Side-chain and backbone ordering in a polypeptide

We report results from multicanonical simulations of polyglutamic acid chains of length of ten residues. For this simple polypeptide we observe a decoupling of backbone and side-chain ordering in the folding process. While the details of the two transitions vary between the peptide in gas phase and in an implicit solvent, our results indicate that, independent of the specific surroundings, upon continuously lowering the temperature side-chain ordering occurs only after the backbone topology is completely formed.     

Side-chain crystallinity. I. Heats of fusion and melting transitions on selected homopolymers having long side chains

Heats of fusion, melting transitions, and the derived entropies of fusion were obtained by differential scanning calorimetry for examples from three homologous series of homopolymers having long side chains. Homopolymers having side-chain lengths between 12 and 22 carbon atoms were chosen from the poly(n-alkyl acrylates), the poly(N-n-alkyl-acrylamides) and the poly(vinyl esters). The data demonstrated that only the outer paraffinic methylene groups were present in the crystal lattice. This was concluded because phase diagrams obtained for mixtures of structurally different monomers and homopolymers, as well as for selected copolymers, showed only isomorphism in the polymeric examples. In addition, scanning curves, reflecting the distribution of crystallite sizes, became narrower as the side chains became longer. The critical chain length required to maintain a stable nucleus in the bulk homopolymers was a constant value for each homologous series. It varied between 9 to 12 carbon atoms. When heats of fusion were determined in the presence of methanol, main-chain restraints were freed, thus permitting more methylene groups to enter the crystal lattice. Hence, the heats of fusion, the crystallinity, and melting points increased above that of the bulk state. The magnitude of the contribution to the heats of fusion by each methylene group indicated that the hexagonal paraffin crystal modification prevailed in these homopolymers, in agreement with x-ray data from the literature.     

Side-chain crystallinity. V. Heats of fusion and melting temperatures on monomers whose homopolymers have long side chains

Heats of fusion and melting temperatures were obtained for selected monomeric n-alkyl acrylates, N-n-alkylacrylamides, and vinyl esters. The corresponding thermodynamic parameters for homopolymers, derived from these monomers, had been reported previously from this laboratory. The α-hexagonal crystal modification was indicated near the melting point for the higher n-alkyl acrylates, but a β form was stable at low temperatures for the entire series. The magnitude of the heats of fusion indicated β polymorphs for vinyl esters in support of x-ray diffraction analysis from the literature. Because hexagonal crystal geometry prevailed in all reported homopolymers having long side chains, greater emphasis was placed on thermodynamic data for monomers exhibiting this crystal modification. Accordingly, a convergence temperature was estimated statistically for the α-hexagonal crystal modification of these systems and appropriate literature values of the n-alkanes and ethyl esters. The convergence temperature was computed to be 135°C, uncorrected for the entropy of disorientation. The anomalously large interfacial end-packing-defect energy of the poly(n-alkyl acrylates) and the poly-N-n-alkylacrylamides was shown to be associated with a high energy barrier to molecular transport in the melt as the vitreous state was approached. In support of this conclusion, similarity of the glass and melting transition temperatures of these homopolymer homologs occurred near their critical side-chain lengths, below which the homopolymers are amorphous. A special critical requirement of nucleus length was not indicated from rough estimations of nucleation parameters for the poly(n-alkyl acrylates). These findings lent increased, but still not unqualified, support to an x-ray diffraction study from the literature. The latter had specified the inclusion of the entire side chain and the main-chain units in the crystal lattices of the higher poly(n-alkyl acrylates).     

Invertible amphiphilic homopolymers

Stimuli-responsive polymers and assemblies are viable candidates for the so-called "smart" materials. In this communication, we report a new class of amphiphilic homopolymers that forms micelle-like structures in polar solvents and inverted micelle-like structures in apolar solvents. We demonstrate that these superstructures are the result of the changes in the molecular-level conformations in the monomer.     

Amphiphilic dendronized homopolymers

A series of second generation of amphiphilic dendronized homopolymers are efficiently synthesized, and their thermoresponsiveness in aqueous solutions and secondary structures in methanol solutions are described. These polymers are constructed in each repeat unit with various generations of hydrophobic 4-aminoproline and hydrophilic oligoethylene glycol (OEG)-based dendrons, and their over-all hydrophilicity is tuned by varying these dendron generations. Polymers with or without the first generation of proline dendron show good water solubility at room temperature, but exhibit typical thermoresponsive behaviors at elevated temperatures as characterized by turbidity measurements using UV-vis spectroscopy, while the polymer with the secondary generation of proline dendron is not soluble in water. All polymers show ordered secondary structures as evidenced by the optical rotation and circular dichroism experiments. Finally, assembly of these amphiphilic homopolymers into porous films via breath figure (BF) technique is described, and polymer structures are found to show significant influence on the morphology of porous film.     

Differential ordering of the protein backbone and side chains during protein folding revealed by site-specific recombinant infrared probes

The time scale for ordering of the polypeptide backbone relative to the side chains is a critical issue in protein folding. The interplay between ordering of the backbone and ordering of the side chains is particularly important for the formation of β-sheet structures, as the polypeptide chain searches for the native stabilizing cross-strand interactions. We have studied these issues in the N-terminal domain of protein L9 (NTL9), a model protein with mixed α/β structure. We have developed a general approach for introducing site-specific IR probes for the side chains (azide) and backbone ((13)C═(18)O) using recombinant protein expression. Temperature-jump time-resolved IR spectroscopy combined with site-specific labeling enables independent measurement of the respective backbone and side-chain dynamics with single residue resolution. We have found that side-chain ordering in a key region of the β-sheet structure occurs on a slower time scale than ordering of the backbone during the folding of NTL9, likely as a result of the transient formation of non-native side-chain interactions.     

Side-chain chlorination of methylquinolines

Methylquinolines 1–4 were chlorinated by heating with phosphorus pentachloride in chlorobenzenes to side-chain halogen derivatives 5–14 . Methyl groups of compounds 1–4 can be chlorinated to chloromethyl, dichloromethyl or trichloromethyl groups depending on their positions and the reaction conditions.     

Styrene-methyl acrylate copolymers and acrylate homopolymers in solution

Molecular weight determinations by light scattering and osmometry and intrinsic viscosity measurements were made in various solvents on fractions of styrene–methyl acrylate copolymers with different compositions and on acrylate homopolymers prepared by free-radical reaction. Relations between intrinsic viscosity [η] and molecular weight M thus established are compared with those reported by other authors. 2-Methylcyclohexanol was found to be a theta solvent for the copolymers and both parent homopolymers, and isoamyl acetate was a theta solvent for poly(methyl acrylate). From theta point viscosity data obtained with these solvents, unperturbed chain dimensions were estimated. The results are compared with the unperturbed dimensions estimated from the [η]–M relations obtained in good solvents. On the basis of the experimental data it was found that the unperturbed dimension depends linearly on the copolymer composition, in contrast to the case of styrene–methyl methacrylate copolymers. Composition dependences of the theta temperature and of the parameter describing the long-range interactions between nonadjacent segments in polymer chains were investigated. The result implies that long-range interactions between monomeric units never disappear even when those between the same monomeric units vanish. The Huggins constant for copolymer is discussed in terms of the excluded volume variable.     

Organogermane homopolymers and copolymers with organosilane

The first high molecular weight soluble, formable organogermane homopolymer (n-Bu₂Ge)_n was synthesized by the sodium coupling of n-Bu₂GeCl₂ in toluene. Soluble organogermane/organosilane copolymers [(X₂Ge)_x(YZSi)_y]_n were prepared by sodium coupling of X₂GeCl₂ and YZSiCl₂ in different molar ratios (X = n-bu, Ph; Y = n-hexyl, cyclohexyl; Z = Me). Germanium-containing polymers and copolymers with organosilanes are highly absorbing between 300–360 nm, with the absorption maxima dependent on the nature of the substituent and the ratio of X₂Ge:YZSi in the polymer. These polymers are photoactive and display bleaching behavior with photoscission.     

Side-chain effects in molecular electronic devices

We discuss the effect of an abundant structural element of molecules on the transmission probabilities of molecular electronic devices. We show that an attachment of side chains to a molecular conductor may lead to zero transmission probabilities. The gaps in the transmission-probability appear approximately at the eigenvalues of the isolated side chains, provided that the corresponding eigenstates are not localized away from the molecular conductor. Simple Hückel-type calculations serve to illustrate the described effect. Furthermore, we show that complex transmission-probability curves, obtained with Kohn-Sham density-functional theory, also exhibit the described side-chain effect.     

Shear induced mixing/demixing: blends of homopolymers,of homopolymers plus copolymers,and blends in solution

Shear may shift the phase boundary towards the homogeneous state (shear induced mixing, SIM), or in the opposite direction (shear induced demixing, SID). SIM is the typical behavior of mixtures of components of low molar mass and polymer solutions, SID can be observed with solutions of high molar mass polymers and polymer blends at higher shear rates. The typical sequence with increasing shear rate is SIM, then occurrence of an isolated additional immiscible area (SID), melting of this island into the main miscibility gap, and finally SIM again. A three phase line originates and ends in two critical end points. Raising pressure increases the shear effects. For copolymer containing systems SID is sometimes observed at very low shear rates, preceding the just mentioned sequence of shear influences.     

Side-chain induced chirality in diketopyrrolopyrrole based polymers

Chiral all-π-conjugated conducting polymers are of significant importance because of their promising aspects ranging from organic electronics, organic spin filters, efficient water splitting materials, magneto-optic materials to circularly polarized light emitters. Here, we report the synthesis and characterization of the first series of diketopyrrolopyrrole based chiral low band-gap alternating copolymers. Alongside characterizations by UV and CD spectroscopy, AFM and FESEM were further performed to understand the morphology of the optically active polymers. The stereochemistry of the side-chains on TDPP unit played a pivotal role in determining chiroptical properties and supramolecular self-assembly of the copolymers in solution which lead to layer-by-layer helical morphology in solid-state. Interestingly, D-A intramolecular charge transfer transition showed Cotton effect with bimodal nature, which is remarkably stronger than the π-π* transition.     

Parent homopolymers of liquid crystalline polyesters

A method that combines multi-temperature powder X-ray diffraction and molecular modelling is used to determine the high temperature crystal structures for poly-(p-phenylene terephthalate) (PPT) and poly-(p-hydroxybenzoic acid) (PHBA). Both have high temperature structures characterized by a degree of rotational disorder. In the case of PHBA there is a distinct high temperature phase above a well defined transition at 350°C, whereas the rotational disorder in ‘as polymerized’ PPT increases gradually between 370 and 475°C. The interchain packing at high temperatures still maintains phenyl edge to phenyl face correlations and, to some degree, carbonyl carbon to carbonyl oxygen contacts. The findings are relevant to the molecular structure and phase behaviour of liquid-crystalline polyesters as a class.     

侧基甲壳效应和甲壳型液晶高分子

侧链液晶高分子体系里,液晶基元可以通过尾接或腰接的方式与主链相连.一般认为,在液晶基元与主链间插入一段长度合适的＂柔性间隔基＂可有效实现主、侧链间的动力学去偶合,从而有利于侧基液晶基元之间的有序排列.作为一类特殊的腰接型侧链液晶高分子,甲壳型液晶高分子中体积较大的侧基（如棒状液晶基元）通过非常短的间隔基或仅通过一个碳-碳键直接横挂至主链上,这导致了强烈的甲壳效应,使得主链被迫伸展.因此,可从与＂柔性间隔基＂完全不同的角度出发,充分利用主链和侧基间的偶合作用,设计甲壳型液晶高分子.本文综述了腰接型侧链液晶高分子中的侧基甲壳效应、甲壳型液晶高分子中由主链与侧基相互作用所导致的特殊构象以及液晶相结构.研究表明,侧基甲壳效应在调控甲壳型液晶高分子的形状、尺寸以及螺旋结构等方面有重要作用.甲壳型液晶高分子可作为刚-柔嵌段共聚物的刚性链段,也可作为主/侧链结合型液晶高分子的主链部分参与到多层次分级超分子有序结构的构筑之中.