Room temperature dynamic polymers based on Diels-Alder chemistry.

Dynamers based on reversible Diels-Alder chemistry have been obtained and shown to undergo dynamic exchange at room temperature. Their study in solution by small-angle neutron scattering indicated the formation of long and highly flexible chains. Polydispersed molecules gave T(g) values below room temperature, permitting the generation of a dynamic elastomer upon introduction of a dynamic cross-linking agent. The use of a system with a low equilibrium constant gives access to materials with interesting self-healing properties.

Supramolecular chemistry: receptors, catalysts, and carriers.

Supramolecular chemistry is the study of the structures and functions of the supermolecules that result from binding substrates to molecular receptors. Macropolycyclic receptors and coreceptors have been designed that form cryptate inclusion complexes and display molecular recognition towards spherical, tetrahedral, and linear substrates of various kinds (metal cations, inorganic anions, and organic or biological cations or anions). Anion binding has led to the development of anion coordination chemistry. Metalloreceptors simultaneously bind organic molecules and metal ions; speleands combine polar and nonpolar binding subunits. Receptors bearing reactive functional groups may act as molecular reagents or catalysts, performing a chemical transformation on the bound substrates (by such reactions as hydrogen transfer, ester cleavage, and protoadenosinetriphosphatase and protokinase activities). Receptors fitted with lipophilic groups can operate as molecular carriers, translocating bound species through a membrane; this transport can be coupled to chemical potentials (proton and redox gradients).

Single-molecule transport in three-terminal devices.

Transport through single molecules has been studied using different test beds. In this paper we focus on three-terminal devices in which a molecule bridges the gap between two gold electrodes and a third electrode-the gate-is able to modulate the conduction properties of the junction. Depending on the electronic coupling, Γ, between the molecule and the gold electrodes, different transport regimes can be distinguished. We show measurements on junctions incorporating different single-molecule systems which demonstrate the distinction between these regimes, as well as the experimental limitations in controlling the exact value of Γ.

Competition-driven selection in covalent dynamic networks and implementation in organic reactional selectivity.

Competition among reagents in dynamic combinatorial libraries of increased complexity leads to reactional self-sorting (improved regioselectivity) in mixtures of aldehydes and oligoamines. High selectivity of a given library component is transferred to a different reacting component of low selectivity through a network of underlying equilibrating reactions which provide component exchange between all species. The selectivity of various carbonyl compounds in reactions with amines was also assessed towards the formation of defined sequences of residues along oligoamine chains. The approach was further exploited for defining selective dynamic protecting groups (DPGs), based on the reversible linkage between the substrate and the protecting group. They represent an intermediate approach between the conventional protecting groups and the protecting-group-free approach in organic synthesis. Removal of the protecting group is effected via dynamic exchange trapping by formation of a more stable product. The establishment of equilibrium eliminates the need for isolation and purification of the dynamically protected intermediate(s) and enables as well the selective sequential derivatisation of oligoamines. The DPG concept can be generalised to other reversible reactions and can thus represent a valuable alternative in the design of total synthesis of complex molecules.

The design of cationic lipids for gene delivery.

Synthetic gene delivery vectors are gaining increasing importance in gene therapy as an alternative to recombinant viruses. Among the various types of non-viral vectors, cationic lipids are especially attractive as they can be prepared with relative ease and extensively characterised. Further, each of their constituent parts can be modified, thereby facilitating the elucidation of structure-activity relationships. In this forward-looking review, cationic lipid-mediated gene delivery will mainly be discussed in terms of the structure of the three basic constituent parts of any cationic lipid: the polar headgroup, hydrophobic moiety and linker. Particular emphasis will be placed on recent advances in the field as well as on our own original contributions. In addition to reviewing critical physicochemical features (such as headgroup hydration) of monovalent lipids, the use of headgroups with known nucleic-acid binding modes, such as linear and branched polyamines, aminoglycosides and guanidinium functions, will be comprehensively assessed. A particularly exciting innovation in linker design is the incorporation of environment-sensitive groups, the intracellular hydrolysis of which may lead to more controlled DNA delivery. Examples of pH-, redox- and enzyme-sensitive functional groups integrated into the linker are highlighted and the benefits of such degradable vectors can be evaluated in terms of transfection efficiency and cationic lipid-associated cytotoxicity. Finally, possible correlations between the length and type of hydrophobic moiety and transfection efficiency will be discussed. In conclusion it may be foreseen that in order to be successful, the future of cationic lipid-based gene delivery will probably require the development of sophisticated virus-like systems, which can be viewed as "programmed supramolecular systems" incorporating the various functions required to perform in a chronological order the different steps involved in gene transfection.

Electron transfer through bridging molecular structures.

Pairs of reducible pentakis(thiophenyl)benzene subunits are linked by different molecular structures as model compounds for reducible molecular-wire-type synthons showing varying electron-transfer ability as a function of the bridging structures, consisting of either para-divinylbenzene, bis-hydrazone, or diacetylene. Their electron-transfer ability from one reducible subunit to the other was investigated by electrochemical and spectroelectrochemical methods. In the case of the bis-hydrazone bridge and the diacetylene bridge, the solid-state structures support the experimental findings. While the para-divinylbenzene bridge completely isolates the reducible subunits (class I system) the diacetylene bridge electronically connects the two reducible structures (class III system), demonstrating its potential application as a "molecular wire." The bis-hydrazone linked dimer displays electronic communication only to a small extend, which was only observed in the spectroelectrochemical investigation. The diacetylene connection as active electron-transfer linker together with poly(thiophenyl)benzene as reducible subunits was used to design more complex molecular architectures. Linear rodlike structures did allow adjustment of the length of these type of molecular wires and investigation of the extent of electron mobility. Cyclic structures addressed the possibility of moving electrons on a bent molecular wire.

Selective adhesion, lipid exchange and membrane-fusion processes between vesicles of various sizes bearing complementary molecular recognition groups.

Equimolar mixtures of large unilamellar vesicles (LUVs) obtained from mixtures of egg lecithin and lipids containing complementary hydrogen bonding head groups (barbituric acid (BAR) and 2,4,6-triaminopyrimidine (TAP)) were shown to aggregate and fuse. These events have been studied in detail using electron microscopy and dynamic light scattering, and by fluorimetry using membrane or water-soluble fluorescence probes. It was shown that aggregation was followed by two competitive processes: a) lipid mixing leading to redispersion of the vesicles; b) fusion events generating much larger vesicles. In order to better understand the nature of the interaction, the effects of ionic strength and surface concentration of recognition lipids on the aggregation process were investigated by dynamic light scattering. Additionally, it was possible to inhibit the aggregation kinetics through addition of a soluble barbituric acid competitor. The study was extended to giant unilamellar vesicles (GUVs) to investigate the size effect and visualise the phenomena in situ. The interactions between complementary LUVs and GUVs or GUVs and GUVs were studied by optical microscopy using dual fluorescent labelling of both vesicle populations. A selective adhesion of LUVs onto GUVs was observed by electron and optical microscopies, whereas no aggregation took place in case of a GUV/GUV mixture. Furthermore, a fusion assay of GUV and LUV using the difference of size between GUV and LUV and calceine self-quenching showed that no mixing between the aqueous pools occured.

Interaction of an acridine dimer with DNA quadruplex structures.

The reactivation of telomerase activity in most cancer cells supports the concept that telomerase is a relevant target in oncology, and telomerase inhibitors have been proposed as new potential anticancer agents. The telomeric G-rich single-stranded DNA can adopt an intramolecular G-quadruplex structure in vitro, which has been shown to inhibit telomerase activity. The C-rich sequence can also adopt a quadruplex (intercalated) structure (i-DNA). Two acridine derivatives were shown to increase the melting temperature of the G- quadruplex and the C-quadruplex at 1 microM dye concentration. The increase in Tm value of the G-quadruplex was associated with telomerase inhibition in vitro. The most active compound, "BisA", showed an IC(50) value of 0.75 microM in a standard TRAP assay.

Molecular recognition of quadruplex DNA by quinacridine derivatives.

The interaction of monomeric and dimeric quinacridines with quadruplex DNA has been investigated using a variety of biophysical methods. Both series of compounds were shown to exhibit a high affinity for the G4 conformation with two equivalent binding sites. As shown from the SPR and dialysis experiments the macrocyclic dimer appears more selective than its monomeric counterpart.

Increasing the oxygen load by treatment with myo-inositol trispyrophosphate reduces growth of colon cancer and modulates the intestine homeobox gene Cdx2.

Preventing tumor neovascularisation is one of the strategies recently developed to limit the dissemination of cancer cells and apparition of metastases. Although these approaches could improve the existing treatments, a number of unexpected negative effects have been reported, mainly linked to the hypoxic condition and the subsequent induction of the pro-oncogenic hypoxia inducible factor(s) resulting from cancer cells' oxygen starvation. Here, we checked in vivo on colon cancer cells an alternative approach. It is based on treatment with myo-inositol trispyrophosphate (ITPP), a molecule that leads to increased oxygenation of tumors. We provide evidence that ITPP increases the survival of mice in a model of carcinomatosis of human colon cancer cells implanted into the peritoneal cavity. ITPP also reduced the growth of subcutaneous colon cancer cells xenografted in nu/nu mice. In the subcutaneous tumors, ITPP stimulated the expression of the homeobox gene Cdx2 that is crucial for intestinal differentiation and that also has an anti-tumoral function. On this basis, human colon cancer cells were cultured in vitro in hypoxic conditions. Hypoxia was shown to decrease the level of Cdx2 protein, mRNA and the activity of the Cdx2 promoter. This decline was unrelated to the activation of HIF1α and HIF2α by hypoxia. However, it resulted from the activation of a phosphatidylinositol 3-kinases-like mitogen-activated protein kinase pathway, as assessed by the fact that LY294002 and U0126 restored high Cdx2 expression in hypoxia. Corroborating these results, U0126 recapitulated the increase of Cdx2 triggered by ITPP in subcutaneous colon tumor xenografts. The present study provides evidence that a chemical compound that increases oxygen pressure can antagonize the hypoxic setting and reduce the growth of human colon tumors implanted in nu/nu mice.

Efficiency, Na+/K+ selectivity and temperature dependence of ion transport through lipid membranes by (221)C10-cryptand, an ionizable mobile carrier.

The kinetics of Na+ and K+ transport across the membrane of large unilamellar vesicles (LUV) were determined at two pH's when transport was induced by (221)C10-cryptand (diaza-1,10-decyl-5-pentaoxa-4,7,13,16,21-bicyclo [8.8.5.] tricosane) at various temperatures, and by nonactin at 25 degrees C and (222)C10-cryptand at 20 and 25 degrees C. The rate of Na+ and K+ transport by (221)C10 saturated with the cation and carrier concentrations. Transport was noncooperative and exhibited selectivity for Na+ with respect to K+. The apparent affinity of (221)C10 for Na+ was higher and less pH-dependent than that for K+, and seven times higher than the affinity for Na+ of nonactin. Its enthalpy was higher than that of (222)C10 for K+ ions (20.5 vs. 1.7 kcal . mole-1). The efficiency of (221)C10 transport of Na+ was pH- and carrier concentration-dependent, and was similar to that of nonactin; its activation energy was similar to that for (222)C10 transport of K+ (35.5 and 29.7 kcal . mole-1, respectively). The reaction orders in cation n(S) and in carrier m(M), respectively, increased and decreased as the temperature rose, and were both independent of carrier or cation concentrations; in most cases, they varied slightly with the pH. n(S) varied with the cation at pH 8.7 and with the carrier for Na+ transport only, while m(M) always depended on the type of cation and carrier. Results are discussed in terms of the structural, physico-chemical and electrical characteristics of carriers and complexes.

In situ generation and screening of a dynamic combinatorial carbohydrate library against concanavalin A.

Dynamic combinatorial chemistry (DCC) is a recently introduced approach that is based on the generation of combinatorial libraries by reversible interconversion of the library constituents. In this study, the implementation of such libraries on carbohydrate-lectin interactions was examined. The dynamic carbohydrate libraries were generated from a small set (four or six compounds) of initial carbohydrate dimers through mild disulfide interchange, and selection was performed under two conditions defining either adaptive or pre-equilibrated libraries. Upon initiation, libraries were formed that contained comparable amounts of 10 or 21 individual dimeric species, dynamically interchanging during the scrambling process. They were probed with respect to binding to the plant lectin concanavalin A, either present during library generation or added after equilibration. The libraries could be generated easily both in the presence and absence of the receptor, and a bis-mannose structure was preferentially bound and selected from the mixture. Scrambling of the library in the presence of the receptor resulted in slightly higher yields than when the receptor was added after scrambling, indicating that the receptor to some extent acts as a thermodynamic trap during library generation. The present results illustrate the extention of the DCC approach to carbohydrate recognition groups, the generation of isoenergetic dynamic libraries, and the implementation of either adaptive or pre-equilibrated procedures.

Virtual combinatorial libraries: dynamic generation of molecular and supramolecular diversity by self-assembly.

Molecular and supramolecular diversity may be generated, respectively, by reversible, covalent or noncovalent self-assembly of basic components whose various potential combinations in number and nature represent a virtual combinatorial library. This concept is applied to the induction of inhibitors of carbonic anhydrase (CA) by reversible recombination of aldehyde and amine components. It is found that the presence of CA favors the formation of those condensation compounds that may be expected to present the strongest binding to the CA active site. The virtual combinatorial library approach may represent a powerful methodology for the discovery of substrates, inhibitors, receptors, catalysts, and carriers for a variety of processes.

Interaction of porphyrin-containing macrotetracyclic receptor molecule with single-stranded and double-stranded polynucleotides. A photophysical study.

Photophysical methods have been used to study the interaction with nucleic acids of a macrotetracyclic cryptand molecule, Pbiph, containing a porphyrin groups, two macrocycles, and a biphenyl bridge. Pbiph binds with a higher affinity to single-stranded polynucleotides than to double-stranded ones. This selectivity, observed by binding and competition studies, using absorption and fluorescence spectroscopy, is pH dependent. Pbiph does not intercalate into double helices and is suggested to bind into the major groove. These features, selective single-strand binding and nonintercalation, are attributed to steric effects of the bulky Pbiph molecule, resulting from the macropolyclic cryptand cage structure.

Photochemical generation of carbon monoxide and hydrogen by reduction of carbon dioxide and water under visible light irradiation.

Visible light irradiation of solutions of Ru(2,2'-bipyridine)(3) (2+), cobalt(II) chloride, and carbon dioxide in acetonitrile/water/triethylamine generates simultaneously carbon monoxide and hydrogen. The reaction involves photoinduced reduction of CO(2) and H(2)O, triethylamine serving as electron donor in the Ru(2,2'-bipyridine)(3) (2+)/Co(2+) system. The amount of gas (CO + H(2)) produced and the selectivity ratio CO/H(2) depend markedly on the composition of the system. Addition of free bipyridine strongly decreases CO generation but increases H(2) production. With different tertiary amines, NR(3), both the quantity (CO + H(2)) and the ratio CO/H(2) increase markedly along the sequence R = methyl, ethyl, propyl. Higher selectivity for CO(2) reduction to CO in preference to water reduction occurs when triethanolamine is used instead of triethylamine. CoCl(2) is the most efficient mediator for both CO and H(2) generation and specifically promotes CO formation, whereas salts of other cations studied only yield H(2). The mechanism of the reaction may involve intermediate formation of Co(I) species. These processes represent an abiotic photosynthetic system allowing simultaneous generation of CO and H(2) and regulation of the CO/H(2) ratio. Mechanistic studies and explorations of other components that may increase efficiency and product selectivity should be carried out. The results obtained are also of significance for solar energy conversion with consumption of a pollutant, CO(2).

pH regulation of divalent/monovalent Ca/K cation transport selectivity by a macrocyclic carrier molecule.

The lipophilic dicarboxylic acid-dicarboxamide macrocycle 1 is an efficient carrier for calcium and potassium transport through a liquid membrane. The process involves competitive Ca2+/K+ symport coupled to proton antiport in a pH gradient. It presents a very pronounced phenomenon of pH regulation of transport selectivity from preferential K+ transport to preferential Ca2+ transport as the pH increases from 2 to 9 in the starting aqueous phase containing the metal ions. The results demonstrate how carrier design allows control of the rate and selectivity of divalent/monovalent M2+/M+ cation transport.