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Les applications des impulsions laser ultra-brèves et accordables en longueur d'onde de l'IR à l'UV constituent des domaines exploratoires actifs et fructueux pour l'étude des processus primaires ultrarapides en photochimie et en photobiologie. La génération des impulsions laser femtoseconde et deux techniques de spectroscopie résolue en temps en fluorescence et en absorption sont brièvement présentées. La génération en optique non-linéaire de la somme de fréquences dans des cristaux permet de sonder à l'échelle de temps femtoseconde la fluorescence des états excités, délivrant des clichés instantanés de la couleur de la fluorescence, révélateurs des propriétés des molécules, de leur micro-environnement et des vitesses des processus intra- et inter-moléculaires. La spectroscopie d'absorption pompe-sonde qui renseigne sur le peuplement et le dépeuplement des états fondamental et excités des molécules est particulièrement utile quand les processus sont non-radiatifs. Quelques applications concernant les processus de transfert de charge intramoléculaire, de transfert d'électron au solvant, de relaxation intra- et inter-moléculaire illustreront les progrès récents des connaissances sur les processus ultrarapides dans l'eau, milieu biologique.

We synthesized a series of N-(N-acetyl-l-cysteinyl)-S-acetylcysteamine (10) analogues bearing various acyl groups on thiol cysteine or cysteamine residues, to investigate the structure−activity relationship for pro-GSH and anti-HIV properties in human macrophages. The S-substituents were ranked in the following order of efficacy:  H ≥ acetyl > isobutyryl > pivaloyl > benzoyl. We found that none of these derivatives had pro-GSH or antiviral activities in vitro higher than that of 10, but several displayed similar levels of anti-HIV activity, making them possible candidates for use as adjuvant therapies in conjunction with HAART, for treating neurological aspects of HIV infection.

It is now widely accepted that neuronal damage in HIV infection results mainly from microglial activation and involves apoptosis, oxidative stress and glutamate‐mediated neurotoxicity. Glutamate toxicity acts via 2 distinct pathways: an excitotoxic one in which glutamate receptors are hyperactivated, and an oxidative one in which cystine uptake is inhibited, resulting in glutathione depletion and oxidative stress. A number of studies show that astrocytes normally take up glutamate, keeping extracellular glutamate concentration low in the brain and preventing excitotoxicity. This action is inhibited in HIV infection, probably due to the effects of inflammatory mediators and viral proteins. Other in vitro studies as well as in vivo experiments in rodents following mechanical stimulation, show that activated microglia and brain macrophages express high affinity glutamate transporters. These data have been confirmed in chronic inflammation of the brain, particularly in SIV infection, where activated microglia and brain macrophages also express glu‐tamine synthetase. Recent studies in humans with HIV infection show that activated microglia and brain macrophages express the glutamate transporter EAAT‐1 and that expression varies according to the disease stage. This suggests that, besides their recognized neurotoxic properties in HIV infection, these cells also have a neuroprotective function, and may partly make up for the inhibited astrocytic function, at least temporarily. This hypothesis might explain the discrepancy between microglial activation which occurs early in the disease, and neuronal apoptosis and neuronal loss which is a late event. In this review article, we discuss the possible neuro‐protective and neurotrophic roles of activated microglia and macrophages that may be generated by the expression of high affinity glutamate transporters and glutamine synthetase, 2 major effectors of glial glutamate metabolism, and the implications for HIV‐induced neuronal dysfunction, the underlying cause of HIV dementia.

The metallothionein active site in vacuo was studied using ab initio methods as a necessary step in order to understand the relationship between the structure and the efficiency of the protein in binding cadmium ions. The natural Cd₄Cys₁₁ cluster located in the protein α-domain was represented by cadmium–thiolate complexes. First, it was shown, by comparison with available experimental results, that the geometry and the binding energy of cadmium coordination complexes can be correctly predicted at the RHF-MP2 theory level using the effective core potentials (ECP) and the double-ζ basis sets of Hay–Wadt, improved with polarization and diffuse functions. The computed geometry of the model system of the α-domain active site was then compared with that extracted from the X-ray crystal structure of the metallothionein. The significant difference between the two structures was attributed to the external constraints imposed in the natural cluster by the surrounding protein structure. The nature of the Cd–thiolate bond was further examined by calculating natural atomic populations and localizing the molecular orbitals. A good correlation between the Cd–S bond length and the amount of charge transferred from the ligand to the cadmium ion was found allowing an explanation of the effects of ligand protonation on the structure of mixed cadmium–thiol/thiolate complexes.

The metallothionein active site in vacuowas studied using ab initiomethods as a necessary step in order to understand the relationship between the structure and the efficiency of the protein in binding cadmium ions. The natural Cd4Cys11cluster located in the protein α-domain was represented by cadmium–thiolate complexes. First, it was shown, by comparison with available experimental results, that the geometry and the binding energy of cadmium coordination complexes can be correctly predicted at the RHF-MP2 theory level using the effective core potentials ECP and the double-ζbasis sets of Hay–Wadt, improved with polarization and diffuse functions. The computed geometry of the model system of the α-domain active site was then compared with that extracted from the X-ray crystal structure of the metallothionein. The significant difference between the two structures was attributed to the external constraints imposed in the natural cluster by the surrounding protein structure. The nature of the Cd–thiolate bond was further examined by calculating natural atomic populations and localizing the molecular orbitals. A good correlation between the Cd–S bond length and the amount of charge transferred from the ligand to the cadmium ion was found allowing an explanation of the effects of ligand protonation on the structure of mixed cadmium–thiolthiolate complexes.

We report a femtosecond laser study of the transient absorption of hydrated electrons generated by 266 nm two-photon ionization of liquid water under very high power laser density (up to 1.5 TW/cm²). The two-photon absorption coefficient of liquid water for femtosecond pulses at 266 nm was determined to be β = (1.8 ± 0.4) × 10^-11m/W. The quantum yield of formation of the hydrated electron per absorbed photon at 266 nm is determined to be 0.26 ± 0.02. We observed, for the first time, that the decay of the hydrated electron produced under 1.5 TW/cm² laser pump power density in pure water is not only due to geminate recombination and that the survival probability of the hydrated electron at 1 ns is 0.16 ± 0.02 under those conditions, where an unprecedented local concentration of the hydrated electron of 0.15 M is produced.

AbstractThe excitation decay kinetics of polar Indandione-1,3 Pyridinium Betaine (IPB) molecules in various environments: solvents, films and crystals, were studied using time-resolved luminescence and transient absorption measurements. A red-shift, depolarization and fast decay of the luminescence intensity were observed on a femtosecond time scale in dilute solutions and explained by excitation relaxation from the initially created intramolecular charge transfer state to the lower energy excited state of the indandione moiety. The first fast relaxation stage is followed by a slower one responsible for the relaxation to the ground state stage with the rate depending on the viscosity of the surrounding. By comparing the excitation properties of IPB in solution and those in the crystal form, the influence of the intermolecular interactions on the excited state dynamics is revealed. Excitation relaxation in the IPB deposited films of different thickness and in a single crystal is described as a competition between different relaxation paths of nonradiative recombination, these paths being the high lying Frenkel exciton states and the low lying intermolecular charge transfer states.

Ultrafast exciton dynamics in polycrystalline films of dipolar N,N-dimethylaminobenzylidene 1,3-indandione (DMABI) molecules was studied by femtosecond luminescence upconversion and transient absorption techniques. An enhanced nonradiative relaxation of the excitons, taking place on the subpicosecond time scale, was observed. It was attributed to the initial localisation and thermalisation of the self-trapped excitons. One-dimensional exciton–exciton annihilation was observed under high excitation conditions.

Oxidative stress and glutathione (GSH) deficit may play an important role in HIV infection pathogenesis, and oral administration of GSH-replenishing drugs such as N-acetylcysteine (NAC) and 2-oxothiazolidine-4(R)-carboxylic acid (OTC) may be associated with an increased survival rate of HIV-infected patients. Nevertheless, beneficial effects of these molecules are restricted in vivo by the high concentrations that are necessary to obtain biological effects, rapid extracellular metabolization, and low availability and plasma concentrations. We synthesized OTC derivatives that are more lipophilic than OTC and theoretically able to overcome these limitations and to generate, in addition to cysteine, other substrates of the γ-glutamyl cycle. Their antiviral effects were investigated in human HIV-1/Ba-L-infected monocyte-derived macrophages. In our experimental conditions, OTC exhibited anti-HIV-1 effects and little cytotoxicity at high doses. None of the nine tested derivatives showed higher cytotoxicity than OTC, nor anti-HIV-1/Ba-L activity.

Nonradiative excited-state relaxation via charge transfer induced twisting of N,N-dimethylaminobenzylidene-1,3-indandione (DMABI) in solution was investigated by means of picosecond transient absorption and femtosecond fluorescence spectroscopy.The analysis of the experimental data allows us to distinguish between different reaction stages of the molecular twisting in excited and ground states, as well as to consider the molecular dynamics along reactive (twisting) and nonreactive (solvation) coordinates. Quantum chemical calculations of the electronic structure of DMABI and its dependence on the molecular conformation are discussed in order to identify the bond(s) involved in the twisting reaction. It is found that such a twisting bond is the C&dbd;C double bond, which is substantially weakened in the excited state.

Time-resolved fluorescence spectra of three amino-substituted coumarin dyes have been recorded in methanol and dimethyl sulfoxide using the fluorescence upconversion technique with an apparatus response function of ≈200 fs fwhm. The three fluorinated coumarins are the 7-amino-4-trifluoromethylcoumarin (C151), the 7-diethylamino-4-trifluoromethylcoumarin (C35), and the rigidified aminocoumarin with a julolidine structure (C153). The dynamic Stokes shifts are found to be dominated by an ultrafast component with a characteristic time shorter than the present time resolution of ≈50 fs. The dynamic Stokes shifts are compared to estimations based on a “Kamlet and Taft” analysis of steady-state data in 20 solvents. It is found that the ultrafast component can be assigned mainly to intramolecular relaxation. The influences of photoinduced changes of solute−solvent hydrogen bonds on the observed spectral shifts are discussed. The breaking of hydrogen bonds at the amino group is very fast in both solvents and embedded in the ultrafast solvent inertial relaxation, while the reformation of hydrogen bonds at the carbonyl group is believed to occur on the 10−20 ps time scale in the hydrogen bond donating (HBD) solvent methanol. However, it is impossible to unambiguously correlate a particular experimental time constant with the breaking or the formation of a hydrogen bond.

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The key steps in the photosynthetic conversion of light to chemical potential energy include not only photodriven charge separation, but also prevention of the back-reaction (charge recombination). Although the first of these steps has been achieved in several biomimetic solar energy conversion systems, retarding the back-reaction has proved more difficult. This may be accomplished by rapidly moving the electron, the hole, or both away from the site of excitation to more stabilizing environments. In photosynthetic membranes, the electron is transferred sequentially over several closely coupled molecules, including tetrapyrroles and quinones1–3. In semiconductor/liquid interfacial systems both the electron and the hole migrate following excitation4,5. We now report that substantial slowing of the back-reaction has been achieved with a tripartite molecule in which a long-lived photodriven charge-separated state of relatively high potential is formed from an excited singlet state in accordance with the above principles. This molecular triad (compound I) consists of a tetraarylporphyrin covalently linked to both a carotenoid and a quinone. In solution, excitation of the porphyrin moiety by visible light results in the rapid (<100 ps) formation of a transient species C+.−P−Q−−., with a lifetime on the μs time scale and an energy more than 1 eV above the ground state.

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