Algebra and Geometry of Rewriting

We present various results of the last 20 years converging towards a homotopical theory of computation. This new theory is based on two crucial notions: polygraphs (introduced by Albert Burroni) and polygraphic resolutions (introduced by François Métayer). There are two motivations for such a theory:

• Providing invariants of computational systems to study those systems and prove properties about them;
• Finding new methods to make computations in algebraic structures coming from geometry or topology.

This means that this theory should be relevant for mathematicians as well as for theoretical computer scientists, since both may find useful tools or concepts for their own domain coming from the other one.

     

Synthesis and liquid crystalline properties of mono-, di- and tri-O-alkyl pentaerythritol derivatives bearing tri-, di- or monogalactosyl heads: the effects of curvature of molecular packing on mesophase formation

Self-organisation and self-assembly are critical to the stability of synthetic and biological membranes. Of particular importance is consideration of the packing arrangements of the various molecular species. Both phospho- and glycolipids can pack in ways in which curvature can be introduced into self-organised or self-assembled systems. For instance, it is known that the degree of curvature can affect the structures of any condensed phases that are formed. In this article we report on a systematic study in which we have varied the shapes of glycolipids and examined the condensed phases that they form. In doing so, we have also unified the shape dependency of lyotropic liquid crystals with those of thermotropic liquid crystals. In order to undertake this systematic study a range of different pentaerythritol derivatives was synthesized, which covers combinations of one to three alkyl chains of different lengths (6,7,9,10,11,12,14,16 carbon atoms) and three to one galactosyl heads. Mono- and di-O-galactosyl derivatives were prepared directly by glycosylation of the corresponding alcohols using 2,3,4,6-tetra-O-benzoyl or acetyl-alpha-D-galactopyranosyl trichloroacetimidate or bromide as the donors; the tri-O-galactosyl derivatives were synthesized from O-alkyl-O-benzyl di-O-galactosyl pentaerythritol intermediates, followed by de-O-benzylation and glycosylation steps. All of the fully deprotected products were obtained by standard methods, and their self-organising and self-assembling properties examined.     

Establishment of mass spectrometric fingerprints of novel synthetic cholesteryl neoglycolipids: The presence of a unique <Emphasis Type="Italic">C</Emphasis>-glycoside species during electrospray ionization and during collision-induced dissociation tandem mass spectrometry

In this study we evaluated the fragmentation pattern of 16 novel amphiphilic neoglycolipid cholesteryl derivatives that can be efficiently used to increase cationic liposomal stability and to enhance gene transfer ability. These neoglycolipids bear different sugar moieties, such as D-glucosamine, N-acetyl-D-glucosamine, N-trideuterioacetyl-D-glucosamine, N-acetyllactosamine, L-fucose, N-allyloxycarbonyl-D-glucosamine, and some of their per-O-acetylated derivatives. Regardless of the structure of the tested neoglycolipid, QqToF-MS analysis using electrospray ionization (ESI) source showed abundant protonated [M+H]+ species. We also identified by both QqToF-MS and low-energy collision tandem mass spectrometry (CID-MS/MS) of the [M+H]+ ion, the presence of specific common fingerprint fragment ions: [Cholestene]+, sugar [oxonium]+, [(Sugar-spacer-OH)+H]+, [oxonium-H2O]+, and [(Cholesterol-spacer-OH)+H]+. In addition, we observed a unique ion that could not be rationally explained by the expected fragmentation of these amphiphilic molecules. The structure of this ion was tentatively proposed with that of a C-glycoside species formed by a chemical reaction between the sugar portion and the cholesterol. MS/MS analysis of this unique [C-glycoside]+ confirmed the validity of the proposed structure of this ion. The presence of an amino group at position C-2 and free hydroxyl groups of the sugar motif is crucial for the formation of a "reactive" sugar oxonium ion that can form the [C-glycoside]+ species. In summary, we precisely established the fragmentation patterns of the tested series of neoglycolipid cholesteryl derivatives and authenticated their structure as well; moreover, we speculated on the formation of a C-glycoside with the ESI source under atmospheric pressure and in the collision cell during MS/MS analysis.     

High performance liquid chromatography coupled to nuclear magnetic resonance spectroscopy and mass spectrometry applied to plant products: Identification of ecdysteroids fromSilene otites

Summary HPLC coupled in parallel to nuclear magnetic resonance (NMR) and mass spectrometry (MS) has been used to obtain¹H NMR and mass spectra of a number of ecdysteroids present in an extract of the plantSilene otites. Reversed phase gradient chromatography was performed using a D₂0-acetonitrile-based solvent system. NMR and mass spectra were obtained for integristerone A, 20-hydroxyecdysone, 2-deoxy-20-hydroxyecdysone and 2-deoxyecdysone to provide structural confirmation using continuous and stopped flow HPLC-NMR. The combined HPLC-NMR-MS system described here provided a more comprehensive analysis of the ecdysteroids present in the extract than HPLC-NMR alone.     

Improvement in the Iatroscan thin-layer chromatographic-flame ionisation detection analysis of marine lipids. Separation and quantitation of monoacylglycerols and diacylglycerols in standards and natural samples.

Mono- and diacylglycerols are important intermediates in glycerolipid biodegradation and intracellular signalling pathways. A method for mass determination of these lipid classes in marine particles was developed using the Iatroscan, which combines thin layer chromatography (TLC) and flame ionisation detection (FID) techniques. We improved existing protocols by adding two elution steps: hexane-diethyl-ether-formic acid (70:30:0.2, v/v/v) after triacylglycerol and free fatty acid scan, and acetone 100% followed by chloroform-acetone-formic acid (99:1:0.2, v/v/v) after 1,2 diacylglycerols. Diacylglycerol isomers 1,2 and 1,3 were separated from each other, as well as from free sterols in standards and marine lipids from sediment trap particles. Monoacylglycerols were separated from pigments and galactosyl-lipids in the same trap samples and in a rich pigment phytoplankton extract of Dunaliella viridis. Quantitation of each class in samples was performed after calibration with 0.5 to 2 micrograms of standards. As many as 17 lipid classes can be identified and quantified in samples using this proposed six-step development.     

Monoglycoconjugated water-soluble phthalocyanines. Design and synthesis of potential selectively targeting PDT photosensitisers

A series of asymmetrically substituted phthalocyanines conjugated to four different carbohydrate units has been designed to be used as photosensitisers for potential selective recognition by targeted cells.     

Isolation of 5 alpha- and 5 beta-dihydrorubrosterone from Silene otites L. (Wib)

5 alpha-Dihydrorubrosterone (2 beta, 3 beta, 14 alpha, 17 beta-tetrahydroxy-5 alpha-androst-7-ene-6-one), a new 19-carbon 5 alpha-ecdysteroid, was isolated together with its 5 beta counterpart from the aerial parts of Silene otites L. (Wib.) (Caryophyllaceae) by a combination of solvent partition, low-pressure column chromatography, thin-layer chromatography (normal-phase and reversed-phase) and finally HPLC. Mass spectrometry and nuclear magnetic resonance spectroscopic procedures were used for compound characterization.     

Matériaux composites à base de graves et de liants polymères—IV. Étude des propriétés mécaniques des matériaux réalisés à base de polyéthylène noble

This study concerns mixtures of calcareous, silica-calcareous, basaltic and sandy granules with polyethylene. The polyethylene (PE) is a low density material or a mixture of recovered PE and PE waxes. The compression resistances (RC) of these composits are related to the kneading temperature, the kneading time, the binder ratio and the fluidity index (I.F.). The two most important factors are the PE ratio and its oxidation rate. On the other hand, I.F. and granules nature are less important factors. In all cases the mechanical performances obtained are always better than those obtained with classical bituminous materials.     

Lectin recognition of liposomes containing neoglycolipids. Influence of their lipidic anchor and spacer length

Eleven neoglycolipids were synthesized with a membrane anchor (cholesterol or Guerbet alcohols) attached, via a hydrophilic spacer, to a potential lectin-recognized sugar moiety (N-acetylglucosamine: GlcNAc). Neoglycolipids G24-0, G28-0 and G32-0, with a C₂₄, C₂₈ and C₃₂ Guerbet alcohol residue, had no spacer, while neoglycolipids G24-3, G28-3, G32-3 contained a triethoxy spacer. Cholesteryl neoglycolipids chol-1 to chol-4 and chol-7 contained one to four and seven ethoxy units. All the cholesteryl neoglycolipids were incorporated into liposomes, while, for the Guerbet derivatives, the presence (or not) of a spacer and the length of their alkyl chains played an important role for obtaining mixed liposomes. The abilities of liposomes to be recognized by wheat germ agglutinin (WGA) were measured as an increase of the absorbance at 450 nm. Significant WGA-induced aggregations were obtained with liposomes containing neoglycolipids chol-3, chol-4, chol-7, G28-3 and G32-3. As neoglycolipid G24-3 was not recognized, the accessibility of its GlcNAc moiety for WGA depended not only on the spacer length but on the nature of the anchor. The WGA-induced aggregations increased with increasing lectin concentrations until a maximum value, which depended on the nature of the neoglycolipid. The highest aggregation obtained with G28-3 and G32-3 occurred for a WGA–neoglycolipid mole ratio superior to the ratio observed with chol-3, chol-4 and chol-7. A preincubation of WGA with free GlcNAc did not inhibit the aggregation between WGA and mixed liposomes, demonstrating a greater affinity of WGA for the neoglycolipid GlcNAc than for free GlcNAc.