ETUDE PAR SPECTROSCOPIE D''ECLAIRS DU TRANSFERT D''ENERGIE CHLOROPHYLLE-CAROTENOIDE

Abstract— Energy transfer from chlorophyll A in its lowest triplet state to carotenoid pigments is demonstrated by rapid flash photolysis experiments.
Two systems are used; the first consists of chlorophyll A and p carotene in organic solvents: in diluted solutions, energy transfer is diffusion controlled. The second consists of chlorophyll A and lutein incorporated into digitonin micelles suspended in water; with this system a very rapid energy transfer is observed (< 0.4 × 10^--⁶ sec).
Energy transfer results in a carotenoid metastable state, which is supposed to be a triplet state; for lutein its half-life is 8·9 × 10^--⁶ sec, and it has an absorption peak at 518 nm. Depopulation of lutein ground state, around 450 nm, can be observed, as well as the reactivity of oxygen towards the metastable state.
Most of these results were obtained with a Q -switch ruby laser as exciting source (6943 Å). A 4350 Å flash can also be obtained by two successive non linear effects. Using this flash for exciting chlorophyll A alone, a strong signal is detected, due to its triplet state. By exciting directly B carotene or lutein, it is not possible to detect any metastable state with our technique.     

De Novo Designed α-Helical Coiled-Coil Peptides as Scaffolds for Chemical Reactions

Coiled coils (CCs) are well-understood protein-folding motifs. They appear in a variety of oligomer states and as homo- and heteromeric assemblies. This versatility and the general accessibility by de novo design makes them ideal building blocks for synthetic biology. This Minireview highlights the efforts being made in designing small peptide catalysts or reaction templates based on the CC scaffold. The first reports described autocatalysis or mediation of peptide ligation based on CC recognition. Over the years, the designs became more advanced, catalyzing ester hydrolysis, acyl transfer and redox reactions with partial enzyme-like reactivity. Due to the ability to control CC assembly, and, in heterodimeric systems, the association and dissociation, the CC motif has become a common peptide tag in chemical biology.     

Linear Relationship between 13C NMR Chemical Shifts and the Bending of sp-Carbon Chains

Polyynes show a strictly linear relationship between the energy impact and the bending of the polyyne chain. The energy, which is necessary to bend the acetylenic chain, decreases with the increasing number of acetylene units. A deviation from linearity in polyynes can be realized in solution by violation of the mutual-exclusion principle between IR and Raman spectra. However, there is still no possibility to measure the extent of the nonlinearity in solution. Herein, we show that the ¹³C NMR spectroscopy represents an appropriate tool for this as we found an almost perfect linear relationship between the bending of the alkyne chain and the change of the chemical shift of the outer acetylenic carbon atoms. By using molecular bows in which the alkyne chain can be bent by switching the azobenzene unit, this correlation can be proved experimentally. In the future, this correlation should enable the determination of the extent of the bending and the strain energy in polyynes. Consequently, polyynes could be employed as probes for measuring further molecular forces.     

F-to-HF reactions. IX. Nonthermal complications for the CH4/C3F6/C2F6 moderated nuclear recoil system

Results are reported from moderated nuclear recoil ¹⁸F experiments with the CH₄/C₃F₆/C₂F₆ mixture system. At a 99.5% confidence level measurement precision of ±3.4%, non-thermal F-to-HF reactions are phenomenologically suppressed at C₂F₆ moderator concentrations in the range of 95.0–99.95 mol-%. Effectively equilibrium reaction conditions can be established in well-designed experiments of this type.     

Three-stage melting of an annelide-type copper complex. A new type of organized phase: Tegma crystals

The solid—solid phase transition of an annlide type copper complex are described. As the temperature is raised, three different constituents of the complex successively melt. In the temperature range 77–95°C, the polyethylene oxide chains and the paraffinic tail are both in a very mobile state while the copper complex sub-units still form a two-dimensional crystalline array.     

CONCERNING THE ORIGIN OF CHARGE AT THE POLYSTYRENE PARTICLE/WATER INTERFACE

The electrophoretic mobility behavior of well-characterized polystyrene latex particles, carrying one type of surface functional endgroups, has been studied as a function of pH. At low pH, the interaction of protons with the functional endgroups increased in the order: Hydroxyl > carboxyl > sulfate; at high pH the order of interaction was reversed; and at intermediate pH no interactions were observed. The particles of the polystyrene latexes in their different forms at the intermediate pH range, dispersed in deionized water, all exhibited the same mobility irrespective of the functional endgroup. The origin of charge in these systems is explained as being the result of either the preferential adsorption of hydroxyl ions or an electron - injection mechanism due to the overlap of local intrinsic molecular - ion states in polystyrene and water. At low concentrations of functional endgroups, the surface properties of the polystyrene latexes are largely dependent upon the hydrophobic nature of the surface.     

≪Monocristaux≫ de copolymères triséquencés styrène/isoprène/styrène présentant la structure cylindrique II. Étude de la déformation par élongation

Résumé On a étudié par diffraction des rayons X aux petits angles des monocristaux de copolymère triséquencé styréne/isopréne/styréne présentant la structure cylindrique. On a porté son attention sur la déformation structurale que subissent ces systèmes lorsqu'ils sont soumis à une contrainte d'élongation dirigée perpendiculairement à l'axe des cylindres. On a montré que le réseau cristallin se déforme réversiblement de manière parfaitement affine.

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Summary This is a low-angle X-ray diffraction study of single-crystals obtained from styrene/isoprene/styrene three-block copolymers presenting the cylindrical structure. The structural deformation has been analyzed as a function of an elongational strain applied to the samples perpendicularly to the axis of the cylinders. It has been shown that the crystal lattice deforms in a reversible and perfectly affine way.

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Zusammenfassung Mit Hilfe der Röntgenkleinwinkelstreuung haben wir die zylindrischen monokristallinen Strukturen der Polystyrol-Polyisopren-Polystyrol-BlockcopolymerenProben untersucht.Wir haben besonders die strukturelle Änderung analysiert, die auftritt, wenn eine Dehnung mit einem Zug senkrecht zur Zylinderachse angewandt wird.Es wird gezeigt, daß das Kristallgitter sich reversibel und streng affin verformt.

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Avec 5 figures et 3 tableaux