Short-range order and F ion diffusion inside the Pb1 − xAlxF2 + x solid solution Part II: Al NMR investigation and proposal of clustering process

A ²⁷Al nuclear magnetic resonance (NMR) investigation at room temperature of the Pb_{1 − x}Al_xF_{2 + x} (0 ≤ x ≤ 0.12) solid solution and of the ordered Pb₉Al₂F₂₄ phase is carried out. Two different types of aluminium ions are identified and their ratio is determined as a function of x. A clustering process based on the formation of column clusters, more and more extended when x increases, is proposed on the basis of results issued from the ¹⁹F (Part I) and ²⁷Al NMR investigations.     

Mouvement du lithium dans deux nouvelles familles d'électrolytes solides: Les boracites Li4+xB7O12+x/2Cl et les halogénoborates de lithium vitreux

The boracites Li_4+xB₇O_12+x/2Cl and the related B₂O₃---xLi₂O---yLiCl glasses have been studied by cw and pulsed NMR between 130 and 500°K. Above 160°K the ⁷Li spectrum is composed of two lines: a broad one due to nonmobile Li⁺ ions and a narrow one due to diffusing Li⁺ with a hopping frequency greater than the dipolar frequency. The activation energy deduced from spin-lattice relaxation time measurements (T₁) is lower than that given by variation of conductivity with temperature. At low temperature T₁ disagrees with the BPP prediction (T₁ ω²_o). The diffusion process may be explained by the existence of a distribution of the local energy barriers.     

Pyrolysis of poly-α-acetoxystyrene,a polymer derived from polystyrene and poly(vinyl acetate)

The pyrolysis of poly-α-acetoxystyrene was investigated by using two methods thermogravimetry, and a reactor working under dynamic conditions coupled with a chromatograph.Thermogravimetry was used firstly with a constant heating rate and secondly under isothermal conditions. Two different degradation steps for the polymer were found, the first between 140 and 200°C and the second above 220°C. The kinetics of the overall pyrolysis were studied for the first thermal degradation step. The degradation order is zero when the weight loss is less than 10%, and unity when the weight loss is between 10 and 32%. The activation energy is about 40 kcal/mole for either order. The results obtained by using dynamic and isothermal thermogravimetry are in good agreement.Volatile products were identified and analysed by using a reactor working under dynamic conditions coupled with a chromatograph. The decomposition is similar to that of poly(vinyl acetate) and different to that of known substituted polystyrenes. Acetic acid was the main component of the volatile products (90–95%), the others being the monomer and acetophenone.     

Cover Picture

The cover picture shows how thrombosis occurs in the deep veins of the lower limbs. Stasis, which results from slow and turbulent blood flow, combined with hypercoagulation, caused, for example, by a surgical procedure, may result in thrombus formation. The synthetic sulfated pentasaccharide shown in part is a potent antithrombotic compound that exerts its effect by activation of the plasma protein antithrombin III. Conformationally locked monosaccharides have now been synthesized to demonstrate that L-iduronic acid, one part of the pentasaccharide, must adopt an unusual distorted conformation to activate antithrombin III. Such conformational effects might be relevant in explaining the unique biological properties of glycosaminoglycans that contain L-iduronic acid. In the background of the picture, a flight of vampire bats is attracted by the pentasaccharide. Vampire was the name given to South American blood-sucking bats (Latin name: desmodus rotundus) in 1761 by the French naturalist Georges Louis Leclerc Comte de Buffon (1707-1788). These bats are known to attack cattle and, very rarely, sleeping human beings. Although their saliva has been shown to contain an anticoagulant compound, they would also be happy to benefit from the pentasaccharide mentioned above, to suck the blood out of the vein more easily. More details about this compound which would be helpful to vampire bats are reported by Petitou, Sina? et al. on p. 1670 ff.     

The design, synthesis and photochemical study of a biomimetic cyclodextrin model of photoactive yellow protein (PYP)

The design, synthesis and study of the photophysical and photochemical properties of the first biomimetic cyclodextrin (CD) model of photoactive yellow protein (PYP) are described. This model bears a deprotonated trans-p-coumaric acid chromophore, covalently linked via a cysteine moiety to a permethylated 6-monoamino β-CD. NMR and UV/Visible spectroscopy studies showed the formation of strong self-inclusion complexes in water at basic pH. Steady-state photolysis demonstrated that, unlike the free chromophore in solution, excitation of the model molecule leads to the formation of a photoproduct identified as the cis isomer by NMR spectroscopy. These observations provide evidence that the restricted CD cavity offers a promising framework for the design of biomimetic models of the PYP hydrophobic pocket.     

Etude par diffraction de neutrons de la solution solide Pb1−1xThxF2+2x

Bragg neutron diffraction studies have been carried out on the fluorite type solid solutions Pb_1?xTh_xF_2+2x (0 < × < 0.25). The distribution of fluorine atoms between normal and interstitial sites as a function of the substitution rate is given. A volume of presence has been determined for interstitial anions. Electrical and structural data are correlated. Two conductivity mechanisms are proposed according to the fact that the pathway is free or blocked up. The best electrical performances are obtained for Pb_0.875Th_0.125F_2.25, which corresponds to a maximum of vacancies.     

Etude des proprietes structurales et electriques de la solution solide Pb1−xBixOxF2−x

An investigation of the PbF₂BiOF system at 600°C has allowed to isolate a Pb_1-xBi_xO_xF_2-x solid solution for x ? 0.67 the structure is of fluorite type and for 0.67 < × ? 0.80 it presents a fluorite derived rhombohedral distortion. The ionic conductivity has been examined. For × ? 0.50 Pb_1-xBi_xO_xF_2-x has electrical performances of the same order than those of the best so far known fluorides. Bragg neutron diffraction investigations have been carried out in order to account for the electrical behavior of the cubic solid solution (0 < x ? 0.67). The distribution of the anions between normal and interstitial sites is given as a function of substitution rate. The structure has been described on the basis of local BiOF-type clusters. The formation of isolated antiprisms BiX₈ (X = O,F) allows to understand the evolution of the electrical properties.     

Etude des proprietes de conductivite ionique des phases appartenant au systeme PbF2-InF3

A detailed study of the transport properties of the disordered fluorite-type solid solution Pb_1-xIn_xF_2+x (0 < × < 0.25) and of the ordered fluorite-type related Pb₂InF₇ (x = 0.333) and Pb₇In₅F₂₉ (x = 0.417) has been carried out. For x = 0.25 the conductivity is shown to decrease when disordering diminishes as a consequence of annealing. The disordered Pb_0.88In_0.12F_2.12 phase is a fairly good anionic conductor.     

Synthesis and characterization of a new red-emitting Ca2+ indicator, calcium ruby

Calcium Ruby m-Cl (X = H, Y = Cl) is a visible-light excited red-emitting calcium concentration ([Ca2+]) indicator dye (579/598 nm peak excitation/emission) with a side arm for conjugation via EDC or click chemistry. Its large molar extinction and high quantum yield rank it among the brightest long-wavelength Ca2+ indicators. Calcium Ruby is a promising alternative to existing dyes for imaging [Ca2+] in multicolor fluorescence applications or in the presence of yellow-green cellular autofluorescence.     

Calcium rubies: a family of red-emitting functionalizable indicators suitable for two-photon ca(2+) imaging

We designed Calcium Rubies, a family of functionalizable BAPTA-based red-fluorescent calcium (Ca(2+)) indicators as new tools for biological Ca(2+) imaging. The specificity of this Ca(2+)-indicator family is its side arm, attached on the ethylene glycol bridge that allows coupling the indicator to various groups while leaving open the possibility of aromatic substitutions on the BAPTA core for tuning the Ca(2+)-binding affinity. Using this possibility we now synthesize and characterize three different CaRubies with affinities between 3 and 22 μM. Their long excitation and emission wavelengths (peaks at 586/604 nm) allow their use in otherwise challenging multicolor experiments, e.g., when combining Ca(2+) uncaging or optogenetic stimulation with Ca(2+) imaging in cells expressing fluorescent proteins. We illustrate this capacity by the detection of Ca(2+) transients evoked by blue light in cultured astrocytes expressing CatCh, a light-sensitive Ca(2+)-translocating channelrhodopsin linked to yellow fluorescent protein. Using time-correlated single-photon counting, we measured fluorescence lifetimes for all CaRubies and demonstrate a 10-fold increase in the average lifetime upon Ca(2+) chelation. Since only the fluorescence quantum yield but not the absorbance of the CaRubies is Ca(2+)-dependent, calibrated two-photon fluorescence excitation measurements of absolute Ca(2+) concentrations are feasible.