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.     

A SHORT-LIVED INTERMEDIATE IN THE PHOTO-ENZYMATIC REDUCTION OF PROTOCHLOROPHYLL(IDE) INTO CHLOROPHYLL(IDE) AT A PHYSIOLOGICAL TEMPERATURE

The reduction of protochlorophyll(ide) into chlorophyll(ide) has been studied by flash absorption spectroscopy at 21°C, with a time resolution of 0.5 µs. The absorption changes have been recorded in the range 670–720 nm after the first and subsequent flashes given to an extract of etiolated bean leaves. At 695 nm the flash-induced absorption increase has its maximum value immediately after the flash and then partly decays with a half-time of about 7–10 µs. A complementary behaviour is observed at 675 nm where the absorption change is very small at time zero and then increases to a stationary value with a half-time of about 6–9 µs. From measurements at several wavelengths it is concluded that a species with an absorption peak around 695 nm is formed immediately after the flash and is then transformed into a stable species with an absorption peak around 675 nm. Measurements at lower temperatures, down to—50°C, show that the transformation is much slowed down at decreased temperatures. The species absorbing at 695 nm (P₆₉₅) is attributed to an intermediate in the photoreduction of protochlorophyll(ide) P_639,650 into chlorophyll(ide) P₆₇₆. When the protochlorophyll(ide) is photoreduced before the flash illumination, the newly formed chlorophyll(ide) gets to a triplet state, which decays with a half-time of 15 µs at 21°C. This result shows that carotenoid molecules do not exert their protective role at this stage of chlorophyll (Chi) formation.     

Impedance measurements on purified silver chloride crystals using ionic vs. electronic contacts

Impedance measurements are made on purified silver chloride crystals covering thicknesses from 0.064 cm to 0.400 cm at temperatures from 293 K to 310 K using constant ionic strength solution contacts with varying silver ion concentrations. Results characterize the macroscopic transport properties from 10 kHz to 0.01 Hz and at d.c. Electrical relaxation times, τ_el, are independent of crystal thickness and bathing solution composition, but dependent on temperature. The resistive component of the impedance, R_∞, is linear with crystal thickness, δ, and the geometric capacitance, C_g, linear with δ^?1. In agreement with Macdonald's theory, R_∞ is equal to the d.c. resistance, R₀. This indicates rapid ion exchange at the surface and no Warburg diffusion. The dimensionless dielectric constant, calculated from C_g and the crystal dimensions, is 15.4. An average activation energy of 0.298 eV, indicative of vacancy transport is found. The parameter α in the equivalent non-ideal Cole-Cole representation is 0.96 indicating a narrow distribution of time constants. These results are compared with the electronic contact measurements typical to solid state physics. Higher values of resistivity and lower values of C_g and conductivity are found with graphite and vapor deposited silver. These differences were as large as 10%. Calculated dielectric constants at 10 kHz were 14.8 and 13.5, respectively, while the most recent published value is 11.14. These differences suggest that ionic solution contact measurements may give more reliable values because surface coverage is more nearly uniform on the molecular level.     

Synthesis of di‐ and tetra‐adducts by addition of polystyrene macroradicals onto fullerene C60

Br‐terminated polystyrenes of controlled molar masses and low polydispersities prepared by atom transfer radical polymerization (ATRP) can be converted to macroradicals using an appropriate catalytic complex (CuBr/bipyridine/100 °C). The addition of this macroradicals PS° to 6–6 bonds of C₆₀ follows a specific atom transfer radical addition mechanism that favors the grafting of even number of chains onto the fullerene core. This peculiar mechanism, resulting from the properties of C₆₀, offers an easy synthetic route toward well‐defined di‐ and tetra‐adducts. In these adducts the disturbance of the electronic structure of the fullerene is kept at its minimum, as only one double bond needs to be opened on the C₆₀ to add two PS chains and only two double bonds are converted to single bonds in the tetra‐adduct. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 3456–3463, 2004     

In situ synchrotron far infrared micro-spectroelectrochemistry with a grazing angle objective

We demonstrate the use of a grazing angle objective attachment to carry out in situ far infrared micro-spectroelectrochemistry at a copper electrode on a nano-scale. A thin-layer spectrochemical cell made out of Teflon was used, fitted with a 20-μm thick Mylar window; the working electrode was 500 μm in diameter. Measurements were carried out in 0.1 M NaOH solution as a function of applied potential between –1.4 and 0 V vs a Hg/Hg₂SO₄ reference electrode. Spectra were obtained with excellent signal to noise ratio for the surface oxide film, formed on copper electrochemically with less than 1 nL of active solution volume. The surface film at 0 V was about 130 nm thick and consisted mainly of CuO, with possibly some Cu(OH)₂ also present. This interpretation is consistent with previous works and thermodynamic calculations. The technique should be useful in other investigations and the further development of electrochemical surface science.     

Hétérocycles contenant du phosphore: XXVII—Etude par Résonance Magnétique Nucléaire de quelques dérivés des dioxa-et dithiaphospholanes-1,3,2 monosubstitués en 4

The ¹H n.m.r. spectra of some derivatives of 4-methyl (or 4-phenyl) 1,3,2-dioxa- and 1,3,2-dithiaphospholanes are analysed. The configurational study of the compounds has been investigated. The torsional angle of the C₄? C₅ fragment is calculated and the ring conformation in solution is discussed.     

Homogeneous Phosphodiesterase and Hybridization Assays Using Europium Cryptate: Oligonucleotide Conjugates

Upon conjugation to single-stranded oligonucleotides, a europium cryptate (Eu³⁺ tris-bipyridine) showed a marked increase in its fluorescence lifetime and was much less sensitive to fluorescence quenching by uric acid. This behavior was shown to be moderately dependent on the length and sequence of the oligonucleotide and all the single-stranded oligonucleotides studied displayed similar behavior. In contrast, a cryptate moiety attached to a double-stranded oligonucleotide did not display such an increase in its fluorescence lifetime and was quenched in presence of uric acid. Taking advantage of this unique behavior characterizing single-stranded K-ODN conjugates, a new concept of dosage based on the modulation of the cryptate fluorescence by a quencher was set up. This fluorescence quenching assay involving a single fluorescent label was applied to the monitoring of hybridization reactions and detection of a phosphodiesterase activity.