Reversible helix sense inversion in surface-grafted poly(beta-phenethyl-L-aspartate) films

The reversible manipulation of the helix screw sense in surface-grafted poly(beta-phenethyl-L-aspartate) (PPELA) films by means of external stimuli was investigated. Ringopening polymerization of beta-phenethyl-L-aspartate N-carboxyanhydride initiated from primary amino-functionalized silicon and quartz substrates results in surface-grafted PPELA films in which the end-grafted polypeptide chains have a right-handed alpha-helical conformation. Upon annealing of the film at 150 degrees C for 30 min, a helix screw sense inversion takes place and the grafted chains adopt a left-handed pi-helical conformation. In the solid state, this left-handed pi-helical form is completely stable and cannot be changed by reheating and/or cooling. Upon immersion of the annealed grafted film in chloroform or other helicogenic solvents, the grafted polypeptide chains completely revert to their original right-handed alpha-helical form. Successive annealing and solvent treatment steps show that this helix sense inversion cycle can be repeated many times.     

Self‐Assembly of a Low‐Symmetry Monodendron Containing Two Asymmetrically Linked Molecular Wedges

Herein, we describe the synthesis of a low‐symmetry monodendron, 3,4‐bis(dodecyloxy)‐5‐[3,4,5‐tris(dodecyloxy)benzyloxy]benzoic acid, following a simple route which starts from gallic acid ethyl ester and does not require any protecting groups. The self‐assembled structures formed by the compound in 3D and 2D were investigated by synchrotron X‐ray scattering and scanning force microscopy (SFM). In 3D, the compound forms a stable crystalline phase with an orthorhombic lattice in which the alkyl chains connected to different benzene rings form crystalline and amorphous domains. Upon cooling from the isotropic melt the compound exhibits a monotropic smectic mesophase. In 100‐nm‐thick films on a neutral substrate the structure loses its biaxiality, adopting a hexagonal columnar structure with the columns oriented parallel to the substrate. By contrast, in ultrathin films on graphite the SFM likely reveals two crystal orientations, which can develop due to the epitaxial adsorption on the substrate of the alkyl chains pertinent to different benzene rings.     

Effect of chloroform on the structure and gas-separation properties of poly(ether imides)

The FTIR spectra of poly(ether imide) films prepared from their chloroform solutions were recorded in a wide temperature interval. The cast films were shown to contain residual solvent. This residual solvent existed in films as unbound chloroform that may be removed by heating to 60–70°C and as bound chloroform that is involved in complex formation with polymers and may be removed by heating at temperatures close to their glass transition temperatures (180°C). Quantum-chemical calculations were performed for structures that model fragments and monomer units of poly(ether imides), as well as their complexes with chloroform. Chloroform was shown to be capable of preferential binding with an oxygen atom in a Ph-O-Ph′ fragment via hydrogen bonds. In this case, the conformational set of poly(ether imide) chains is changed. The above evidence is invoked to explain changes in transport characteristics with time for poly(ether imide) films cast from chloroform solutions.     

The effect of casting solvent on the material properties of poly(γ-methyl-D-glutamate)

The effect of casting solvent on the material properties of poly(γ-methyl-D -glutamate), PMDG, was investigated. The specific solvents used were chloroform, trifluoroacetic acid, dichloroacetic acid, methylene chloride, hexafluoroisopropanol, and tetrachloroethane. The different nature of these solvents controlled the degree of α, β, or random coil contents of the final film. The effect of the morphology on material properties induced by the respective solvents was investigated by dynamic mechanical measurements of the dry films, stress strain behavior of both wet and dry films as well as by wide-angle x-ray diffraction, small-angle light scattering, and optical microscopy. Infrared spectroscopy was used to help determine α or β content. It was found that the casting solvent has considerable influence on material behavior and morphology. These differences are reflected in both the dynamical mechanical (small strain) and stress–strain (large strain) measurements as well as the x-ray scattering and optical microscopy. It was noted by light scattering that all films gave rise to anisotropic rod scattering with the exception of the β film cast from trifluoroacetic acid. This latter film appeared to be optically isotropic.     

Stability and phase transformations of the mesomorphic form of isotactic polypropylene in stereodefective polypropylene

A study of the thermodynamic stability and the related polymorphic transformations induced by thermal treatments of the mesomorphic form that crystallizes in stereodefective metallocene isotactic polypropylene (iPP) is presented. We show that the mesomorphic form of the more isotactic samples is stable at room temperature, whereas the mesomorphic form crystallizing in the more stereoirregular sample is unstable and crystallizes at room temperature in the crystalline α form. In any case, the mesomorphic form transforms during heating or by annealing at temperatures higher than 60–80 °C always in the α form, regardless of the stereoregularity, even in the case of stereoirregular samples generally crystallizing from the melt in the γ form. These data confirm the proposed model of structure of the mesomorphic form as small aggregates of chains in three-fold helical conformation packed with lateral correlations similar to the α form of iPP.     

High-resolution NMR spectra and phase equilibria in poly(phenylacetylene)-diluent mixtures

The C-13 NMR spectra of partly crystalline poly(phenylacetylene) (PPA) in CDCl₃ CCl₄ are rather well resolved and the peaks can be matched with those of 1, 3,5-triphenylbenzene. A different, less-well-resolved C-13 spectrum is characteristic of a disordered PPA obtained by heating. We conclude that crystalline PPA has the chain conformation of a cis-cis-oid helix. This interpretation is consistent with the proton NMR spectra and is supported by the fluorescence spectra, which can display two bands, one concluded to be characteristic of the cis-cis-oid conformation, the other of chain conjugation in the disordered polymer. Phase equilibria of PPA in the presence of chloroform were determined and are represented as those of the quasiternary mixture cis-cis-oid helix, disordered polymer, and chloroform.     

Conformational transitions of poly(β-benzyl L-aspartate)

Conditions that affect the conformation of poly(β-benzyl L-aspartate) in films have been studied. A slow transition from the left-handed to the right-handed α-helical form is observed under certain conditions in half-dried film cast from chloroform solutions and after exposure of amorphous films of the left-handed β-helical form to chloroform vapor. This transition is caused by adsorption of moisture. Thermal transitions of these films were examined by differential scanning calorimetry and infrared spectroscopy. The right-handed α-helices change to left-handed ω-helices around 140°C, and sharp exothermic peak is observed in this region for some films.     

A deuterium NMR investigation of polymorphism in benzene pizzanes

The polymorphism of three members of the homologous series hexa(p-alkoxyphenoxymethyl) benzene (benzene pizzanes) with 5, 6 and 7 carbons in the alkoxy chains and several of their deuteriated isotopomers have been investigated by differential scanning calorimetry, polarizing optical microscopy, X-ray diffraction and deuterium NMR spectroscopy. These homologues exhibit several solid phases and a high temperature M phase, which is isomorphic in the three homologues and whose nature is discussed. In the solid phases, the benzene cores of the molecules remain rigid, but the side chains are mobile, as reflected by rapid pi-flips of the benzene rings in the side chains. It is found that there are two types of such benzene rings, differing in the rates of flips. In the M phase the molecules undergo fast overall reorientation and the side chains are even more disordered than in the solid phases. However the X-ray measurements do not provide a clear cut determination as to whether this phase is crystalline or mesomorphic. Mixing of the benzene pizzanes with p-xylene yields lyomesophases, which appear to belong to the Dho class.     

Conformations of branched poly(α-amino acids) in solution and at the air—water interface

The validity of Silberberg's hypothesis concerning the conformation of branched polypeptides was tested by studies on solutions and on monolayers. The poly(α-amino acids) investigated consisted of poly-L -lysine backbones with side chains of poly(gamma;-benzyl L -gutamate) and poly(β-benzyl L -aspartate). In solution, the conformation was studied in the mixed solvent chloroform–dichloroacetic acid. Optical rotatory dispersion measurements demonstrated the existence of a helix–coil transition which is a function of the length of the side chain. A significant “transition curve” appeared between 9 and 15 residues. From the results, it may be assumed that the poly-L -lysine backbone is extended in a coiled conformation and the side chains are coiled or helical, depending on the number of glutamate residues. At the air–water interface, a discontinuity in the surface-pressure curve appears between 9 and 15 residues per side chain. The similarity with the results obtained in solution appears significant; however, this has to be confirmed with methods more specific for structural modifications.     

Computer simulation and structure analysis of polypeptide derivatives of cyclotriphosphazene

A quantum-chemical analysis and experimental investigation of the structure of poly(γ-methyl L-α-glutamate) and poly(γ-benzyl L-α-glutamate) linked with the triphosphazene ring via aminophenoxy groups were performed. The computer simulation showed that these compounds with a total degree of polymerization of amino acid sequences of n ≤ 60 are close to the rigid-rod conformation in struture, whereas the rod is distorted to the arch architecture at n > 60. According to X-ray data, the α helices of polypeptide segments are arranged into two-dimensional hexagonal packing. By means of the DSC technique, the glass transition associated with the freezing-out of the mobility of side chains was revealed, transitions to the mesophase were determined, and it was established that the helix type experiences a local change characteristic of poly(γ-benzyl glutamate) derivatives of cyclotriphosphazene in the solid state.     

Structure of a poly(2,5‐benzimidazole)/phosphoric acid complex

As‐cast films of poly(2,5‐benzimidazole) exhibit uniplanar orientation in which the planes of the aromatic rings lie parallel to the film surface. Upon doping with phosphoric acid, the original crystalline order is lost, but the doped film can be stretched to produce films with uniaxial orientation. After thermal annealing at 540 °C, nine Bragg reflections are resolved in the fiber diagram, and these are indexed by an orthorhombic unit cell with the dimensions a = 18.1 Å, b = 3.5 Å, and c = 11.4 Å, containing four monomer units of two chains. The absence of odd‐order 00l reflections points to a 2₁ chain conformation, which is probably planar so that the aromatic units can be stacked along the b axis. The water and phosphoric acid contents of the crystalline structure cannot be determined exactly because of the presence of extensive amorphous regions that probably have different solvation. The best agreement between the observed and calculated intensities is for an idealized structure containing two phosphoric acids and two water molecules per unit cell. However, the phosphoric acid is probably present mainly in the form of pyrophosphoric acid and its higher oligomers. In addition, the X‐ray data are consistent with a more disordered structure containing chains with random (up and down) polarity and a lack of c‐axis registry. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2576–2585, 2004     

alpha and piDL helical states of alternating poly(gamma-benzyl D-L-glutamate) in solution.

As in solid state, strictly alternating poly(gamma-benzyl D-L-glutamate) in solution can adopt two different helical conformations. Besides the alpha helix, a second helical conformation is found at higher temperatures in dioxane and chloroform, the properties of which correspond to that of the piDL4 helix. As the molecules have a finite length a screw sense is favored for both helical forms thus giving rise to optical activity allowing the study of the transconformation by optical rotatory dispersion and circular dichroism besides infrared and dielectric measurements. Thus, as the temperature is raised the equilibria right-left handed alpha helices and alpha-piDL helical forms can be followed. The favored screw senses are determined by the number of interacting side chains for the alpha helix and by the number of hydrogen bonds which are formed in the piDL helical conformation. The side chain-side chain interactions in the alpha helix are experimentally shown to be attractive.     

Heat capacity and vibrational dynamics of α poly(β-benzyl-L-aspartate)

Poly(β-benzyl-L-aspartate) (PBLA) is an unusual polypeptide, which is capable of going into four different conformations, namely, left-handed α helix, right-handed α helix, ω helix, and β pleated sheet. The present work is a complete study of normal modes and their dispersion in the unusual left-handed α form. A special feature of some of the dispersion curves is their tendency to bunch in the neighborhood of helix angle. This is attributed to the presence of strong intramolecular interactions. Crossing and repulsion between the dispersion curves is also observed. The N-deuterated analogue of PBLA has been studied to check the validity of assignments and force field (Urey Bradley). Specific heat has been obtained from dispersion curves via density of states. A comparative study of left-handed and right-handed forms is presented. © 1996 John Wiley & Sons, Inc.     

Extra meridional reflections of drawn polyoxymethylene

Extra meridional reflections (00l: l = 3, 5, 13, 15, 21) were observed in drawn polyoxymethylene (POM). The 003, 0015, and 0021 x-ray diffraction intensities increase on annealing, while the 005 and 0013 intensities decrease. It is concluded that crystalline regions are of two kinds: in the first, which generates the 003, 0015, and 0021 reflections, the molecular conformation is slightly distorted from the uniform helix by intermolecular steric hindrance between the CH₂ groups; and in the second, which generates 005 and 0013 reflections, the molecular conformation is distorted in another way. The second form is converted into the first by annealing. The displacements in the c direction of the CH₂ groups and the oxygen atoms from the uniform helix in the first form are about 1.0% and 0.75% of the interval between a CH₂ group and an oxygen atom along helix axis of the POM crystal; thus the correct conformation in the stable state is known. The nature of distortion in the second region is not clarified but several characteristics are offered.     

Poly(amino acid) derivatives of cyclotriphosphazene with luminescent fragments

Poly(amino acid)s with a central triphosphazene cycle and a peripheral pyrene moiety were synthesized from hexakis(4-aminophenoxy)cyclotriphosphazene and 1-pyrenebutanoic acid and have been characterized. The introduction of the luminophore resulted in the formation of highly ordered structures containing fragments with three α helices in the trans position to the phosphazene cycle. The character of luminescence spectra suggests the presence of pyrene dimers; their appearance may be associated with the intermolecular aggregation of peptide chains with chromophore end groups. The DSC data revealed the glass transition due to the freezing out of the mobility of side chains, as well as an alteration in the helix type of the poly(amino acid) fragments.     

Spontaneous birefringence in films of some phenyl-containing polymers

Spontaneous birefringence in films prepared by free evaporation of a solvent from solutions of sulfated phenyl-containing polymers has been investigated in relation to the thickness of films and the incidence angle of a polarized light beam. It has been demonstrated that not only the value but also the sign of the coefficient of surface birefringence depends on the mode of incorporation of benzene rings into a polymer chain. The structural and orientation factors determining the spontaneous birefringence in the films have been distinguished. The results are compared with the data available for the films of water-soluble polysaccharides. The degree of order in the near-surface layers of polymer films cast from solutions is determined by the statistical segment length of macromolecules and is independent of the chemical nature of chains in the number of water-soluble polymers under consideration.     

Structure and phase transitions in poly[bis-(2,2,3,3-tetrafluoropropoxy) phosphazene]

The structure and phase transitions in poly[bis-(2,2,3,3-tetrafluoropropoxy)phosphazene] have been studied by differential scanning calorimetry (DSC) and x-ray diffraction. Two crystalline phases and one mesomorphic phase are found, denoted I, II, and III, respectively. These phases convert reversibly one into the other on heating and cooling. The Phase I–Phase II transition occurs in a temperature range from 5 to 30°C whereas the Phase II mesophase (Phase III) transition proceeds above 80°C. Heats of transitions are measured to be about 29.0 J/g and 3.6 J/g, respectively. Crystalline Phase I is characterized by a monoclinic unit cell with the parameters: α = 24.4 Å, b = 9.96 Å, c = 4.96 Å, γ = 123°. The axes of both chains, traversing the unit cell, are directed along the “c” axis, the main chains having cis-trans conformation. Phase I is the common crystalline structure for the main chain and side chains. The structure of Phase II is controlled mainly by packing of the side chains. Transition of Phase II into mesomorphic Phase III is accompanied with distortion of packing of the side chains. Only regular packing of the main chains of macromolecules in the plane perpendicular to their axes exists in Phase III. Mesomorphic phase III is stable up to the degradation temperature of the polymer. A significant effect of stress on the Phase II–III transition in oriented samples was found.     

Pi-helix preference in unsolvated peptides

Ion mobility measurements have been used to examine helix formations in the gas phase for a series of alanine/glycine-based peptides that incorporate a glutamic acid (E) and lysine (K) at various positions along the backbone. Incorporation of an EK pair lowers the percent helix for all positions (presumably because hydrogen bonding between the backbone and the E and K side chains stabilize the nonhelical globular conformations). The largest percent helix is found when the EK pair is in an i,i+5 arrangement, which suggests that the preferred helical conformation for these peptides is a pi-helix. This conclusion is supported by comparison of cross sections deduced from the ion-mobility measurements to average cross sections calculated for conformations obtained from molecular dynamics simulations. The glutamic acid and lysine may form an ion pair that is stabilized by interactions with the helix macro-dipole.     

Polymer‐Assisted Solution‐Phase Synthesis and Neurite‐Outgrowth‐Promoting Activity of 15‐Deoxy‐Δ12,14‐PGJ2 Derivatives

An efficient solution‐phase synthesis of rac‐15‐deoxy‐Δ^12,14‐PGJ₂ (15dPGJ₂) derivatives that contain variable α and ω chains based on a polymer‐assisted strategy and their neurite‐outgrowth‐promoting activity are described. The strategy for the synthesis of PGJ₂ derivatives involves the use of a vinyl iodide bearing cyclopentenone as a key intermediate, which undergoes Suzuki–Miyaura coupling and subsequent Lewis acid catalyzed aldol condensation for incorporation of the ω and α chains, respectively. For easy access to the PGJ₂ derivatives, a polymer‐supported catalyst and scavengers were adapted for use in these four diverse steps, in which workup and purification can be performed by simple filtration of the solid‐supported reagents. By using this methodology, we succeeded in the synthesis of 16 PGJ₂ derivatives with four alkyl boranes and four aldehydes. The neurite‐outgrowth‐promoting activity of the 16 synthetic compounds in PC12 cells revealed that the side‐chains play a major role in modulating their biological activity. The carboxylic acid on the α chain improved the biological activity, although it was not absolutely required. Furthermore, a PGJ₂ derivative with a phenyl moiety on the ω chain was found to exhibit an activity comparable to that of natural 15dPGJ₂.     

Solid-state photopolymerization of α,α′-bis(4-acetoxy-3-methoxybenzylidene)-p-benzenediacetonitrile

One crystalline form of α,α′-bis(4-acetoxy-3-methoxybenzylidene)-p-benzenediacetonitrile is photopolymerized by a four-center reaction to give a polymer containing alternating benzene and cyclobutane rings. The polymer is highly crystalline and insoluble in most solvents. Location of groups around the cyclobutane ring is deduced from the products obtained on thermal decomposition. Chain contained benzene rings are located in positions 1 and 3; aromatic groups alternate sides of the cyclobutane ring. Special characterization and quantum yield of the polymerization are reported, as well as polymer solution viscosity and alkaline depolymerization of the polymer.