High Degree Of Internal Order Research Articles

Toward a Two-Dimensional Supramolecular Organic Framework with High Degree of Internal Order via Amphiphilic Modification

Solution-phase self-assembly of two-dimensional (2D) networks with a high degree of internal order and long-range periodicity is a great challenge. Herein, we report a rational design to improve 2D...

Macrocyclization of a Class of Camptothecin Analogues into Tubular Supramolecular Polymers.

Nanostructured supramolecular polymers (SPs) are filamentous assemblies possessing a high degree of internal order and have important uses in regenerative medicine, drug delivery, and soft matter electronics. Despite recent advances in functional SPs, a challenging topic is the development of robust assembly protocols enabling the incorporation of various functional units without altering its supramolecular architecture. We report here the robust tubular assembly of camptothecin (CPT) analogues into functional SPs. Covalent linkage of two CPT moieties to various short hydrophilic segments (e.g., nonionic, cationic, anionic, and zwitterionic) leads to a class of CPT analogues that self-assemble in water into tubular SPs. Systemic administration of nonionic SPs effectively suppresses tumor growth. Furthermore, these tubular SPs act as universal dispersing agents in water for low-molecular-weight hydrophobes.

Supramolecular Polymers Capable of Controlling Their Topology.

One important class of supramolecular materials is one-dimensionally elongated supramolecular polymers, in which monomers are associated by reversible intermolecular interactions, yielding a fibrous morphology. Unlike frequently reported conventional supramolecular polymers based on, for instance, host-guest interactions, those composed of one-dimensionally stacked π-conjugated molecules can be encoded with high degrees of internal order by cooperative association of the rigid aromatic monomers, endowing such supramolecular polymers with extraordinary properties and functionality. However, their internal order has not yet been exploited to manipulate the complex landscape of well-defined states of the supramolecular polymer backbone, which may induce new functionalities beyond the intrinsic properties of the backbones. This Account will focus on the inceptive phase of our research on supramolecular polymers with high degrees of internal order able to impart intrinsic curvature to their backbones. Initially, we developed a naphthalene molecule functionalized with barbituric acid, which forms uniform toroidal short fibers with diameters of approximately 16 nm via the formation of hydrogen-bonded cyclic hexamers (rosettes). As we thought the uniformity of the toroid size to arise from the intrinsic curvature generated upon stacking of the rosettes, we exploited this intrinsic curvature to design continuously curved extended supramolecular polymers by extension of such molecular π-systems. The intrinsic curvature produced by the monomers with more expanded π-systems indeed gave us access to higher-order structures (topologies) ranging from randomly folded to helically folded coils in extended supramolecular polymers. We will discuss the kinetic aspects of the generation of intrinsic curvature for topology control, including the formation of toroidal structures resulting from ring-closing processes. For extended supramolecular polymers with well-defined topologies, we will discuss manipulation of a complex landscape of well-defined states by external stimuli. The incorporation of a photoresponsive azobenzene chromophore in the original naphthalene molecular scaffold allowed us to reversibly destroy or recover the curvature of the main chain through trans- cis photoisomerization. By means of this photocontrollable curvature, we have demonstrated light-induced unfolding of helically folded structures into entirely stretched structures. Furthermore, a direct extension of the π-conjugated core provided us with access to unprecedented supramolecular polymers with emergent time-dependent topology transitions. Molecules with a naphthalene core conjugated with two phenylene units kinetically afforded supramolecular polymers that consist of helically folded and misfolded domains. Upon aging the supramolecular polymer solution, we observed spontaneous folding of the misfolded domains in a time scale of days, eventually obtaining a supramolecular polymer topology analogous to the tertiary structure of proteins. These supramolecular polymers with unrivaled and active topologies provide new prospects for supramolecular polymers as one-dimensional nanomaterials.

Threading carbon nanotubes through a self-assembled nanotube.

Achieving the co-assembly of more than one component represents an important challenge in the drive to create functional self-assembled nanomaterials. Multicomponent nanomaterials comprised of several discrete, spatially sorted domains of components with high degrees of internal order are particularly important for applications such as optoelectronics. In this work, single-walled carbon nanotubes (SWNTs) were threaded through the inner channel of nanotubes formed by the bolaamphiphilic self-assembly of a naphthalenediimide-lysine (NDI-Bola) monomer. The self-assembly process was driven by electrostatic interactions, as indicated by ζ-potential measurements, and cation-π interactions between the surface of the SWNT and the positively charged, NDI-Bola nanotube interior. To increase the threading efficiency, the NDI-Bola nanotubes were fragmented into shortened segments with lengths of <100 nm via sonication-induced shear, prior to co-assembly with the SWNTs. The threading process created an initial composite nanostructure in which the SWNTs were threaded by multiple, shortened segments of the NDI-Bola nanotube that progressively re-elongated along the SWNT surface into a continuous radial coating around the SWNT. The resultant composite structure displayed NDI-Bola wall thicknesses twice that of the parent nanotube, reflecting a bilayer wall structure, as compared to the monolayer structure of the parent NDI-Bola nanotube. As a final, co-axial outer layer, poly(p-phenyleneethynylene) (PPE-SO3Na, M W = 5.76 × 104, PDI - 1.11) was wrapped around the SWNT/NDI-Bola composite resulting in a three-component (SWNT/NDI-Bola/PPE-SO3Na) composite nanostructure.

Redox-Responsive Dimerization in a Ferrocene-Ureidopyrimidinone Supramolecular Assembly

At one extreme, supramolecular polymers can have a high degree of randomness, existing as entangled coils with the mechanical properties of plastics and elastomers. At the other extreme, supramolecular polymers can be formed through directional intermolecular interactions among designed molecular subunits to achieve dynamic behavior as well as high degrees of internal order. The strength of these intermolecular interactions can be altered via external signals such as the absorption of light, temperature, pH, or the voltage of an electrode. Such is one of the important goals in supramolecular chemistry: the development of stimuli-responsive systems where the strength of intermolecular interactions can be perturbed through the use of external signals. Applications of supramolecular chemistry include self-healing polymers and gels, controlled release of entrapped molecules, and smart materials. In this presentation the results of electrochemical studies with a ferrocene-containing ureidopyrimidone (UPy) will be described. The UPy system – introduced by Meijer – can form two self-complementary 4 hydrogen bond arrays (AADD or ADAD) which dimerize in relatively non-polar organic solvents. By attaching an electroactive substituent such as ferrocene, the strength of electrostatic interactions can be perturbed to create a redox-responsive ureidopyrimidinone. Following earlier studies by Graham and co-workers, a ferrocene-functionalized ureidopyrimidinone UPy(Fc) was prepared. Concentration and scan-rate-dependent cyclic voltammetric studies indicate a square scheme type mechanism in dichloromethane. In the starting UPy(Fc) state, the AADD dimer is preferred. Oxidation to UPy(Fc+) results in conversion to another form with a less positive E1/2. Preference for this new form increases at lower concentrations which suggests that UPy(Fc+) exists as a monomeric species. This would indicate that oxidation breaks the dimer apart as desired. Alternatively it is possible that the observed concentration dependence results from rate-limiting dissociation of the dimer, but the monomer then tautomerizes to the ADAD enol form and re-dimerizes. To distinguish between these possibilities the relative diffusion coefficients (D’s) of UPy(Fc+) to UPy(Fc) were measured by steady-state voltammetry at a microdisk electrode. They were then compared to the relative D’s of ferrocenium to ferrocene. If UPy(Fc) is a dimer in both oxidation states, the DUPy(Fc+)/DUPy(Fc) ratio should be similar to that observed for ferrocene (DFc+/DFc), which will be a monomer in both oxidation states. However, if UPy(Fc+) is a monomer, the DUPy(Fc+)/DUPy(Fc) ratio will be larger than DFc+/DFc, since the D of the UPy(Fc+) monomer will be relatively smaller than the D of the UPy(Fc) dimer due to its smaller size. The latter is observed, supporting the conclusion that oxidation breaks apart the UPy(Fc) dimer at mM concentrations in dichloromethane. The results of these studies support the conclusion that UPy(Fc+) is monomeric at milli-molar concentrations in dichloromethane, meaning that oxidation of the UPy(Fc) breaks apart the dimer as hypothesized. Furthermore the process appears to be completely reversible. CV simulations were performed to determine if the square mechanism can quantitatively explain the observed voltammetry. The resulting simulated CV data supported the use of a square scheme mechanism to explain the cyclic voltammetry. However, the mechanism only accounts for the predominant keto AADD tautomer and excludes the enol ADAD tautomer. HNMR studies mimicking experimental conditions in cyclic voltammetry have been performed, revealing the presence of the enol ADAD tautomer in dichloromethane. Further HNMR studies in dichloromethane are underway, along with a comparative study in chloroform which does not appear to contain the enol ADAD tautomer. Cyclic voltammetry of the Upy(Fc) system will also be performed in chloroform. The results of these experiments will determine if the enol ADAD contributes to the redox mechanism of UPy(Fc). In addition, the synthesis of covalently linked UPy(Fc) oligomers is being initiated with the goal of creating a redox-responsive supramolecular polymer.

Light-induced unfolding and refolding of supramolecular polymer nanofibres

Unlike classical covalent polymers, one-dimensionally (1D) elongated supramolecular polymers (SPs) can be encoded with high degrees of internal order by the cooperative aggregation of molecular subunits, which endows these SPs with extraordinary properties and functions. However, this internal order has not yet been exploited to generate and dynamically control well-defined higher-order (secondary) conformations of the SP backbone, which may induce functionality that is comparable to protein folding/unfolding. Herein, we report light-induced conformational changes of SPs based on the 1D exotic stacking of hydrogen-bonded azobenzene hexamers. The stacking causes a unique internal order that leads to spontaneous curvature, which allows accessing conformations that range from randomly folded to helically folded coils. The reversible photoisomerization of the azobenzene moiety destroys or recovers the curvature of the main chain, which demonstrates external control over the SP conformation that may ultimately lead to biological functions.

Asymmetric Peptide Nanoribbons.

Asymmetry in chemical structure or shape at molecular, nanoscale, or microscopic levels is essential to a vast number of functionalities in both natural and artificial systems. Bottom-up approaches to create asymmetric supramolecular nanostructures are considered promising but this strategy suffers from the potentially dynamic nature of noncovalent interactions. We report here on supramolecular self-assembly of asymmetric peptide amphiphiles consisting of two different molecularly linked domains. We found that strong noncovalent interactions and a high degree of internal order among the asymmetric amphiphiles lead to nanoribbons with asymmetric faces due to the preferential self-association of the two domains. The capture of gold nanoparticles on only one face of the nanoribbons demonstrates symmetry breaking in these supramolecular structures.

Synthesis, optical properties and alignment of poly(9,9-dioctylfuorene) nanofibers

Poly(9,9-dioctlylfluorene) (PFO) nanofibers were fabricated by solution template wetting of anodic alumina membranes. Nanofibers with controlled thickness of 23 nm and length between 0.8 and 10 μm, were obtained, regulated by the dimensions of the used template. Nanofibers displayed spectroscopic characteristics associated with the formation of significant percentages of planar and elongated β phase within the amorphous PFO glassy-phase. Optical polarized microscopy displayed high birefringence resulting from the high degree of internal order induced by β phase generation within the fibers. The structural intra-chain reorganization associated with formation of β phase was promoted by the strong geometrical confinement imposed on the material by the porous template during polymer wetting and solvent evaporation. Flow and shear force alignment techniques were used to control the orientation of fabricated PFO nanofibers, yielding to formation of large oriented nanofiber arrays on transparent substrates.

Highly directional co-assembly of 2,6-pyridinedicarboxylic acid and 4-hydroxypyridine based on low molecular weight gelators

The highly directional interactions among complex gelators of 2,6-pyridinedicarboxylic acid and 4-hydroxypyridine are investigated, which lead to the formation of ordered one-dimensional fibers with diameters of 500nm~2μm. By scanning electron microscopy (SEM), Fourier transform infrared (FT-IR) spectroscopy, 1H nuclear magnetic resonance (1H NMR) spectroscopy, ultraviolet–visible (UV–vis) spectroscopy, and X-ray powder diffraction (XRD), the co-assembly of the complex into straight fibers under the function of multiple directional hydrogen bonds and π–π stackings is proved. The strong and directional interactions lead to rich dynamic behavior and high degrees of internal order among the complex gelators. The ordered one-dimensional co-assemblies possess useful functions in templated synthesis, molecular recognition, enzyme immobilization, and other biochemistry fields.

Functional supramolecular polymers.

Supramolecular polymers can be random and entangled coils with the mechanical properties of plastics and elastomers, but with great capacity for processability, recycling, and self-healing due to their reversible monomer-to-polymer transitions. At the other extreme, supramolecular polymers can be formed by self-assembly among designed subunits to yield shape-persistent and highly ordered filaments. The use of strong and directional interactions among molecular subunits can achieve not only rich dynamic behavior but also high degrees of internal order that are not known in ordinary polymers. They can resemble, for example, the ordered and dynamic one-dimensional supramolecular assemblies of the cell cytoskeleton and possess useful biological and electronic functions.

Integration of self-assembled discotic-based fibres into field-effect transistors: a comparison of preparation approaches

The role played by the preparation method upon the morphological and electrical properties of alkyl substituted thio-triphenylene-based self-assembled fibres is explored by comparing two processing approaches, termed solvent vapour annealing (SVA) and solvent induced precipitation (SIP). Both approaches led to fibres having widths of several hundred nanometres and lengths of tens of micrometres. SVA formed isolated fibres which were tens of nanometres high, flat, and tapered at the ends. Conversely, SIP fibres exhibited nearly matching heights and widths, but organized into bundles. Despite these morphological differences, the same intermolecular packing is found by XRD in each type of structure, albeit with differing degrees of long-range order. The fibres were integrated into bottom-gate bottom-contact field-effect transistors. The density and configuration of the fibres with respect to the electrodes and gate dielectric were found to play an important role in the transport properties. SIP devices yielded the highest mobilities compared to SVA and spin-coated devices, largely owing to their high degree of internal order and the possibility to achieve high fibre densities within the transistor channel.

Problems in fitting high resolution structures into electron microscopic reconstructions

Great advances have been made in electron microscopy (EM) over the past decade, with the result that a number of protein complexes have been solved at near-atomic resolution using EM imaging. However, only a limited number of such complexes are expected to have the high degree of internal order needed to achieve this type of resolution. Many other complexes and polymers will be visualized and reconstructed by EM at an intermediate level of resolution, where the polypeptide chain cannot be directly traced. Crystal and nuclear magnetic resonance structures for components or subunits of these higher-order assemblies are frequently available. One of the greatest strengths of EM continues to be the ability to dock high-resolution structures of components into low or intermediate resolution reconstructions of assemblies to build pseudoatomic models for quaternary structure. This review discusses the strengths and limitations of this approach, with particular emphasis on protein polymers. I discuss how limitations in resolution can lead to ambiguities in building models, and these cannot be always be resolved with available data. The use of homology models for quaternary structure are particularly problematic, given accumulating evidence for the divergence of quaternary structures at the same time that tertiary structure can be conserved.

Nanopatterning self-assembled nanoparticle superlattices by moulding microdroplets

Highly ordered arrays of nanoparticles exhibit many properties that are not found in their disordered counterparts. However, these nanoparticle superlattices usually form in a far-from-equilibrium dewetting process, which precludes the use of conventional patterning methods owing to a lack of control over the local dewetting dynamics. Here, we report a simple yet efficient approach for patterning such superlattices that involves moulding microdroplets containing the nanoparticles and spatially regulating their dewetting process. This approach can provide rational control over the local nucleation and growth of the nanoparticle superlattices. Using DNA-capped gold nanoparticles as a model system, we have patterned nanoparticle superlattices over large areas into a number of versatile structures with high degrees of internal order, including single-particle-width corrals, single-particle-thickness microdiscs and submicrometre-sized 'supra-crystals'. Remarkably, these features could be addressed by micropatterned electrode arrays, suggesting potential applications in bottom-up nanodevices.

Peptide amphiphile nanofibers with conjugated polydiacetylene backbones in their core.

The coupling of electronic and biological functionality through self-assembly is an interesting target in supramolecular chemistry. We report here on a set of diacetylene-derivatized peptide amphiphiles (PAs) that react to form conjugated polydiacetylene backbones following self-assembly into cylindrical nanofibers. The polymerization reaction yields highly conjugated backbones when the peptidic segment of the PAs has a linear, as opposed to a branched, architecture. Given the topotactic nature of the polymerization, these results suggest that a high degree of internal order exists in the supramolecular nanofibers formed by the linear PA. On the basis of microscopy, the formation of a polydiacetylene backbone to covalently connect the beta-sheets that help form the fibers does not disrupt the fiber shape. Interestingly, we observe the appearance of a polydiacetylene (PDA) circular dichroism band at 547 nm in linear PA nanofibers suggesting the conjugated backbone in the core of the nanostructures is twisted. We believe this CD signal is due to chiral induction by the beta-sheets, which are normally twisted in helical fashion. Heating and cooling shows simultaneous changes in beta-sheet and conjugated backbone structure, indicating they are both correlated. At the same time, poor polymerization in nanofibers formed by branched PAs indicates that less internal order exists in these nanostructures and, as expected, then a circular dichroism signal is not observed for the conjugated backbone. The general variety of materials investigated here has the obvious potential to couple electronic properties and in vitro bioactivity. Furthermore, the polymerization of monomers in peptide amphiphile assemblies by a rigid conjugated backbone also leads to mechanical robustness and insolubility, two properties that may be important for the patterning of these materials at the cellular scale.

Crystallographic Order in Multi‐Walled Carbon Nanotubes Synthesized in the Presence of Nitrogen

Multi-walled carbon nanotubes were synthesized by chemical vapor deposition from pure toluene and toluene/diazine mixtures using ferrocene as a catalyst precursor at 760 degrees C. As recently announced, characterization of the resulting nanotube films showed that, unlike pure carbon nanotubes, those grown in the presence of nitrogen have an extremely high degree of internal order, both in terms of the uniform chirality in the nanotube walls and of the crystallographic register between them. Here, the structure, defects, and morphology of the nanotubes were analyzed in depth using advanced electron microscopy techniques, and compared with existing models and observations. Nitrogen, which seems to be responsible for the dramatic structural order, was found to segregate preferentially within the core of the nanotubes.

Layered Double Hydroxide Minerals as Possible Prebiotic Information Storage and Transfer Compounds

One of the fundamental difficulties when considering the origin of life on Earth is the identification of an emergent system that not only replicated, but also had the capacity to undergo discrete mutation in such a way that following generations might inherit and pass on the mutation. We speculate that the layered double hydroxide (LDH) minerals are plausible candidates for a proto-RNA molecule. We describe a hypothetical LDH-like system which, when intercalated with certain anions, forms crystals with a high degree of internal order giving rise to novel information storage structures in which replication fidelity is maintained, a concept we use to propose an explanation for interstratification in terephthalate LDHs. The external surfaces of these hypothetical crystals provide active sites whose structure and chemistry is dictated by the internal information content of the LDH. Depending on the LDH polytype, the opposing external surfaces of a crystal may give rise to reactive sites that are either complementary or mirror images of each other, and so may be chiral. We also examine similarities between these proposed "proto-RNA" structures and the DNA that encodes the hereditary information in life today, concluding with a hypothetical scenario wherein these proto-RNA molecules predated the putative RNA-world.

Ascus and ascospore development in Eleutherascus peruvianus. 2. Nuclear cytology and early ascus ontogeny

A study was undertaken of the cytological events leading to ascus initiation through delimitation of the ascospores prior to spore wall deposition in Eleutherascus peruvianus Huang. As revealed by Giemsa staining, nuclei in asci which are derived from croziers in the classical manner undergo the three divisions, with each division occurring in a different plane. The result is a globose ascus with eight nuclei displaying a high degree of internal order with respect to the arrangement of the ascospores. Electron microscopy reveals that subsequent to the final nuclear division, the ascus becomes filled with several elongate segments of double membrane underlying the ascus plasmalemma. These membrane segments appear to result from marked focal invaginations of the ascus plasmalemma and ultimately fuse together to form the ascus vesicle. Occasional observations of nuclei outside of the ascus vesicle membranes are believed to account for the often observed phenomenon of asci with odd numbers of ascospores. The significance of these findings relative to those for some close allies is discussed.

Structure, Conformation, and Mechanism in the Formation of Polysaccharide Gels and Networks

It is noted that the gel is the most typical state for polysaccharides, both in biological and artificial systems. The polymer chains usually form an interconnected network that gives rise to characteristic texture and properties. Polysaccharide gels have various biological functions essentially in the wall of the young plant-cell, in animal fluids and connective tissues, and in the bacterial capsule. They also have widespread commercial uses, particularly in foodstuffs, cosmetics, paper, and textiles. The gel state is considered characteristic of polysaccharides in the way that polypeptides and globular proteins can characteristically occur as compact particles having a high degree of internal order, or nucleic acids can occur as chain pairs in highly ordered and complementary association. Polysaccharides also make a distinctive contribution to natural-polymer chemistry in showing, in terms of molecular structure, interaction of chain molecules in three-dimensions to provide physical and biological properties of gels.

An electron microscope study of the growth and structure of spherulites in polyethylene

AbstractA surface‐etching technique has been devised which reveals the spherulites in molded Polyethylene for study with the electron microscope. By means of this technique it is possible to observe the effect of various heating cycles and polymer structures on the size and perfection of the spherulites. The etching technique shows that two different types of large spherulites may be formed in polyethylene. One, which may be formed on molding, contains a relatively low amount of internal order. The other, which forms on long heat aging or annealing, contains a high degree of internal order. Heat aging at temperatures well below the melting point will induce the poorly ordered type of spherulite to break up and regrow in the highly ordered form. Spherulite structure was observed to depend not only on heat treatment, but also on chain branching and molecular weight. Increased linearity induces the formation of a more regular, sheetlike or laminated structure within the spherulites, while lower molecular weight causes the lamellae and spherulites as well to be larger in size. Extremely high molecular weight linear resins showed no spherulitic structure as such, but were fibrillar in appearance. Annealing or long heat aging of these resins induced the formation of a row type aggregate structure rather than the typical spherulites observed in normal molecular weight resins. Electron microscope studies of stress‐cracked surfaces indicate that a high degree of internal spherulitic order such as is formed on long heat aging may well be the primary cause of stress‐cracking in polyethylene. This is further indicated by resins such as the ethylene/propylene copolymers, in which highly ordered spherulites do not form with heat aging. These resins are very resistant to stress cracking. Structural studies show that the ethylene/propylene copolymers are very nonrandom, and it is believed that this interferes with the formation of perfect, and consequently crack‐prone, spherulites.