Showing papers in "Chemistry: A European Journal in 1997"
TL;DR: This article showed that covalently bound fluorine hardly ever acts as a hydrogen-bond acceptor, which is in accord with results of other physicochemical studies and with the physical properties of fluorinated organic compounds.
Abstract: Statistical analysis of structural data and detailed inspection of individual crystal structures culled from the Cambridge Structural Database and the Brookhaven Protein Data Bank show that covalently bound fluorine (in contrast to anionic fluoride) hardly ever acts as a hydrogen-bond acceptor. The weakness of covalently bound fluorine as hydrogen-bond acceptor is backed by results of new molecular orbital calculations on model systems using ab initio intermolecular perturbation theory (IMPT), and is in accord with results of other physicochemical studies and with the physical properties of fluorinated organic compounds. Factors influencing the strength of hydrogen bonding in extended systems are discussed.
826 citations
TL;DR: In this paper, the dependence of the aurophilic attraction in perpendicular model systems of the type [(ClAuPH3)2] on the ab initio method, basis set and different pseudopotentials used, and on relativity was studied.
Abstract: We study the dependence of the aurophilic attraction (AuI–AuI) in perpendicular model systems of the type [(ClAuPH3)2] on the ab initio method, basis set and different pseudopotentials used, and on relativity. The effects of varying the „halogen” (X = F, H, Cl, Me, Br, -C≡CH, I), the „phosphine” (L = PH3, PMe3, -N≡CH) and the metal (M = Cu, Ag, Au) on the M–M′ interaction of the [(XML)2] dimer are also studied. The depth of the interaction potential increases with the softness of the group X. It decreases by 27% for M = Au, X = Cl and L = PH3 if relativistic effects are omitted at fixed geometry.
375 citations
TL;DR: In this article, the reduction of the bond length alternation (BLA) in the polyenic chain has been investigated, and unprecedented quadratic hyperpolarizabilities (p) have been achieved (up to p(0) =I500 x 10-30esu) by reduction.
Abstract: Donor-acceptor polyenes of various lengths, and that combine aro- matic electron-donating moieties with powerful heterocyclic electron-withdraw- ing terminal groups, have been synthe- sized and characterized as efficient non- linear optical (NLO) chromophores. Their linear and nonlinear optical proper- ties have been investigated, and variations in these properties have been related to ground-state polarization (dipole p) and structure. In particular, unprecedented quadratic hyperpolarizabilities (p) have been achieved (up to p(0) =I500 x 10-30esu) by reduction of the bond- length alternation (BLA) in the polyenic chain. In each series of homologous com-
362 citations
TL;DR: Cyclopalladated complexes of the general formula [Pd2(μ-L)2(PC)2] (L = bridging ligand, e.g. OAc, Cl, Br, I; PC = cyclometallated P donor) are highly efficient catalysts for the Heck vinylation of aryl halides.
Abstract: Cyclopalladated complexes of the general formula [Pd2(μ-L)2(PC)2] (L = bridging ligand, e.g. OAc, Cl, Br, I; PC = cyclometallated P donor, e.g. o-CH2C6H4P(o-Tol)2 or o-CH2C6H2(CH3)2-P(Mes)2) are highly efficient catalysts for the Heck vinylation of aryl halides. The isolated complexes are easily accessible from palladium(II) acetate by spontaneous metallation of ortho-methyl-substituted arylphosphines. They display improved activity and stability compared to conventional catalyst mixtures (e.g. [Pd(OAc)2] +nPPh3), and also exhibit a higher stability towards air than conventional Pd0-based systems (e.g. [Pd(PPh3)4]). Turnover numbers (TON) of up to 1000000 and turnover frequencies (TOF) in the range of 5000-20000 are achieved in catalytic coupling reactions of aryl bromides. Even technically interesting aryl chlorides undergo the Heck reaction (TON = 600-40000) if promoting salts are added to the catalyst ((NBu4)Br, LiBr). The new structural type for catalysts is compared to palladacycles formed in situ from mixtures of [Pd(OAc)2] + P(o-tolyl)3 and the established [Pd(OAc)2] + nPPh3 system. The scope of the new CC coupling catalysts is outlined for the vinylation of aryl halides by the use of different mono- and disubstituted olefins. Mechanistic consequences for the Heck reaction in general are discussed.
362 citations
TL;DR: It is concluded that the mechanism of the reaction varies with catalysis or substituent effects, and the exceptional isotope effect observed experimentally was shown to be due to the rate-determining carbonyl addition caused by autocatalysis.
Abstract: The Baeyer-Villiger reaction of p-anisaldehyde with peroxyacetic acid in nonpolar solvents to give p-anisylformate was examined on the basis of ab initio molecular orbital calculations. To explain the experimental observations, the free-energy change was evaluated for each case in the absence and in the presence of an acid catalyst. It was found that, without catalysts, the rate-determining step corresponds to the carbonyl addition of peroxyacetic acid to p-anisaldehyde and the reaction hardly occurs. Acetic acid was found to catalyze the carbonyl addition and change the rate-determining step from the carbonyl addition to the migration of the carbonyl-adduct intermediate. Trifluoroacetic acid was observed to catalyze both the carbonyl addition and migration, and the carbonyl addition was demonstrated to be a rate-determining step. The results provided a convincing explanation of the complex kinetics seen experimentally. Further calculations were performed for the reaction of benzaldehyde with peroxyacetic acid to give phenylformate. Migratory aptitude was found to depend on the catalyst. Isotope effects were also investigated, and the exceptional isotope effect observed experimentally was shown to be due to the rate-determining carbonyl addition caused by autocatalysis. It is concluded that the mechanism of the reaction varies with catalysis or substituent effects.
244 citations
226 citations
TL;DR: In this paper, a model was proposed for the arrangement of the molecules in the fibers, in this model the bisurea molecules aggregate through hydrogen-bond formation into long ribbons, which assemble into sheets.
Abstract: New low molecular weight gelators based on the structure R-NHCONH-X-NHCONH-R have been synthesized and tested for their ability to cause gelation of organic solvents. Compounds 2 (R = n-dodecyl, X = -(CH2)(9)-), 3 (R = n-dodecyl, X = -(CH2)(12)-), 4 (R = n-dodecyl, X = 4,4'-biphenyl), and 5 (R = benzyl, X = -(CH2)(9)-) form thermoreversible gels with a wide range of organic solvents, at concentrations well below 10 mg mL(-1). Depending on the nature of the R and X groups, the solvents that undergo gelation include hexadecane, p-xylene. 1-octanol, n-butyl actetate, cyclohexanone, and tetralin. The gels are stable up to temperatures well above 100 degrees C, but are easily disrupted by mechanical agitation. Light microscopic investigations revealed that compounds 2-5 spontaneously aggregate to form thin flat fibers, which can be several hundreds of micrometers long and only 2-10 mu m wide. Depending on the solvent, multiple twists in the fibers are observed. In the gels, these fibers form an extended three-dimensional network, which is stabilized by multiple mechanical contacts between the fibers. Electron microscopy and X-ray powder diffraction revealed that the fibers consist of stacks of sheets. The thickness of the sheets is 3.65 and 3.85 nm for 2 and 3, respectively. Scanning tunneling microscopic investigations of 2 absorbed on graphite showed that 2 forms ion ribbons with a width of 5.0 nm. In the ribbons the molecules have a parallel arrangement, with the long molecular axis perpendicular to the long ribbon axis. The two urea groups within a given molecule are each part of mutually parallel extended chains of hydrogen bonds, Based on these observations a model is proposed for the arrangement of the molecules in the fibers, In this model the bisurea molecules aggregate through hydrogen-bond formation into long ribbons, which assemble into sheets. In these sheets the ribbons are tilted. Finally, the sheets stack to form lone thin fibers. This model is supported by molecular dynamics simulations.
217 citations
TL;DR: Terrylenimides 3 and 4 represent a new class of blue colorants, exhibiting absorption maxima at 650 to 700 nm and fluorescence emissions in the NIR region, and can be modified to serve either as soluble dyes or as insoluble pigments.
Abstract: Terrylenimides 3 and 4 represent a new class of blue colorants, exhibiting absorption maxima at 650 to 700 nm and fluorescence emissions in the NIR region (673 to 750 nm). The terrylenimides were synthesized by means of various organometallic coupling reactions, catalyzed by transition metal complexes (Ni(o) , Pd(o) ) and starting from the aromatic bromides, boronic acids, or organotin compounds. The terrylenimides have all the properties expected of excellent fluorescent dyes: high extinction coefficients, high fluorescence quantum yields, and very good thermal, chemical, and photochemical stabilities. Owing to its extended π system, 3 can reversibly accept four negative charges. By varying the substituents, 3 and 4 can be modified to serve either as soluble dyes or as insoluble pigments.
190 citations
189 citations
TL;DR: In this article, the concept of constitutive packing models was introduced for interpenetrated diamondoid coordination networks based on crystalline adducts of 4,4′-biphenyldicarbonitrile with silver(I) salts.
Abstract: The ability to predict and subsequently control solid-state structure has been identified as a major challenge in the field of crystal engineering. Here we suggest the concept of constitutive models as a tool for understanding crystal packings and for designing new solid-state structures. Such models are intended to relate molecular interactions and their geometrical constraints with solid-state organization. These models will most likely be of greatest use for crystals consisting of supramolecular networks, that is, infinite assemblies of small molecules associating through strong, directional, and selective noncovalent interactions. The concept of the constitutive packing model is illustrated for interpenetrated diamondoid coordination networks based on crystalline adducts of 4,4′-biphenyldicarbonitrile with silver(I) salts. Observed structural deformations induced by counterions of varying size may be understood in terms of the interference of two supramolecular networks within this system: the diamondoid metal–ligand coordination network and face-to-face aromatic stacks of the organic ligand. The constitutive model developed here has been applied to other diamondoid coordination networks in the literature and is found to be general.
170 citations
TL;DR: In this paper, the authors studied the long-range limit of the aurophilic attraction at MP2 and Hartree-Fock levels for the model system (X-Au-L)n.
Abstract: We study the nature of the aurophilic attraction (AuI–AuI) at its long-range limit for the model systems [(X-Au-L)n] (n = 2, 3; X = Cl, I, L = PH3, PMe3; X = H, L = -N≡CH) at the ab initio MP2 and Hartree-Fock levels. The nature of the interactions and nonadditive effects at various orientations are related to simple electrostatic induction and dispersion expressions involving the individual properties of each monomer.
TL;DR: In this article, photochemical control of a self-assembled supramolecular 1:1 pseudorotaxane (formed between a tetracationic cyclophane, namely the tetrachloride salt of cyclobis(paraquat-p-phenylene), and 1,5-bis[2-(2-(1-hydroxy)ethoxy)thoxy)ethoxide]naphthalene) has been achieved in aqueous solution.
Abstract: Photochemical control of a self-assembled supramolecular 1:1 pseudorotaxane (formed between a tetracationic cyclophane, namely the tetrachloride salt of cyclobis(paraquat-p-phenylene), and 1,5-bis[2-(2-(2-hydroxy)ethoxy)ethoxy]naphthalene) has been achieved in aqueous solution. The photochemical one-electron reduction of the cyclophane to the radical trication weakens the noncovalent bonding interactions between the cyclophane and the naphthalene guest—π-π interactions between the π-electron-rich and π-electron-poor aromatic systems, and hydrogen-bonding interactions between the acidic α-bipyridinium hydrogen atoms of the cyclophane and the polyether oxygen atoms of the naphthalene derivative—sufficiently to allow the guest to dethread from the cavity; the process can be monitored by the appearance of naphthalene fluorescence. The radical tricationic cyclophane can be oxidized back to the tetracation in the dark by allowing oxygen gas into the system. This reversible process is marked by the disappearance of naphthalene fluorescence as the molecule is recomplexed by the tetracationic cyclophane. This supramolecular system can be chemically modified such that the π-electron-rich unit, either a naphthalene derivative or a hydroquinone ring, and the tetracationic cyclophane are covalently linked. We have demonstrated that the π-electron-rich residue in this system is totally “self-complexed” by the cyclophane to which it is covalently attached. Additionally, the self-complexation can be switched “off” and “on” by electrochemical two-electron reductions and oxidations, respectively, of the tetracationic cyclophane component. Thus, we have achieved the construction of two switches at the nanoscale level, one driven by photons and the other by electrons.
TL;DR: Electrospray mass spectrometry (ESI-MS) is used to analyze in detail the structure of the first five generations of the poly(propylene imine) dendrimers with 4-64 end groups and the defects in the divergent growth patterns are compared to those observed in the Merrifield synthesis of polypeptides.
Abstract: Electrospray mass spectrometry (ESI-MS) is used to analyze in detail the structure of the first five generations of the poly(propylene imine) dendrimers with 4-64 end groups. Although the reaction sequence to prepare these dendrimers is highly optimized, an accumulation of statistical defects is observed at higher generations. A simple statistical simulation allows the assignment of all peaks in the deconvoluted spectrum and the determination of yields for each side reaction in the formation of each generation. The fifth generation of the poly(propylene imine) dendrimer containing amine end groups, which is formed after 248 consecutive reaction steps, possesses a dendritic purity of approx. 20%, which corresponds to a polydispersity of approximately 1.002. The defects in the divergent growth patterns of dendrimers are compared to those observed in the Merrifield synthesis of polypeptides.
TL;DR: In this paper, an orifice was opened on the framework of C60 in the form of a cobalt complex having the metal sitting on top of the opening, which is used for the synthesis of endohedral metallofullerenes.
Abstract: We have been investigating the synthesis of endohedral metallofullerenes since we successfully opened an orifice on the framework of C60 in the form of a cobalt complex having the metal sitting on top of the opening. A second approach aims at the synthesis of spherically-shaped acetylenic macrocycles, which are expected to rearrange to endohedral metal complexes of fullerenes in a controlled process analogous to the gas-phase coalescence of mono- and polycyclic polyynes during fullerene formation by the graphite evaporation method. Since the potential benefits from obtaining endohedral metal complexes of C60 are enormous, both approaches are being actively pursued in our group.
TL;DR: The current status of absolute asymmetric synthesis in a chiral crystalline environment has been reviewed in this article, where a number of topochemically controlled four-center type photocycloadditions are described for a series of unsymmetrically substituted diolefin crystals and CT crystals.
Abstract: The current status of absolute asymmetric synthesis in a chiral crystalline environment has been reviewed. A number of topochemically controlled four-center type photocycloadditions are described for a series of unsymmetrically substituted diolefin crystals and CT crystals. This concept has been applied to intramolecular photoreactions, and several successful absolute asymmetric syntheses have been achieved, involving Norrish Type II reaction, di-π-methane rearrangement, electrocyclization, thietane formation, oxetane formation, hydrogen abstraction by thiocarbonyl and alkenyl groups, and radical-pair intermediates.
TL;DR: In this article, the authors used the shielding effect of the dendrimer branches on the Ru-bipyridine core, which limits the quenching effect of molecular oxygen.
Abstract: Several synthetic strategies have been explored to prepare dendrimers having the [Ru(bpy)3]2+ complex as their core (bpy = 2,2′-bipyridine). Dendritic ligands have been synthesized by attaching branches in the 4,4′-positions of bpy. The largest dendritic bipyridine ligand contains 54 peripherical methylester units. Four RuII dendritic complexes have been prepared. Their absorption and emission spectra are very similar to those of the unsubstituted parent RuII–bipyridine complexes. The large dendritic complexes, however, exhibit a more intense emission and a longer excited-state lifetime than [Ru(bpy)3]2+ in aerated solutions. This is due to the shielding effect of the dendrimer branches on the Ru–bipyridine core, which limits the quenching effect of molecular oxygen. For the largest dendritic complex, which contains 54 peripherical methylester units, the excited-state lifetime in aerated acetonitrile solution is longer than 1 μs, and the rate constant for dioxygen quenching is twelve times smaller than for [Ru(bpy)3]2+.
TL;DR: In this article, a new family of carbohydrate-containing calixarene derivatives named calixsugars were introduced by glycosylation of the phenolic hydroxyl groups by means of a Mitsunobu reaction.
Abstract: Model O-glycosylation reactions at either rim of calix[4]arenes are described with the aim of providing access to a new family of carbohydrate-containing calixarene derivatives named calixsugars. One or two sugar moieties (D-mannofuranose and D-glucopyranose) were introduced at the lower rim of the parent calix[4]arene by glycosylation of the phenolic hydroxyl groups by means of a Mitsunobu reaction. Tetrapropoxy calix[4]arenes bearing two or four hydroxymethyl groups at the upper rim were coupled with perbenzoylated thioethyl D-galactoside and D-lactoside in the presence of the thiophilic promoter copper(III) triflate. In this way β-linked bis- and tetrakis-O-galactosyl calix[4]arenes were obtained in good yield, the latter showing some solubility in water. For the O-lactosyl derivatives only the bis-substituted compound could be obtained because of the competing formation of an intramolecular ether linkage between 1,3-hydroxymethyl groups. Preliminary binding studies showed some affinity of the galactose-containing calixsugars toward charged carbohydrates and dihydrogen phosphate anion.
TL;DR: In this article, the authors used NMR and Raman spectra in conjunction with the acute Si-Si-Si angles found in their molecular structures indicate mainly ionic bonding.
Abstract: Transmetalation reactions between bis(hypersilyl)zinc Zn[Si(SiMe3)3]2 and alkali metals have already been established as a facile route to powders of the solvent-free potassium, rubidium, and cesium derivatives of tris(trimethylsilyl)-silane (hypersilane, (Me3Si)3SiH).[1,2] By the use of boiling n-heptane as the solvent, the hitherto unknown NaSi(SiMe3)3 (1) along with the previously synthesized KSi(SiMe3)3 (2) have now been obtained as colorless crystalline materials. Information from NMR and Raman spectra in conjunction with the acute Si-Si-Si angles found in their molecular structures indicate mainly ionic bonding. This was verified by population analyses of suitable model systems. Both hypersilanides[2] consist of cyclic dimers [MSi(SiMe3)3]2 (1a, M = Na; 2a, M = K) with almost planar M2Si2 rings (Na-Si = 299 pm (av); K-Si = 339 pm (av)), which are linked up into coordination polymers. In a similar manner to the related rubidium and cesium derivatives, a pentane suspension of the potassium compound afforded a yellow solution on addition of benzene, from which the crystalline, bright yellow tris(benzene) solvate 2·(benzene)3 (2b) was isolated. Complex 2b consists of monomers containing short K-Si bonds (332–334 pm) and three η6-bonded benzene molecules. No solvate of 1 was obtained under these conditions. However, on crystallization from pure benzene, crystals of (1)2·benzene (1b) were isolated (Na-Si = 302 pm (av)). Benzene was found to be intercalated between rods of coordination polymers of (1)2. In addition, the influence of π or s donors on the molecular and crystal structures of hypersilylrubidium (3) and cesium (4) was investigated. The structures of the tetrahydrofuran solvate (4)2·THF (4c), the biphenylene/pentane complex (4)2·biphen·(pentane)0.5 (4b) along with the known toluene solvates (3)27dot;toluene (3a) and (4)2·(toluene)3 (4a)[1a] are compared. Finally, an example is presented of how the alkali metal hypersilanides could be utilized in preparing extremely bulky stannanide anions.
TL;DR: It was shown that the interior of compounds 1 a-c preferentially adopts a C3 symmetrical conformation owing to strong intramolecular H-bonding, which gives rise to an extended core that induces strong interactions between molecules, leading to mesophases of enhanced thermal stability.
Abstract: A new type of disc-shaped molecule (1 a-c) has been synthesised and characterised. The molecules were built up by linking three lipophilic, N-monoacylated 2,2'-bipyridine-3,3'-diamine wedges to a central 1,3,5-benzenetricarbonyl unit. They show liquid crystalline behaviour, as shown by DSC, polarisation microscopy and X-ray diffraction. In all cases the mesophase was characterised as a Dho phase. From (1) H NMR results it was shown that the interior of compounds 1 a-c preferentially adopts a C3 symmetrical conformation owing to strong intramolecular H-bonding, which gives rise to an extended core. This large core induces strong interactions between molecules, leading to mesophases of enhanced thermal stability.
TL;DR: In this paper, a pseudorotaxane formed in acetonitrile solution by self-assembly of a wire-type electron donor based on the tetrathiafulvalene unit and the cyclobis(paraquat-p-phenylene) tetracationic electron acceptor was investigated.
Abstract: The electrochemical and spectroscopic properties of a pseudorotaxane formed in acetonitrile solution by self-assembly of a wire-type electron donor based on the tetrathiafulvalene unit and the cyclobis(paraquat-p-phenylene) tetracationic electron acceptor have been investigated. We show that a) reversible dethreading/rethreading cycles of the pseudorotaxane can be performed by either oxidation and successive reduction of the electron-donor wire or reduction and successive oxidation of the electron-accepting tetracationic cyclophane, and b) because of this special behavior, the input (electrochemical)/output (absorption spectrum) characteristics of this molecular-level system correspond to those of an XNOR logic gate.
TL;DR: In this paper, the stereoelectronic information imprinted in the components is crucial in controlling the extent of the formation of the complexes and compounds in the first place; moreover, it has a very significant influence on the relative orientations and motions of the components.
Abstract: As a result of cooperative noncovalent bonding interactions (namely, π–π stacking, [CH…O] hydrogen bonding, and [CH…π] interactions) supramolecular complexes and mechanically interlocked molecular compounds—in particular pseudorotaxanes (precatenanes) and catenanes—self-assemble spontaneously from appropriate complementary components under thermodynamic and kinetic control, respectively. The stereoelectronic information imprinted in the components is crucial in controlling the extent of the formation of the complexes and compounds in the first place; moreover, it has a very significant influence on the relative orientations and motions of the components. In other words, the noncovalent bonding interactions—that is, the driving forces responsible for the self-assembly processes—live on inside the final superstructures and structures, governing both their thermodynamic and kinetic behavior in solution. In an unsymmetrical [2]catenane, for example, changing the constitutions of the aromatic rings or altering the nature of substituents attached to them can drive an equilibrium associated with translational isomerism in the direction of one of two or more possible isomers both in solution and in the solid state. Generally speaking, the slower the components in mechanically interlocked compounds like catenanes and rotaxanes move with respect to each other, the easier it is for them to self-assemble.
TL;DR: In this article, the steric repulsion of the PR2 groups within the chelating fragment was found to significantly influence the coordination geometry of [(P2)Rh], depending on the nature and length of the backbone (X).
Abstract: The complexes [(P2)Rh(hfacac)] 1 [P2 = R2P-(X)-PR2] are introduced as model compounds for the investigation of the intrinsic steric properties of the [(P2)Rh] fragment. The ligand exchange processes that occur during the syntheses of 1 from [(cod)Rh(hfacac)] and the appropriate chelating diphosphanes 3 were studied by variable-temperature multinuclear NMR spectroscopy. The molecular structures of eight examples of 1 with systematic structural variations in 3 were determined by X-ray crystallography. The steric repulsion of the PR2 groups within the chelating fragment was found to significantly influence the coordination geometry of [(P2)Rh], depending on the nature and length of the backbone (X). A linear correlation between the P-Rh-P angles in the solid state and the 103Rh chemical shifts reveals a similar geometric situation in solution. A unique molecular modeling approach was developed to define the accessible molecular surface (AMS) of the rhodium center within the flexible [(P2)Rh] fragment. The potential of this model for application in homogeneous catalysis was exemplified by the use of 1 as catalysts in a test reaction, the hydrogenation of CO2 to formic acid. Complexes 1 were found to be the most active catalyst precursors for this process in organic solvents known to date.
TL;DR: In this paper, a saccharide-containing polymers, which are highly branched and water-soluble, were proposed for dendrimers, which can find applications of a biological nature as well as in the context of new materials.
Abstract: Dendrimers coated with carbohydrates on their exterior surfaces have been constructed by using both convergent and divergent synthetic routes. Alternatively, cluster glycosides in the form of highly branched oligosaccharides can serve as dendritic wedges in the subsequent elaboration of fully carbohydrate dendrimers. It is anticipated that these novel saccharide-containing polymers, which are highly branched and water-soluble, will find applications of a biological nature as well as in the context of new materials.
TL;DR: Oligonucleotides constructed of 1′, 5′-anhydrohexitol nucleoside building blocks are completely stable towards 3′-exonuclease and form very stable self-complementary duplexes as well as sequence-selective stable duplexe with the natural DNA and RNA.
Abstract: Oligonucleotides constructed of 1′, 5′-anhydrohexitol nucleoside building blocks (hexitol nucleic acids, HNA) are completely stable towards 3′-exonuclease and form very stable self-complementary duplexes as well as sequence-selective stable duplexes with the natural DNA and RNA. Triple-helix formation has also been observed. These hybridisation characteristics are highly dependent on the base sequence and the experimental conditions. When using a phosphate buffer containing 0.1M NaCl, a homopurine HNA dodecamer gives a δTm of +3.0 °C/ base pair with RNA as complement. These oligomers may therefore be of considerable interest as antisense constructs.
TL;DR: In this article, the complexation of three guests containing 4,4′-bipyridinium redoxactive residues by β-cyclodextrin (β-CD) and its heptakis-(2,6-Odimethyl) analogue (DM-β -CD) was investigated by means of voltammetric techniques.
Abstract: The complexation of three guests containing 4,4′-bipyridinium redoxactive residues by β-cyclodextrin (β-CD) and its heptakis-(2,6-O-dimethyl) analogue (DM-β-CD) was investigated by means of voltammetric techniques. The three 4,4′-bipyridinium (viologen) derivatives used as guests were designed to be water-soluble in all three accessible oxidation states. The N-substituents chosen to enhance aqueous solubility were: 2-(2-(2-ethoxy)ethoxy)ethanol (guest 12+), 6-hexanoate (guest 2), and 3-propanesulfonate (guest 3). Detailed analysis of the voltammetric results by digital simulation techniques revealed that the oxidized forms of the guests did not interact appreciably with either CD host; the two-electron reduced guests formed extremely stable inclusion complexes, with association constants in the range 103–104M−1, while the cation radical forms exhibited intermediate binding affinities (≈102M−1). In all cases, DM-β-CD was found to form more stable complexes than unmodified β-CD.
TL;DR: In this paper, the definition and reasons for desiring extralarge pore, crystalline molecular sieves are enumerated and a historical perspective on the development of these materials is presented with emphasis on critical features important for practical application.
Abstract: The definition and reasons for desiring extralarge pore, crystalline molecular sieves are ennumerated. A historical perspective on the development of these materials is presented with emphasis on critical features important for practical application. Based on the known physiochemical properties of extra-large pore materials, several avenues for their synthesis are suggested. Finally, a few issues of concern when dealing with this class of materials are presented.
TL;DR: Modifications of DAB-dendr-(NH2)x with biologically active carbohydrates affords a new and simple approach to high molecular weight compounds that may be considered as neoglycoconjugates with perfectly symmetrical structures and that offer much promise as multivalent ligands involved in carbohydrate-protein interactions.
Abstract: The use of preformed poly-(propylene imine) dendrimers (DAB-dendr-(NH2)x) for the rapid and facile construction of high molecular weight carbohydrate-coated dendrimers (glyco-dendrimers) is presented. An efficient attachment of spacer-armed derivatives of D-galactose and lactose to the primary amino end groups of DAB-dendr-(NH2)x has been achieved by means of amide bond formation, using the N-hydroxysuccinimide coupling procedure. Acetate protecting groups have been employed in order to avoid side reactions at the coupling stage. Deacetylation leads to the target glycodendrimers. The reactivity of all the available DAB-dendr-(NH2)x (generations 1–5) has been investigated and a series of homologous carbohydrate-coated dendrimers have been synthesized. In addition, the attachment of larger saccharide moieties has been demonstrated by the condensation of a trisgalactoside cluster with DAB-dendr-(NH2)x carrying both four and eight primary amino groups. The regularity of the glycodendrimers has been proven by NMR spectroscopy, and the molecular weights of the low-generation carbohydrate-coated dendrimers have been determined by mass spectrometry. Modifications of DAB-dendr-(NH2)x with biologically active carbohydrates affords a new and simple approach to high molecular weight compounds that may be considered as neoglycoconjugates with perfectly symmetrical structures and that offer much promise as multivalent ligands involved in carbohydrate-protein interactions.
TL;DR: In this paper, the 3-helical conformation of 1 was shown to be so stable that it was restored spontaneously within 400 ps after unfolding helical structures after molecular dynamics simulations.
Abstract: Peptides consisting entirely of homochiral (a derived from the NMR studies were T = 298, 350, 400 K; with and without equally well satisfied by both the re- strained and the unrestrained room-tem- perature molecular dynamics simulations. The 3,-helical conformation of 1 was shown to be so stable that it was restored spontaneously within 400 ps after unfold- helical structures - molecular dynam- ics simulations * NMR spectroscopy * peptides