Carbohydrate-carbohydrate interaction prominence in 3D supramolecular self-assembly

Self-association in water of biologically significant carbohydrate molecules is a controversial topic due to the strong solvation of these molecules in this solvent and the difficulty to experimentally detect these very weak intermolecular forces by biophysical techniques. Herein we report the tremendous ability of amphiphilic carbohydrate molecules to form complex three-dimensional architectures. We have experimentally observed the 3D self-assembly into multilayers of disaccharide neoglycolipid dimers on graphite by means of noncontact AFM and we have also theoretically modeled the interaction between two dimers in order to learn about the structure and composition of these layers. A simple bilayer structure as observed for many amphiphilic lipids was discarded by the experiments. Instead, based on the good agreement between experiments and calculations, we propose that multilayer formation takes place through the assembly of building blocks consisting of two dimers each. The fundamental key in the formation of this supramolecular structure is the complementarity between the van der Waals surfaces of the amphiphilic carbohydrate molecules, a result which differs from the most common idea that H-bonding interactions are prominent in carbohydrate-mediated interactions.     

Syntheses and self-assembly behaviors of the azobenzenyl modified beta-cyclodextrins isomers

A couple of analogues of azobenzenyl beta-cyclodextrins 1 and 2 with self-locked and self-unlocked conformations have been synthesized via the Huisgen cycloaddition from the same reactants, but in distinct reaction conditions (i.e., the hydrothermal synthesis and the "click" reaction, respectively), their conformations were sufficiently proved by X-ray crystal structural analysis, molecular modeling study, and 2D NMR spectroscopy, and their self-assembly behaviors in aqueous solution were also investigated by NMR spectroscopy. Interestingly, the self-locked conformer 1, which could be regarded as a new type of [1]rotaxane, self-assembled to a novel bimolecular capsule, where its azobenzene substituent was included in both its own cavity and the counterpart's cavity, in aqueous solution and in the solid state. In contrast, by adjusting the conformation of 1 to a self-unlocked one, the resulting conformer 2 was found to self-assemble to a linear supramolecule. These studies have shown the stronger impact of the reaction condition changing in cyclodextrin's modification products and will provide a new access to control the structure of supramolecular assemblies by tuning the conformation of building blocks.     

A facile approach toward multicomponent supramolecular structures: selective self-assembly via charge separation

A novel approach toward the construction of multicomponent two-dimensional (2-D) and three-dimensional (3-D) metallosupramolecules is reported. Simply by mixing carboxylate and pyridyl ligands with cis-Pt(PEt(3))(2)(OTf)(2) in a proper ratio, coordination-driven self-assembly occurs, allowing for the selective generation of discrete multicomponent structures via charge separation on the metal centers. Using this method, a variety of 2-D rectangles and 3-D prisms were prepared under mild conditions. Moreover, multicomponent self-assembly can also be achieved by supramolecule-to-supramolecule transformations. The products were characterized by (31)P and (1)H multinuclear NMR spectroscopy, electrospray ionization mass spectrometry, and pulsed-field-gradient spin echo NMR techniques together with computational simulations.     

Synthesis of very small TiO2 nanocrystals in a room-temperature ionic liquid and their self-assembly toward mesoporous spherical aggregates

In this Communication, we describe the synthesis of 2-3 nm sized titania nanocrystals in a room-temperature ionic liquid under mild conditions and their self-assembly toward mesoporous TiO2 spheres. The resulting structures combine the convenient handling of larger spheres with a considerable high surface area and narrow pore size distribution and are expected to have potential in solar energy conversion, catalysis, and optoelectronic devices.     

Versatile approach toward the self-assembly of heteromultimetallic salen structures

A general route is presented toward the template-directed preparation of self-assembled heteromultimetallic salen structures using noncovalent coordinative metal-ligand interactions. Various higher order assemblies have been studied in detail using a combination of NMR spectroscopy and X-ray crystallography.     

Preparation of Au/Ag multilayers via layer-by-layer self-assembly in spherical polyelectrolyte brushes and their catalytic activity

Spherical polyelectrolyte brushes(SPBs) consisting of polystyrene(PS) core and poly(2-aminoethyl methacrylate hydrochloride)(PAEMH) shell were prepared by photo-emulsion polymerization. Au nanoparticles(Au-NPs) with controlled size and size distribution were synthesized in situ using SPBs as nanoreactors. Via layer-by-layer deposition technique on the surface of SPBs, nano-composite particles with Au/Ag-NPs bilayer and Au/Ag/Au-NPs trilayer were prepared. The structures of the as-prepared Au/Ag multilayer SPBs were characterized by UV-Vis spectroscopy, TEM, ICP-AES and DLS. The charge reversal of the nano-composite particles observed by zeta potential confirmed the success of layer-by-layer assembly. The Au/Ag-NPs bilayer nano-composite particles showed high catalytic efficiency with an apparent activation energy of about 41.2 k J/mol in the reduction reaction of 4-nitrophenol to 4-aminophenol in the existence of sodium borohydride monitored. The catalytic activity of Au/Ag-NPs multilayer SPBs close to that of Au-NPs SPBs and much higher than that of Ag-NPs SPBs reveals its potential applications in cost-effective catalysts with high-performance.     

Influence of the guests, the type and degree of substitution on inclusion complex formation of substituted beta-cyclodextrins

The stability of inclusion complexes (i.e. the measure of molecular recognition between a guest and (beta-cyclodextrin, CD) is highly influenced by the fit of the guest into the cavity of the host and by secondary bonds among the functional groups getting in close connection. The guests themselves influence these interactions by their sizes, shapes and functional groups. On the other hand, both the size of the cavity and the reactivity of beta-CDs are altered when the hydroxy groups are substituted. As best models, the interactions among hydroxypropylated CDs of different average degree (and pattern) of substitution (DS) and phenolphthalein (as a model for 'large' guests) or p-nitrophenol/p-nitrophenolate (p-NP/p-NPate) couple (as for 'small' ones) have been studied. The formation constants of phenolphthalein-hydroxypropyl-beta-CD (HP-CD) complexes are continuously decreasing, while those of p-NP and p-NPate ones show a maximum with increasing DS. Similarly, the pattern of substitution has a significant effect on the quality of the interaction, too. The change of the DS on O(6) position alters the type of the interactions most, and a series of different findings prove that this change is the basis in the chiral selectivity of different CD derivatives, too. The ratio of the average DS on primary and secondary hydroxy rims [R(DS)=DS(6)/DS(2,3)] is recommended as the simplest possibility for characterizing the substitution pattern.     

Supramolecular surface-confined architectures created by self-assembly of triangular phenylene-ethynylene macrocycles via van der Waals interaction

At the liquid/graphite interface triangular and rhombic phenylene-ethynylene macrocycles substituted by alkyl chains self-assemble to form porous two-dimensional (2D) molecular networks of honeycomb and Kagomé types, respectively, or close-packed non-porous structures via alkyl chain interdigitation as the directional intermolecular linkages. Factors that affect the formation of the 2D molecular networks, such as alkyl chain length, solvent, solute concentration, and co-adsorption of guest molecules, were elucidated through a systematic study. For the porous networks, various molecules and molecular clusters were adsorbed in the pores reflecting the size and shape complementarity, exploring a new field of 2D host-guest chemistry.     

Columnar self-assembly of rhomboid macrocyclic molecules via step-like intermolecular interaction. Crystal formation and gelation

Two macrocyclic compounds with a rhomboid molecular shape, composed of a π-conjugated framework and an imine or amine functionality, were synthesized. The amine-containing macrocycle crystallizes with step-like interaction of each molecule, forming a columnar arrangement, although dispersion of the imine gelates upon ultrasonification.     

Studies toward a synthesis of C3-epimauritine D: construction of the macrocycle

We successfully synthesized a 14-membered cyclic enamide of C3-epimauritine D from the cis-2,3-pyrrolidinediol derivative. Treatment of the pyrrolidinediol with nitrobenzonitrile in an S_NAr reaction efficiently installed the desired aryl-alkyl ether linkage on the N-heterocyclic system. Macrocyclization was successfully achieved by the use of TFFH as the peptide coupling reagent in the presence of HOAt.     

Absorption of 10-hydroxycamptothecin on Fe3O4 magnetite nanoparticles with layer-by-layer self-assembly and drug release response

In this paper, the structural and zeta potential properties of 10-hydroxycamptothecin (HCPT) were investigated by FT-IR and zeta potential analyzer under different pH. The anticancer drug HCPT as a model drug was used to prepare a high-performance and relatively easy-to-fabricate system on Fe(3)O(4) magnetite nanoparticles by using a polystyrene sulfonate (PSS) and HCPT interlayer self-assembly method. The results obtained from FT-IR and XRD confirmed that HCPT was molecularly dispersed into the nanoparticles. The method holds not only environment-friendly characteristics and the ability to mimic the self-organization process in biological systems but also greatly decreases adjuvant polymers. In addition, the system has an ideal drug payload for the delivery of insoluble HCPTs.     

From discrete particles to spherical aggregates: a simple approach to the self-assembly of Au colloids

Here we demonstrate a simple, template-free approach to the formation of spherical gold aggregates through the reduction of HAuCl4 by NaBH4, in the presence of cysteine (Cys) as a capping agent. The resulting aggregates are quite stable in solution. The pH of the solution and the molar ratio of Au:Cys are two key empirical factors in the formation of such highly ordered aggregates. At slightly alkaline pH (7-10) and with Au:Cys ratios ranging from 1:0.5 to 1:2, spherical Au aggregates of 30-80 nm are formed. At lower Cys ratios (Au:Cys> or =1:0.5) very loosely linked aggregates are formed; however, at very high Cys ratios (Au:Cys< or =1:3), highly dispersed Au particles of 2-4 nm are obtained, which are virtually indistinguishable from the original colloidal form. Aggregate size is influenced markedly by component concentration; a 3-fold increase in standard levels resulted in Au spherical aggregates of a larger size, 200-500 nm. In addition, we used a combination of Cys and lysine (Lys) as a capping agent/cross-linker and found that the morphology of the Au colloid aggregates can be easily manipulated from a linear to a spherical form by adjusting the proportions of Cys and Lys in the capping agent/cross-linker mixture. The introduction of mercapto (SH)-containing organic acids reduced the cross-linking ability of Cys, especially in the case of long-chain acids. Complete disruption of the spherical aggregates highlights the importance of Cys per se. An explanation of this ordered self-assembly process is proposed, in the context of the known surface chemistry of Au colloids.     

Molecular architecture via coordination: self-assembly of pseudohexagonal A2(3)X2(3)-macrocycles

[formula: see text] The interaction of two complementary ditopic building blocks, each incorporating 120 degrees angles between the active coordination sites, in methylene chloride at room temperature results in the spontaneous self-assembly of platinum-based assemblies of hexagonal shape.     

Multiple equilibria in the complexation of dibenzylimidazolium bromide salts by cyclodextrins: toward controlled self-assembly

In aqueous solution, dibenzylimidazolium bromide salts form dimeric assemblies by T-stacking between an acidic proton of the imidazolium and the benzyl aromatic ring of another cation. This dimeric association can be disturbed by the addition of native cyclodextrins. The control of the majority species in solution can be made by the judicious choice of cyclodextrin concentration and its macrocycle size. The dimer is complexed directly by beta-cyclodextrin, whereas in the presence of alpha-cyclodextrin, the dimer is dissociated to form a 1:1 inclusion complex; at higher concentration, this 1:1 complex can dimerize.     

Synthesis and properties of magnetite Fe3O4 via a simple hydrothermal route

Magnetic Fe₃O₄ crystals were synthesized by a simple hydrothermal method using K₄Fe(CN)₆ as single precursor. The chemical compositions and morphologies of the as-prepared samples were characterized in detail by X-ray diffraction (XRD), X-ray photoelectron spectra (XPS) and scanning electron microscopy (SEM). The experimental results show that the magnetic Fe₃O₄ particles are facilely prepared by the hydrothermal reaction of K₄Fe(CN)₆ and NaOH at 200 °C. Regular octahedral Fe₃O₄ crystals are obtained when Na₂S₂O₃ is added to the reaction system. The possible mechanism was discussed to elucidate the formation of the magnetic Fe₃O₄ crystals. Besides, the magnetic investigations show that the values of saturation magnetization and coercivity of octahedral Fe₃O₄ are about 87 emu/g and 184 Oe, respectively. Octahedral Fe₃O₄ also exhibits relatively good catalytic performance in the oxidation of styrene.     

The inclusion of the drug diflunisal by alpha- and beta-cyclodextrins. A nuclear magnetic resonance and ultraviolet spectroscopic study

¹⁹F nmr and ultraviolet spectroscopic studies show that the inclusion of the anion of the drug diflunisal (DF) by alpha- and beta- cyclodextrins (CD and CD) in water produces the complexes: DF.CD, DF.CD and DF.(CD)₂ characterized by stability constants of 17, 1.81×10⁵ and 3.07×10³ dm³mol^–1 respectively.     

Synthesis and surface self-assembly of [3]rotaxane-porphyrin conjugates: toward the development of a supramolecular surface tweezer for C60

Surface immobilization of pristine C60 by supramolecular interactions is an attractive way to introduce C60 on surfaces since the pi-electron network and the electronic properties of C60 remain intact. Several hosts have been developed for surface complexation of C60. With few exceptions, the hosts reported to date are "electronically inert", limiting the potential applications of pristine C60-based devices. In this study, we present the synthesis and self-assembly of a potential tweezer-like host for C60 having a light-harvesting moiety and an electron-donating unit. More precisely, an azide-containing [3]rotaxane scaffold having ferrocene moieties as blocking group and thioctic acid as anchoring group for a gold surface has been synthesized. This [3]rotaxane has been self-assembled on gold in its protonated (NH2+) (1p) and neutral (NH) (1n) forms and characterized using electrochemistry, XPS, and contact angle measurements. The SAMs were functionalized with free-base and zinc porphyrin using copper-catalyzed 1,3-dipolar cycloaddition in optimized conditions. In combination with C60, this new host is expected to form a triad that could potentially be used as active building block in the preparation of nanostructured electrodes for photoelectrochemical application.     

Control of self-assembly of a 3-hexen-1,5-diyne derivative: toward soft materials with an aggregation-induced enhancement in emission

The supramolecular architectures of a fluorophore are controlled through the design of a conjugated polycatenar molecule, the self-assembly of which can be addressed toward a columnar liquid-crystalline phase and organogels. Thus, depending on the environmental conditions for self-assembly, compound CA9 organizes into an unprecedented hexagonal columnar mesophase in the condensed state, in which half a molecule constitutes the slice of the column, or into a rectangular mesomorphic-like organization in the presence of apolar solvents such as cyclohexane and dodecane, at a concentration in which fibers form and gelling conditions are fulfilled. In this Col(r)-type arrangement, the organization within the columns depends on the solvent. All of the materials prepared show luminescence, and moreover, a remarkable 3-fold increase in fluorescence intensity was observed in going from the solution to the gel state.     

Experimental self-assembly: the many facets of self-assembly

The self-assembly of matter is manifested throughout the hierarchy of the universe in a myriad of modes, both biotic and abiotic. It is not only an area of deep interest as a fundamental prerequisite to life, but it is now emerging as a strategy to many new unanticipated synthetic architectures, unique properties and commercial applications. In this issue, 11 authors review recent advances in each of these realms. Considering the vastness of the subject, we have chosen to focus on selected highlights in three general areas, namely: (i) synthesis; (ii) dynamics/characterization/external parameters affecting assembly; and (iii) theoretical/biological perspectives. Current Opinion in Colloid & Interface Science 1999, 4:3–5 Electronic identifier: 1359-0294-004-00003 ©1999 Elsevier Science Ltd. All rights reserved. ISSN 1359-0294