Spacer effects in dimeric cationic surfactants

A series of dimeric cationic surfactants (gemini surfactants), which have spacer groups of varying length and flexibility, was synthesized. The series is derived from the parent compounds dodecyltrimethylammonium chloride or benzyldodecyldimethylammonium chloride. Characteristic surfactant properties of the dimeric ammonium compounds such as surface activity, micellization, viscosity effects, foaming, and solubilization, were studied with respect to the influence of the spacer group on the surfactant. For all properties, the influence of the length of the spacer group was predominant though the chemical nature of the spacer cannot be neglected.     

Synthesis and properties of cationic oligomeric surfactants

Three series of new oligomeric cationic surfactants were synthesized. These amphiphiles are trimeric and tetrameric oligomeric quaternary ammonium chlorides, with spacer groups of different lengths separating the individual surfactant fragments. The properties of the compounds, such as Krafft temperatures, surface activity, micellization, viscosifying effects, foaming and solubilizing capacity, are studied. The influence of the degree of oligomerization and of the spacer group on the surfactant properties is discussed, in comparison with the analogous standard monomeric and dimeric ("gemini") surfactants. Typically, the evolution of the properties observed from standard to dimeric surfactants progresses with the trimers and tetramers, resulting for instance in extremely low critical micellization concentrations.     

Photoreduction of 4,4'-bipyridine by amines in acetonitrile-water mixtures: influence of H-bonding on the ion-pair structure and dynamics

The photoreduction of 4,4'-bipyridine (44BPY) by diazabicyclo[2.2.2]octane and triethylamine (TEA) is investigated by using picosecond transient absorption and time-resolved resonance Raman spectroscopy in various acetonitrile-water mixtures. The results are interpreted on the basis of a preferential solvation effect resulting from the presence of a specific interaction between 44BPY and water by hydrogen bonding. Below 10% water, the free 44BPY species is dominant and leads upon photoreduction to a contact ion pair that undergoes efficient intrapair proton transfer if TEA is the amine donor. Above 10% water, most of the 44BPY population is H-bonded and leads upon photoreduction to a hydrated ion pair in which the intrapair proton transfer is inhibited. Instead, the 44BPY(-*) species is protonated by water through the hydrogen bond with a rate constant that increases by more than 3 orders of magnitude on going from 10% to 100% water. The dependence of this rate constant on the solvent mixture composition suggests that the reaction of intracomplex proton transfer is controlled by the hydration of the residual OH(-) species by three molecules of water, leading to a trihydrated HO(-)(H(2)O)(3) species.     

On the geometric structure of independence systems

A bouquet of matroids is a combinatorial structure that generalizes the properties of matroids. Given an independence system, there exist several bouquets of matroids having the same family of independent sets. We show that the collection of these geometries forms in general a meet semi-lattice and, in some cases, a lattice (for instance, when is the family of the stable sets in a graph). Moreover, one of the bouquets that correspond to the highest elements in the meet semi-lattice provides the smallest decomposition of into matroidal families, such that the rank functions of the different matroids have the same values for common sets. In the last section, we give sharp bounds on the performance of the greedy algorithm, using parameters of some special bouquets in the semi-lattice.     

High-throughput modeling of human G-protein coupled receptors: amino acid sequence alignment, three-dimensional model building, and receptor library screening

The current study describes the development of a computer package (GPCRmod) aimed at the high-throughput modeling of the therapeutically important family of human G-protein coupled receptors (GPCRs). GPCRmod first proposes a reliable alignment of the seven transmembrane domains (7 TMs) of most druggable human GPCRs based on pattern/motif recognition for each of the 7 TMs that are considered independently. It then converts the alignment into knowledge-based three-dimensional (3-D) models starting from a set of 3-D backbone templates and two separate rotamer libraries for side chain positioning. The 7 TMs of 277 human GPCRs have been accurately aligned, unambiguously clustered in three different classes (rhodopsin-like, secretin-like, metabotropic glutamate-like), and converted into high-quality 3-D models at a remarkable throughput (ca. 3s/model). A 3-D GPCR target library of 277 receptors has consequently been setup. Its utility for "in silico" inverse screening purpose has been demonstrated by recovering among top scorers the receptor of a selective GPCR antagonist as well as the receptors of a promiscuous antagonist. The current GPCR target library thus constitutes a 3-D database of choice to address as soon as possible the "virtual selectivity" profile of any GPCR antagonist or inverse agonist in an early hit optimization process.     

Reversal of enantioselectivity in the asymmetric rhodium- versus iridium-catalyzed hydroboration of meso substrates

The Ir-catalyzed asymmetric hydroboration of bicyclic hydrazines with ee and chemical yields up to 64% is reported. The switch from rhodium to iridium leads systematically to opposite enantiomers in this desymmetrization reaction.     

Discriminating bacteria from other atmospheric particles using femtosecond molecular dynamics

We investigated femtosecond pump–repump depletion schemes in biological fluorophors (tryptophan and riboflavin) in order to discriminate bioaerosols from organic interferents emitted by combustion (traffic related urban aerosols). Although fluorescence depletion is significative for riboflavin (Rbf, Vitamin B2), the most striking results have been obtained for the amino acid tryptophan (Trp). By using a 270 nm-pump 810 nm-repump femtosecond excitation, we showed that Trp exhibits fluorescence depletion up to 50%, contrary to naphthalene (<2%), despite almost identical absorption/emission spectra. We demonstrate that this process in Trp is so robust that it still occurs in living bacteria (Bacillus subtilis, Escherechia coli and Enterococcus fæcalis) but is absent for pure diesel fuel. This remarkable difference between biological and organic aerosols can be exploited to discriminate among them.     

Titanium dioxide plasma treatment

After a review of the literature on titanium production under plasma conditions, the main parameters controlling the reactions (residence time, type and relative quantity of reducer, reaction temperature, etc.) are emphasized. The controlled-atmosphere chamber where the rutile or anatase particles are heated and partly reduced in a crucible by a d.c. plasma torch with Ar-H₂ as plasma gas is described. Study of the temperature dependence of the Gibbs free energy allows one to explain, at least qualitatively, the experimental results obtained which correspond at best to 10% (weight) of TiO₂ reduced to TiO by carbon.     

Squash inhibitors: from structural motifs to macrocyclic knottins

In this article, we will first introduce the squash inhibitor, a well established family of highly potent canonical serine proteinase inhibitors isolated from Cucurbitaceae. The squash inhibitors were among the first discovered proteins with the typical knottin fold shared by numerous peptides extracted from plants, animals and fungi. Knottins contain three knotted disulfide bridges, two of them arranged as a Cystine-Stabilized Beta-sheet motif. In contrast to cyclotides for which no natural linear homolog is known, most squash inhibitors are linear. However, Momordica cochinchinensis Trypsin Inhibitor-I and (MCoTI-I and -II), 34-residue squash inhibitors isolated from seeds of a common Cucurbitaceae from Vietnam, were recently shown to be macrocyclic. In these circular squash inhibitors, a short peptide linker connects residues that correspond to the N- and C-termini in homologous linear squash inhibitors. In this review we present the isolation, characterization, chemical synthesis, and activity of these macrocyclic knottins. The solution structure of MCoTI-II will be compared with topologically similar cyclotides, homologous linear squash inhibitors and other knottins, and potential applications of such scaffolds will be discussed.     

Palladium-catalyzed cross-coupling of aryl electrophiles with dimethylalkynylaluminum reagents

[reaction: see text] Alkynyldimethylaluminum reagents are easily available from terminal alkynes and trimethylaluminum via a triethylamine-catalyzed metalation. These compounds can react with various aromatic and heterocyclic halides in the presence of palladium in a fast and efficient way. This catalyzed cross-coupling reaction provides a simple entry to numerous internal alkynes, using a readily available, inexpensive, and nontoxic metalating agent.