Effect of delay time and grid voltage changes on the average molecular mass of polydisperse polymers and polymeric blends determined by delayed extraction matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

The dependence of the calculated average molecular mass of a polyethylene glycol with a large polydispersity on the instrumental parameters adopted in the acquisition of mass spectra using delayed extraction matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (DE MALDI-TOFMS) was investigated. It has been shown that a combined effect of delay times and potential gradients can act on the ion cloud in the source chamber affecting both mass resolution and average molecular mass value of the analyzed polymeric sample. Also examined was a blend of two different polymers (a PEG and a PMMA commercial sample having a similar average molecular mass), which presents an additional problem concerning the discrimination among the different polymer species as a function of the experimental conditions. In this work, the best instrumental conditions to obtain both good resolution and a correct average molecular mass for the examined polydisperse sample are reported.     

Structure de la gouregine,alcaloïde orginal apparente aux cularines

The structure of guoregine, a new isoquinoline alkaloid from $\text{G}$$\text{u}$$\text{a}$$\text{t}$$\text{t}$$\text{e}$$\text{r}$$\text{i}$$\text{a}$$\text{o}$$\text{u}$$\text{r}$$\text{e}$$\text{g}$$\text{o}$$\text{u}$, Annonaceae, has been deduced by spectral analysis and confirmed by an X-ray structure determination. It is the first member of a new class of cularine-related alkaloids (α-$\text{g}$$\text{e}$$\text{m}$-dimetyltetradehydrocularines).     

Reactivity of amino acids in nitrosation reactions and its relation to the alkylating potential of their products

Nitrosation reactions of amino acids with an -NH(2) group [namely, six alpha-amino acids (glycine, alanine, alpha-aminobutyric acid, alpha-aminoisobutyric acid, valine, and norvaline); two beta-amino acids (beta-alanine and beta-aminobutyric acid), and one gamma-amino acid (gamma-aminobutyric acid)] were studied. Nitrosation was carried out in aqueous acid media, mimicking the conditions of the stomach lumen. The rate equation was r = k(3)(exp)[amino acid][nitrite](2), with a maximum k(3)(exp) value in the 2.3-2.7 pH range. The existence of an isokinetic relationship supports the argument that all the reactions share a common mechanism. A nitrosation mechanism is proposed, and the following conclusions are drawn: (i) Nitrosation reactions of amino acids with a primary amino group in acid media occur with dinitrogen trioxide as the main nitrosating agent. The finding that the nitrosation rate is proportional to the square of the nitrite concentration suggests that the yield of nitrosation products in the stomach would increase sharply with higher nitrate/nitrite intakes. (ii) Stomach hypochlorhydria could be a potential enhancer of in vivo amino acid nitrosation. (iii) The reactivity (k(3)()(exp)) [alpha-amino acids > beta-amino acids > gamma-amino acids] is the same as that found in a previous work for the alkylating potential of lactones formed from nitrosation products of the same amino acids. This implies that the nitrosation reactions of the most common natural amino acids are the most efficient precursors of the most powerful alkylating agents. (iv) The order of magnitude (10(7)-10(8) M(-1) s(-1)) of the bimolecular rate constants of nitrosation shows that such reactions occur through an encounter process.     

Effect of pH on the Encapsulation of the Salicylic Acid / Salicylate System by Hydroxypropyl-β-Cyclodextrin at 25 °c. A Fluorescence Enhancement Study in Aqueous Solutions

The encapsulation of the acid/base conjugated system salicylic acid(HSA)/salicylate (SA^-) by hydroxypropyl--cyclodextrin (HPBCD) has beenstudied through fluorescence emission enhancement measurements in aqueoussolutions at 25 °C. With the aim of analyzing the crucial importance ofa proper and cautious choice of the pH of the medium (i.e. choice of thebuffer), the study has been carried out at pH = 1, 2, 4 and 7. Since the pK_a of the HSA/SA^- system is 2.95 at 25 °C, the presence of the protonated(HSA) and non-protonated (SA^-) forms suitable for inclusion by cyclodextrinvary appreciably within the different pH conditions: 99% HSA,1% SA^- at pH = 1, 90% HSA, 10% SA^- at pH = 2,10% HSA, 90% SA^- at pH = 4, and 0% HSA,100% SA- at pH =7. The association constants K CD : HSA and K _{CD: SA-} have been determined in all cases by using a nonlinear regressionanalysis of the experimental data at three different ^em. The effectof the pH of the medium on all the equilibria involved as well as in the Kvalues is fully discussed. The 8-anilino-1-naphthalene sulfonate (ANS) +-cyclodextrin (-CD) system, widely reported in literature, hasinitially been studied to check the experimental protocol and the numericalmethod.     

Covalent Cross-Linking of 2H-MoS2 Nanosheets

The combination of 2D materials opens a wide range of possibilities to create new-generation structures with multiple applications. Covalently cross-linked approaches are a ground-breaking strategy for the formation of homo or heterostructures made by design. However, the covalent assembly of transition metal dichalcogenides flakes is relatively underexplored. Here, a simple covalent cross-linking method to build 2H-MoS₂–MoS₂ homostructures is described, using commercially available bismaleimides. These assemblies are mainly connected vertically, basal plane to basal plane, creating specific molecular sized spaces between MoS₂ sheets. Therefore, this straightforward approach gives access to the controlled connection of sulfide-based 2D materials.     

A Water Molecule Triggers Guest Exchange at a Mono-Zinc Centre Confined in a Biomimetic Calixarene Pocket: a Model for Understanding Ligand Stability in Zn Proteins

In this study, the ligand exchange mechanism at a biomimetic Zn^II centre, embedded in a pocket mimicking the possible constrains induced by a proteic structure, is explored. The residence time of different guest ligands (dimethylformamide, acetonitrile and ethanol) inside the cavity of a calix[6]arene-based tris(imidazole) tetrahedral zinc complex was probed using 1D EXchange SpectroscopY NMR experiments. A strong dependence of residence time on water content was observed with no exchange occurring under anhydrous conditions, even in the presence of a large excess of guest ligand. These results advocate for an associative exchange mechanism involving the transient exo-coordination of a water molecule, giving rise to 5-coordinate Zn^II intermediates, and inversion of the pyramid at the Zn^II centre. Theoretical modelling by DFT confirmed that the associative mechanism is at stake. These results are particularly relevant in the context of the understanding of kinetic stability/lability in Zn proteins and highlight the key role that a single water molecule can play in catalysing ligand exchange and controlling the lability of Zn^II in proteins.     

Determination of Paint Solvents by Vapour Phase Fourier Transform Infrared Spectrometry.

A fast procedure has been developed for the direct determination of paint solvents. The method is based on the injection of small volumes of untreated solvent mixtures into a heated Pyrex glass reactor in which the sample is volatilized and introduced by means of a flow of nitrogen into an IR multiple pass gas cell and the spectrum in the mid-IR region is registered as a function of time. Data found for samples are interpolated in calibration graphs obtained by injecting different volumes of pure compounds which constitute the solvent mixture. A methylisobutylketone (MIBK) toluene mixture was used as test system to develop the proposed procedure. The method provides a limit of detection of the order of 1–4 μg, a relative standard deviation of the order of 0.4 to 2% for five independent measurements, and recovery percentage values from 99.8 till 103.6 %. A commercial sample of polyurethane paint solvent has been analysed by the developed procedure.