Helical superstructures from charged Poly(styrene)-Poly(isocyanodipeptide) block copolymers

Self-Assembly of ABA Amphiphilic Triblock Copolymers into Vesicles in Dilute Solution

Self-assembly of an ABA amphiphilic triblock copolymer into vesicles in dilute solution was studied by successfully combining experimental methods and a real-space self-consistent field theory in three-dimensional space. It was found experimentally that vesicle size was sensitive to the initial copolymer concentration in the organic solvent. Also, the aggregate morphologies and vesicles sizes were found to be dependent on the annealing time. A number of complex vesicles, such as global, long-style, trigonal, and necklacelike vesicles, were obtained in our experiments. Moreover, the corresponding microstructures were produced in our simulations. The results show that various vesicles in dilute solution are formed solely on account of the inhomogeneous density distribution in the local region in nature. Our simulations confirm that the structural complexity coexisting behavior in the single-amphiphile systems is largely attributed to the metastability rather than the polydispersity of the triblock copolymer. These metastable states should strongly depend on the pathway of the system on the free energy landscapes, which is governed by the initial condition.

Self-assembly in mesoscopic dimension and artificial supramolecular membranes

The development of mesoscopic supramolecular architectures is an area of growing interest. The field has grown from early works on bilayer membranes, and the design of large (super- or giant-) amphiphiles, hybrid amphiphiles and supramolecular membranes have now been described. Impartment of amphiphilicity to a unit supermolecule allows their hierarchical self-assembly to the mesoscopic structures. A supramolecular combinatorial approach is useful in the development of functional self-assemblies. In addition, self-assembly in ionic liquids has been introduced as a promising area in materials chemistry.

Soluble Oligoaramide Precursors?A Novel Class of Building Blocks for Rod-Coil Architectures

A new synthetic route is described that allows the reversible conversion of the inherently insoluble oligo-p-benzamides into soluble materials through the formation of imidoyl chlorides. Syntheses of the corresponding dimer, trimer, and tetramer are reported; these compounds can easily be purified by crystallization and are accessible on the multigram scale. Structural proof was obtained by single-crystal X-ray structures of the trimer and tetramer precursors. They can be selectively functionalized into amides or esters at the terminal carboxylic acid group followed by hydrolysis of the imidoyl chlorides to the parent amides. This new class of compounds gives access to strongly aggregating rigid rodlike materials in few synthetic steps, as is demonstrated by the preparation of poly(ethylene glycol)-co-oligo(p-benzamide) rod-coil block copolymers.

?-Helical Polypeptide Microcapsules Formed by Emulsion-Templated Self-Assembly

alpha-Helical peptide microcapsules were prepared by the emulsion-templated self-assembly of amphiphilic poly(gamma-benzyl L-glutamate)s (PBLG) 1. By mixing solutions of 1 in dichloromethane (in the form of a sodium salt) with water, oil-in-water emulsions were obtained. Spontaneous stripping of the dichloromethane phase caused a decrease in the diameter of the microdroplets and finally stable microcapsules formed. The microcapsules contain an inner aqueous phase as observed by confocal laser scanning microscopy (CLSM). Binding of hydrophobic pyrene molecules to the polypeptide shell was also demonstrated. The present polypeptide microcapsules are stable even after drying in air and they would serve as supramolecular vehicles for both hydrophobic and water-soluble molecules.

Solid phase synthesis of biohybrid block copolymers

A new versatile route to synthesise biohybrid block copolymers is presented in which an amine terminated polymer is attached to an aldehyde functionalised resin, from which in subsequent steps the desired peptide can be grown using standard procedures.

Helical Poly(3-methyl-4-vinylpyridine)/Amino Acid Complexes: Preparation, Characterization, and Biocompatibility

Helical poly(3-methyl-4-vinylpyridine) (P3M4VP)/amino acid complexes have been prepared via acid-base reaction of the achiral polymer with D and L amino acids: alanine, leucine, valine, serine and phenylalanine. The circular dichroism (CD) spectra of P3M4VP/D- and L-alanine complexes in CH(3)OH/H(2)O show opposing (near mirror image) Cotton effect signals at 278.4, 274.8 and 270.8 nm, indicating the formation of enantiomeric secondary structures. The formation of the enantiomeric structures is supported by observed [alpha](D)(25) values of -3.0 and +3.0 for the P3M4VP/D-alanine and P3M4VP/L-alanine complexes, respectively. The preparation of helical P3M4VP/amino acid complexes has been carried out in CH(3)OH and H(2)O at pH 1.8 and 2.7. The intensities of the Cotton effect signals were good. For example, for the P3M4VP/L-alanine complexes in CH(3)OH/H(2)O and H(2)O (pH 1.8), the second Cotton effect signal around 275-277 nm show [theta;] values of 49 980 and 79 210 deg . cm(2) . dmol(-1), respectively. The formation of the helical secondary structure is rapid. The acid-base reaction between P3M4VP and L-alanine in CH(3)OH/H(2)O, in 10 min, show a CD spectrum with Cotton effect signals at 274 and 272 nm with [theta] values of 27,000 deg . cm(2) . dmol(-1) and -36,000 deg . cm(2) . dmol(-1), respectively. P3M4VP permits ready conformational reorientation on complexation with amino acids, but once the helical P3M4VP/amino acid complexes are formed, it is stable at room temperature. P3M4VP is not compatible with HeLa ovarian cancer cells, but the helical P3M4VP/amino acid complexes are compatible with HeLa cells. The complexes minimally interfere with the adhesion and growth of HeLa cells on complex surfaces. Helical poly(3-methyl-4-vinylpyridine)/D- and L-alanine complexes support the attachment and growth of HeLa cells. The micrographs shows HeLa cells after three days: left panel: on P3M4VP/L-alanine complex; right panel: on P3M4VP/D-alanine complex.

Toroidal Triblock Copolymer Assemblies

A stable phase of toroidal, or ringlike, supramolecular assemblies was formed by combining dilute solution characteristics critical for both bundling of like-charged biopolymers and block copolymer micelle formation. The key to toroid versus classic cylinder micelle formation is the interaction of the negatively charged hydrophilic block of an amphiphilic triblock copolymer with a positively charged divalent organic counterion. This produces a self-attraction of cylindrical micelles that leads to toroid formation, a mechanism akin to the toroidal bundling of semiflexible charged biopolymers such as DNA. The toroids can be kinetically trapped or chemically cross-linked. Insight into the mechanism of toroid formation can be gained by observation of intermediate structures kinetically trapped during film casting.

Self-assembly and properties of diblock copolymers by coarse-grain molecular dynamics

Block-copolymer amphiphiles have been observed to assemble into vesicles and other morphologies long known for lipids but with remarkably different properties. Coarse-grain molecular dynamics (CG-MD) is used herein to elaborate the structures and properties of diblock copolymer assemblies in water. By varying the hydrophilic/hydrophobic ratio of the copolymer in line with experiment, bilayer, cylindrical and spherical micelle morphologies spontaneously assemble. Varying the molecular weight (MW) with hydrophilic/hydrophobic ratio appropriate to a bilayer yields a hydrophobic core thickness that scales for large MW as a random coil polymer, in agreement with experiment. The extent of hydrophobic-segment overlap in the core increases nonlinearly with MW, indicative of chain entanglements and consistent with the dramatic decrease reported for lateral mobility in polymer vesicles. Calculated trends with MW as well as hydrophilic/hydrophobic ratio thus agree with experiment, demonstrating that CG-MD simulations provide a rational design tool for diblock copolymer assemblies.

Stimuli-responsive polypeptide vesicles by conformation-specific assembly

In biology, lipids are well known for their ability to assemble into spherical vesicles. Proteins, in particular virus capsids, can also form regular vesicle-like structures, where the precise folding and stable conformations of many identical subunits directs their self-assembly. Functionality present on these subunits also controls their disassembly within the cellular environment, for example, in response to a pH change. Here, we report the preparation of diblock copolypeptides that self-assemble into spherical vesicular assemblies whose size and structure are dictated primarily by the ordered conformations of the polymer segments, in a manner similar to viral capsid assembly. Furthermore, functionality was incorporated into these molecules to render them susceptible to environmental stimuli, which is desirable for drug-delivery applications. The control of assembly and function exhibited in these systems is a significant advance towards the synthesis of materials that can mimic the precise three-dimensional assembly found in proteins.

Oxidation-responsive polymeric vesicles

Vesicles formed in water by synthetic macro-amphiphiles have attracted much attention as nanocontainers having properties that extend the physical and chemical limits of liposomes. We sought to develop ABA block copolymeric amphiphiles that self-assemble into unilamellar vesicles that can be further oxidatively destabilized. We selected poly(ethylene glycol) (PEG) as the hydrophilic A blocks, owing to its resistance to protein adsorption and low toxicity. As hydrophobic B blocks, we selected poly(propylene sulphide) (PPS), owing to its extreme hydrophobicity, its low glass-transition temperature, and most importantly its oxidative conversion from a hydrophobe to a hydrophile, poly(propylene sulphoxide) and ultimately poly(propylene sulphone). This is the first example of the use of oxidative conversions to destabilize such carriers. This new class of oxidation-responsive polymeric vesicles may find applications as nanocontainers in drug delivery, biosensing and biodetection.

Morphology of microphase separated domains in rod–coil copolymer melts

We investigate the morphology of microphase separated domains in diblock copolymers where each chain consists of a stiff rod block and a flexible coil block. A simplified phenomenological model system is introduced, which is coarse-grained in terms of the local concentration difference between the two blocks and the local director field of the rod part. Computer simulations of this set of time-evolution equations in two dimensions show in the weak segregation regime that the elastic energy in the rod-block rich domains affects drastically the structures of microphase separated domains. A coil-to-rod transition is incorporated into the model system to examine the elastic and anisotropic effects. The effects of the external electric field are also investigated to control the domain morphology.

Silica-supported biomimetic membranes

The hybridization of lipid membranes with inorganic silica-based framework results in mechanically stable biomembrane mimics. This account describes three types of silica-based biomimetic membranes. As the first example, a Langmuir monolayer of dialkylalkoxysilane was polymerized and immobilized onto a porous glass plate. Permeability through the monolayer-immobilized glass was regulated by phase transition of the immobilized monolayer. In the second example, spherical vesicles covalently attached to a silica cover layer (Cerasome) were prepared. The Cerasome was stable enough to be assembled into layer-by-layer films without destruction of its vesicular structure. This material could be an example of the multicellular assembly. Mesoporous silica films densely filling peptide assemblies (Proteosilica) are introduced as the third example. The Proteosilica was synthesized as a transparent film through template sol-gel reaction using amphiphilic peptides.

Image analysis of tendon helical superstructure using interference and polarized light microscopy

Wave-like structures (WLS also known as crimp) have generally been reported to be planar structures. However, there is evidence that a helical superstructure, rather than a planar one, should be considered. Conditions dictated by supramolecular chemistry, molecular recognition and self-assembly favor the idea of a helical arrangement for collagen bundles in a supramolecular structure. The aim of this work is to provide additional data in support of a helical superstructure for collagen bundles in tendons. Cryosections of fixed flexor bovine tendons and sections of resin-embedded peeled rat tail were studied using polarized light, interference, and phase contrast microscopy. Image analysis was used to find appropriate mathematical descriptors for WLS. Interference colors due to the dispersion of birefringence allowed the detection of a gradual, intertwined twisted fiber organization in WLS, as the angle of the tendon axis was rotated relative to the polarizers. Helical movements of the images of the WLS bands were produced using animation methods. Interference microscopy revealed interference colors associated with different orientations and dry mass concentrations in the fibers, especially in tendon cross-sections, which also exhibited Maltese-cross birefringence images. Similar images were detected by interference microscopy, suggesting a spiral organization of fibers in the section plane. The helical orientation of the fibers was detected by focusing through different planes of sections. Based on a comparison of this superstructure with mesophases, the twisted grain boundary concept is considered to be the most appropriate for the classification of tendon WLS.

Synthesis of Poly(ε-caprolactone)-b-Poly(γ-benzyl-l-glutamic acid) Block Copolymer Using Amino Organic Calcium Catalyst

A biodegradable two block copolymer, poly(epsilon-caprolactone)-b- poly(gamma-benzyl-L-glutamic acid) (PCL-PBLG) was synthesized successfully by ring-opening polymerization of N-carboxyanhydride of gamma-benzyl-L-glutamate (BLG-NCA) with aminophenyl-terminated PCL as a macroinitiator. The aminophenethoxyl-terminated PCL was prepared via hydrogenation of a 4-nitrophenethoxyl-terminated PCL, which was novelly obtained from the polymerization of epsilon-caprolactone (CL) initiated by amino calcium 4-nitrobenzoxide. The structures of the block copolymer and its precursors from the initial step of PCL were confirmed and investigated by 1H NMR, FT-IR, GPC, and FT-ICRMS analyses and DSC measurements.

Nanotubes from the self-assembly of asymmetric crystalline-coil poly(ferrocenylsilane-siloxane) block copolymers

Block copolymers with a high asymmetry normally give spherical starlike micelles in a solvent selective for the longer block. We have discovered that samples of poly(ferrocenyldimethylsilane-b-dimethylsiloxane) (PFS-b-PDMS) with block ratios of 1:12 form nanotubes in n-hexane and n-decane, which are poor solvents for PFS. Two block copolymer samples PFS(40)-b-PDMS(480) (M(n) = 45 300, PDI = 1.01) and PFS(80)-b-PDMS(960) (M(n) = 90 500, PDI = 1.01) were synthesized by sequential anionic polymerization. When self-assembly occurs, the PFS blocks aggregate and crystallize to form a shell with a cavity in the middle of the tube, while the PDMS blocks form the corona. The nature of these structures was elucidated by conventional transmission electron microscopy and dark-field scanning transmission electron microscopy. Time- and temperature-dependence studies revealed that a variety of morphologies are formed initially depending on the conditions of sample preparation, but most of them eventually rearrange to form nanotubules. The lengths of the tubes can be varied with time and with the choice of solvents. We have been able to grow nanotubes with lengths reaching 0.1 mm. The presence of the hollow core was confirmed by trapping tetrabutyllead in the cavity and performing energy-dispersive X-ray measurements on the resulting structure.

Polymer vesicles

Vesicles are microscopic sacs that enclose a volume with a molecularly thin membrane. The membranes are generally self-directed assemblies of amphiphilic molecules with a dual hydrophilic-hydrophobic character. Biological amphiphiles form vesicles central to cell function and are principally lipids of molecular weight less than 1 kilodalton. Block copolymers that mimic lipid amphiphilicity can also self-assemble into vesicles in dilute solution, but polymer molecular weights can be orders of magnitude greater than those of lipids. Structural features of vesicles, as well as properties including stability, fluidity, and intermembrane dynamics, are greatly influenced by characteristics of the polymers. Future applications of polymer vesicles will rely on exploiting unique property-performance relations, but results to date already underscore the fact that biologically derived vesicles are but a small subset of what is physically and chemically possible.

Spontaneous assembly of a polymeric helicate of sodium with LVO(2) units forming the strand: photoinduced transformation into a mixed-valence product

The anionic cis-dioxovanadium(V) complex species LVO(2)(-) of a tridentate ONS ligand (H(2)L) can bind sodium ion in a bis-monodentate fashion like a bridging carboxylate group. The product [LVO(2)Na(H(2)O)(2)](infinity) (1) is a water soluble polymeric compound in which the complementary units are held together by the simultaneous use of hydrogen bonding and Coulombic interactions. Crystallographic characterization reveals that 1 is a single stranded helicate with LVO(2)(-) units forming the strand which surrounds the labile sodium ions that occupy the positions on the axis. In solution of protic solvents, viz. water and methanol, 1 is quite stable as indicated by electrical conductivity and (1)H NMR measurements. In aprotic solvents, viz. CH(3)CN, DMF, or DMSO, however, the extended hydrogen bonded network in 1 breaks apart and the helical structure collapses when irradiated with visible light. The product is a mixed-oxidation vanadium(IV/V) species obtained by photoinduced reduction as confirmed by EPR, time dependent (1)H NMR, and electronic spectroscopy. Compound 1 is a rare example of a nonnatural helix where hydrogen bonding interactions play a crucial role in stabilizing the single stranded polymeric structure such as that frequently observed in the biological world.

Poly(ethylene glycol) block copolymers

The ubiquitous use of poly(ethylene glycol) in the biomaterials field has also boosted the research activity in the chemical derivatization of this polymer. We focused our interest on the preparation of tailor-made poly(ethylene glycol)-based structures and on the study of structure-activity relationships for its functionalization, as preliminary steps for the preparation of smart functional materials. More specifically, amphiphilic and cationic block copolymers were prepared for prospective use in the preparation of self-assembled carriers, and Michael-type addition of thiols onto acrylates was studied as a model for end-group reaction leading to hydrogel formation.