Gaétan Munger

Excess free energies of interaction of chlorophyll a with monogalactosyldiacylglycerol and phytol a mixed monolayer study

Abstract The surface pressure isotherms of chlorophyll a , monogalactosyldiacylglycerol and phytol at the air-water interface were studied on a Langmuir trough at 20.0±0.5° C . The subphase was a phosphate buffer, 10 −3 M at pH 8.0. The extrapolated limiting areas per molecule are 115, 82 and 38 A 2 /molecule, respectively. The isotherms of eight mixtures of chlorophyll a with monogalactosyldiacylglycerol and eight mixtures of chlorophyll a with phytol, covering in both cases the whole range of molar fractions have also been measured. The results for the mixed monolayers were analysed in terms of the additivity rule. They show that a small negative deviation with respect to ideality is observed upon mixing chlorophyll a with monogalactosyldiacylglycerol. However, chlorophyll a forms an ideal two-dimensional solution when mixed with phytol. The excess free energies of mixing of chlorophyll a with monogalactosyldiacylglycerol as a function of concentration were calculated from the surface pressure isotherms at 10, 15, 20 and 25 mN·m −1 . The values are negative, reflecting the interaction prevailing between these components in the monolayers. For the four surface pressures studied, the excess free energies are symmetrical with respect to the mode fraction. The values for an equimolar mixture range from −300 to −540 J·mol −1 at 10 and 25 mN·m −1 , respectively. A comparison between the thermodynamics of mixing of chlorophyll a with monogalactosyldiacylglycerol and phytol suggests that the polar head of monogalactosyldiacylglycerol together with the polar groups of chlorophyll a are probably involved in the interaction. However, this does not completely rule out the possibility that structural effects due to a different packing of chlorophyll a with monogalactosyldiacylglycerol and phytol may also be involved. Furthermore it is shown that the small interactions between these constituents are not inconsistent with the specific coupling existing between the apoprotein of the chlorophyll a -protein complexes and chlorophyll a .

Monolayers and Langmuir-Blodgett films of a semifluorinated tetrathiafulvalene derivative

Abstract A new semifluorinated tetrathiafulvalene (SFTTF) derivative was prepared and its behaviour at the air-water interface (π−A and ΔV−A isotherms) and in multilayer films was studied. The deposition (Y type) of such a pure SFTTF monolayer (i.e. without fatty acid) onto solid substrates was successful with a good transfer ratio. X-ray experiments and IR dichroism suggests a tilt angle (ca.35°) of the semifluorinated chains with respect to the normal of the substrate. Under an atmosphere saturated with iodine vapours, the Langmuir-Blodgett films exhibit a brown colour with a broad charge transfer band around 8500 cm−1 and a low d.c. conductivity, suggesting the formation of totally ionized TTF-type dimers. After iodination and under air exposure, iodine is rapidly lost by the films and a new charge transfer band around 3400 cm−1 appears, indicating the formation of partially ionized dimers. These changes are analysed through a reversible iodination-deiodination cycle.

Adjustable speed and vibration‐free hydraulic system for Langmuir–Blodgett films deposition

Conventional apparatus for the dipping and raising of slides usually uses an electric motor with the slides fixed in a rigid holder. Most of the devices create a great deal of vibrations and this produces a nonuniform film deposition. We have used a new apparatus, which does not produce any vibrations of the slide, to build up Langmuir–Blodgett films. To move the slide, we employ a column of water and use the siphon principle, with speeds that can be varied between 3.0 and 120.0 mm min−1. An electronic circuit controls the automatic dipping and raising of the slide. A relay switches the solenoid valve to move the piston in the opposite direction. A machined Teflon trough and antivibration mount are used for the monolayer preparation. With this method, the deposition ratio usually obtained for the cadmium arachidate and chlorophyll a is 1.00±0.05. The absorption spectrum of multilayers of chlorophyll a give a linear relation between the absorbance at 680 nm and the number of layers up to 10. The nonlinear ...

Light energy conversion with chlorophyll a and pheophytin a monolayers at the optically transparent SnO2 electrode: artificial photosynthesis

Abstract The photoelectrochemical behavior of chlorophyll a and pheophytin a monolayers, deposited on tin oxide (SnO2) optically transparent electrodes (OTE) by means of the Langmuir-Blodgett technique, has been investigated. The photocurrent action spectra coincide with the absorption spectra of photosynthetic pigment monolayers at the SnO2/solution interfaces. Anodic and cathodic photocurrents were measured at various electrode potentials. Effects of electrolyte concentration, pH, electrode potentials, type of SnO2 OTE and the nature of redox reagents on the magnitude of the photocurrent in solar energy conversion are discussed. A mechanism for the electron transfer in thin films of photosynthetic pigments in monolayers at the SnO2 OTE is proposed. On the other hand, the absorption and fluorescence spectral characteristics, and fluorescence lifetime measurements of pheophytin a in monolayers and thin films are also discussed in view of the aggregation properties of the photosynthetic pigments.

On some physicochemical properties of sucrose esters and the stability they confer to membrane proteins

Abstract A homologous series of fatty acid monoesters of sucrose whose chain length varies from 8 to 22 carbon atoms has been examined with respect to some physicochemical properties. In this series, the critical micelle concentration is comparable to that of lysophospholipids having similar hydrophobic moiety. The behavior of a spin probe dispersed in micelles of sucrose esters and their surface pressure-area isotherms at the air/water interface indicate that, despite a relatively large hydrophilic head, they form increasingly compact structures as their chain length increases. Finally, when used as dispersing medium for the purified intrinsic protein rhodopsin, long-chain esters confer to the pigment a thermal stability which is close to that of its membrane-bound state. We conclude that these detergents may be among those best suited to purified intrinsic membrane protein studies.

Second-harmonic generation investigation of Langmuir–Blodgett 2-docosylamino-5-nitropyradine films

With the use of second-harmonic generation, we report the observation of a better alignment of Langmuir–Blodgett-deposited films of 2-docosylamino-5-nitropyradine for a low number of monolayers. An estimation of the nonlinear susceptibility of the molecule is also given. We show that the refractive indices of the Langmuir–Blodgett films has to be taken into account for thicknesses greater than 25 nm.

The light-dependent oxygen reduction by monolayers of hydrated chlorophyll a oligomer

Abstract The phenomenon of light-dependent O2 uptake by monolayers of hydrated chlorophyll a oligomer deposited by the Langmuir-Blodgett technique on an SnO2 optically transparent electrode has been observed. Spectra of cathodic photocurrents coincided with the absorption spectrum of hydrated oligomer of chlorophyll a. In the presence of an artificial electron donor, hydroquinone, and an oxygen electron acceptor, both the cathodic and the anodic photocurrent caused by dry and wet chlorophyll a molecules of monolayer were measured under illumination in the range 400–800 nm. The effects of electrode potentials and redox reagents on the magnitude of solar energy conversion by the chlorophyll a monolayer at the optically transparent electrode are discussed. ESR and circular dichroism spectra show that hydrated oligomer of chlorophyll a consists of six molecules of chlorophyll a bonded with water molecules.

The standard gibbs free energy of adsorption and isotherms of adsorption of amphiphilic molecules and clusters at the oil/water and gas/water interfaces: Adsorption of dry and hydrated chlorophyll

Abstract The thermodynamics of adsorption of large surface active compounds at the interface between two immiscible liquids is considered. These molecules are supposed to substitute at the interface a number of adsorbed molecules of both solvents. Gibbs free energy and the isotherm of adsorption of surface active amphiphilic compounds were calculated. The adsorption isotherms of dry and wet chlorophyll a at the octane/water and air/water interfaces are measured. The difference between the adsorption behavior of chlorophyll a dissolved in dry and wet octane is discovered. The thermodynamic parameters of chlorophyll monomer and hydrated oligomer of chlorophyll a adsorption at octane/water interface are determined.

Hydrated chlorophyll a oligomers in solutions, monolayers, and thin films

Abstract The interfacial tension measurements of dry and wet chlorophyll a at an octane—water interface show differences in adsorption behavior of the pigment depending on its aggregation state. Optical properties were also investigated using absorption and Fourier transform-infrared spectroscopy, along with fluorescence (excitation and emission) measurements in non-polar solvents and in thin films. The absorption and fluorescence spectral characteristics, and fluorescence lifetime measurements of dry and wet samples clearly indicate the difference in aggregation properties of this photosynthetic pigment. A strong decrease in the energy of Q y electronic transitions (Δν = 1700cm −1 ), due to the formation of hydrated oligomers, was accompanied by a decrease in fluorescence lifetime from 5.75 ± 0.02 ns (diluted dry chlorophyll a in n -hexane) to 0.12 ± 0.03 ns (wet chlorophyll a in thin films). Discussion of these results is based on the key role of the central magnesium atom in the porphyrin ring in intermolecular interactions and self-organization of chlorophyll a molecules.

Characterization of monolayers and Langmuir-Blodgett films of dry and wet chlorophyll a

Abstract The surface pressure-area (π- A ) isotherms of dry and wet chlorophyll a (Ch1 a ) at the nitrogen-buffer interface were measured. It was observed that the properties of Ch1 a from dry non-polar organic solvents differed from those of wet non-polar organic solvents. These properties of dry and wet Ch1 a were discussed based on the absorption, fluorescence and Fourier transform infrared (FTIR) spectroscopies of monolayer and multilayer Langmuir-Blodgett (LB) films.