Main Chain Structure Research Articles

Comparative Studies on Structure-Property Relationships of the Polyimide Films Derived from the Dianhydride containing Alicyclic cis-Ethylenic Double Bond

An ester dianhydride bearing alicyclic cis-ethylenic double bond, bicyclo [2.2.1]hept-5-ene-2, 3-diylbis(methylene)bis(1, 3-dioxo-1, 3-dihydroisobenzofuran-5-carboxylate) (BHEM) and its hydrogenated analogue bicyclo[2.2.1]heptane-2, 3-diylbis(methylene)bis(1, 3-dioxo-1, 3-dihydroisobenzofuran-5-carboxylate) (BHSM) were synthesized and polymerized with three aromatic diamines, respectively, to prepare two analogous series of polyimide (PI) films. Comparative studies were carried out to elucidate how the unsaturated bicylo[2.2.1]heptenyl structure and the saturated bicyclo[2.2.1]heptyl structure in polymer main chain influence the properties of the PI films. The structural characteristics of the two analogous alicyclic structures including reactivity, bulkiness, configuration, polarity were found to impact the solubility, hydrophobicity, surface morphology, and the thermal, optical and dynamic dielectric properties of PI films. The aggregation state of both series of PI films was analyzed by wide-angle X-ray scattering (WAXD) and computed tomography (CT) fluorescence, which reveals the structural characteristics differences between unsaturated alicyclic containing cis-ethylenic double bond with planar sp2 hybrid configuration and saturated alicyclic with wholly tetrahedral sp3 hybrid configuration. It indicates that the thermally crosslinkable alicyclic cis-ethylenic double bonds in BHEM incline to induce a closer polarized chain packing via inter-chain Π-Π complex, while the wholly tetrahedral sp3 configuration of the bulkier saturated alicyclic in BHSM inhibits the chain aggregation in the PI film and leads to greater chain mobility.

Crystal Structures of Protein-Bound Cyclic Peptides.

Cyclization is an important post-translational modification of peptides and proteins that confers key advantages such as protection from proteolytic degradation, altered solubility, membrane permeability, bioavailability, and especially restricted conformational freedom in water that allows the peptide backbone to adopt the major secondary structure elements found in proteins. Non-ribosomal synthesis in bacteria, fungi, and plants or synthetic chemistry can introduce unnatural amino acids and non-peptidic constraints that modify peptide backbones and side chains to fine-tune cyclic peptide structure. Structures can be potentially altered further upon binding to a protein in biological environments. Here we analyze three-dimensional crystal structures for 211 bioactive cyclic peptides bound to 65 different proteins. The protein-bound cyclic peptides were examined for similarities and differences in bonding modes, for main-chain and side-chain structure, and for the importance of polarity, hydrogen bonds, hydrophobic effects, and water molecules in interactions with proteins. Many protein-bound cyclic peptides show backbone structures like those (strands, sheets, turns, helices, loops, or distorted variations) found at protein-protein binding interfaces. However, the notion of macrocycles simply as privileged scaffolds that primarily project side-chain substituents for complementary interactions with proteins is dispelled here. Unlike small-molecule drugs, the cyclic peptides do not rely mainly upon hydrophobic and van der Waals interactions for protein binding; they also use their main chain and side chains to form polar contacts and hydrogen bonds with proteins. Compared to small-molecule ligands, cyclic peptides can bind across larger, polar, and water-exposed protein surface areas, making many more contacts that can increase affinity, selectivity, biological activity, and ligand-receptor residence time. Cyclic peptides have a greater capacity than small-molecule drugs to modulate protein-protein interfaces that involve large, shallow, dynamic, polar, and water-exposed protein surfaces.

Synthesis and properties of shape memory poly(aryl ether ketone)s

Thermally responsive shape memory polymers have received great attention in the fields of biomedicine and aerospace. A new type of thermoplastic shape memory polymer with excellent mechanical and thermal properties was synthesized. A new reactable monomer with flexible alkyl chain was first designed and synthesized, then introducing it into the main chain structure of poly(aryl ether ketone)s. The alternating chain structure of the soft and hard segments and the H-Bonding between the carboxyl groups on the side chains both serve as physical cross-linking points in the polymer network, thereby imparting shape memory properties to the polymers. Polymers with more flexible segments exhibit better shape memory performance in terms of high shape fixing ratio and high shape recovery ratio.

Preparation of methyl alginate and its application in acidified milk drinks.

A series of methyl alginate with different degree of esterification (DE) were prepared with low cost, and its application in acidified milk drinks (AMDs) was investigated. The gel strength, molecular weight, solution apparent viscosity and optical rotation value of methyl alginate all decreased with the increase of DE. Methyl alginate with DE equal or larger than 60.7% was more effective in stabilizing AMDs than high-methoxyl pectin (HMP) under the same conditions, and the higher the DE, the better its stabilizing ability. The methyl alginate had better synergistic action for stabilizing AMDs with propylene glycol alginate (PGA) than HMP. The reason for this could be attributed to that methyl alginate and PGA had the same main chain structure, and the same levorotatory optical activity. The results indicated that the methyl alginate had a potential industry application prospect for stabilizing AMDs as an alternative to HMP.

Improved De Novo Main-Chain Tracing Method Mainmast for Multi-Chain Modeling, Local Refinement, and Graphical User Interface

The significant progress of the cryo-EM poses a pressing need for software for structural interpretation of EM maps. Particularly, protein structure modeling tools are needed for maps determined around 4 Å resolution, where finding main-chain structure and assigning the amino acid sequence into EM map are still challenging problems. We have developed a de novo modeling tool named MAINMAST (MAINchain Model trAcing from Spanning Tree, http://kiharalab.org/mainmast/) for EM maps for this resolution range (Nature Communications, 2018). MAINMAST builds main-chain traces of a protein in an EM map from a tree structure constructed by connecting points with a high density in the map without referring to known protein structures or fragments.Here, we report substantial improvements of MAINMAST in three aspects. The largest improvement is that the method now can perform automatic map segmentation and structure modeling for symmetrical multi-chain complexes. The tree structure that connects dense points are traced for multiple chains simultaneously in a symmetric fashion. Moreover, the accuracy of a model is significantly improved by a new implementation of local sequence matching and structure refinement. The local matching protocol is also useful for identifying missing regions in a structure model, i.e. regions with a low density, in an EM map. Finally, we developed a software plugin of MAINMAST for the UCSF Chimera, so that users can monitor structures at each step of a modeling procedure. The major functionalities include to generate and to display tree structures from local dense points in the map, main-chain traces, and reconstructed all-atom models. Through the interface, users can easily control parameters of MAINMAST and save and restore sessions.

Valencene as a naturally occurring sesquiterpene monomer for radical copolymerization with maleimide to induce concurrent 1:1 and 1:2 propagation

Valencene, a naturally occurring sesquiterpene that can be obtained from various citrus fruits, was copolymerized as an unconjugated 1,1-disubstituted bulky vinyl monomer by radical copolymerization with conjugated monomers possessing electron-withdrawing substituents, such as maleimides, acrylates, methacrylates, acrylonitrile, and methacrylonitrile, in toluene and PhC(CF3)2OH. The fluorinated alcohol increased the copolymerizability, as in the radical copolymerization of other unconjugated non-polar olefins with those electron-acceptor comonomers. In particular, the copolymerization of valencene and N-phenylmaleimide in PhC(CF3)2OH proceeded via concurrently occurring 1:1 and 1:2 propagation, in which the monomer reactivity ratio of maleimide and valencene to the maleimide radical with the penultimate valencene unit was close to 1 according to a kinetic analysis of the copolymerization based on the penultimate model. Therefore, the copolymerizability of valencene was higher than that of limonene, which is a similar unconjugated 1,1-disubstituted vinyl monomer with a smaller cyclohexenyl substituent that undergoes selective 1:2 radical copolymerization with maleimide derivatives under the same conditions. Reversible addition-fragmentation chain transfer RAFT copolymerization using a trithiocarbonate resulted in copolymers with controlled molecular weights and chain-end groups. The biobased copolymers of valencene and N-phenylmaleimide showed relatively high glass transition temperatures (Tg) of approximately 220–240 °C and relatively high thermal stability with 5% decomposition temperatures (Td5) of approximately 330–340 °C due to their rigid main-chain structures.

Self-organization of cholesterol-side-chain liquid crystalline polymers by tailoring the main chain structure and flexible spacer length

Liquid crystal polymers with cholesterol side-chains formed different smectic A phases under the influence of the main-chain structure and flexible spacer length.

Characterization and flocculation evaluation of a novel carboxylated chitosan modified flocculant by UV initiated polymerization

In this work, carboxylated chitosan modified flocculant (CC-g-PCD) was prepared by graft copolymerization technique to enhance the charge-attracting and adhesion of bridges and net-sweeping capacity of flocculants. The dimethyldiallylammonium chloride (DMDAAC), carboxylated chitosan (CMCS), and 3-chloro-2-chloropropyltrimethylammonium chloride (CTA) were utilized for synthesis of CC-g-PCD via photopolymerization techniques. The synthesized CC-g-PCD was characterized by 1H NMR, SEM, XRD, and FTIR, and the characteristic groups on the main chain and surface morphological structure of CC-g-PCD were investigated. The obtained results indicated that CTA and DMDAAC were successfully grafted into the CPCTS. In ordered to evaluate the flocculation performance of CC-g-PCD at various dosages, stirring intensity (G value), and pH value by detecting Chl a, COD, and turbidity, the actual lake water that contains algae was used for flocculation assessment tests. The experimental results of the water sample with flocculation showed that the maximum flocculation efficiency of turbidity (91.1%), Chl a (97.2%), and COD (97.0%) can be achieved by CC-g-PCD at pH 7, G value 200 s−1, and 4.0 mg/L. The comparison results demonstrated that CC-g-PCD had better flocculation efficiency than commercial flocculants. Finally, based on the analysis of algae removal in combination with Zeta potential measurements, the flocculation mechanisms in actual lake water at various dosages and pH values were adsorption bridging and electrical neutralization.

One-Pot Multicomponent Tandem Reactions and Polymerizations for Step-Economic Synthesis of Structure-Controlled Pyrimidine Derivatives and Poly(pyrimidine)s

The development of new polymerization methodologies for the construction of π-conjugated polymers with unique electronic and photophysical properties is of great importance. Multicomponent tandem polymerizations (MCTPs), featured with high synthetic efficiency and convenience, have proved their potential in the synthesis of polymers with π-conjugated structures. Herein, a new multicomponent tandem reaction of alkyne, guanidine hydrochloride, DMSO, and O2 was reported through the combination of four sequential Glaser coupling–nucleophilic addition–heterocyclization–oxidation reactions in a one-pot procedure. The corresponding MCTP of diyne, guanidine hydrochloride, DMSO, and O2 was also developed in the presence of CuCl, N,N,N′,N′-tetramethylethylenediamine, and Cs2CO3 to afford conjugated poly(pyrimidine) with well-defined structure, high yield of 87%, and high molecular weight of 25300 g/mol. A mechanistic study of the reaction and a kinetic study of the polymerization were conducted, enabling facile modulation of polymer mainchain structure from rigid conjugated backbone to flexible nonconjugated backbone by simply tuning the atmosphere of the MCTP from air to nitrogen. The poly(pyrimidine)s enjoy outstanding thermal stability and good solubility, whose hydrophobicity and photophysical property are influenced by their subtle structural difference. This MCTP directly converts monomers with simple structure to complex heterocycle-containing products in a step-economic manner and constructs new functional substituted pyrimidine rings embedded in the polymer main chain, providing a compact approach for the efficient and convenient synthesis of structurally controllable poly(heterocycle)s and the construction of polymer materials with potential high-tech applications.

Network-Stabilized Bulk Heterojunction Organic Photovoltaics

Bulk heterojunction organic photovoltaic (OPV) devices are multilayer organic devices that can be fabricated using low-cost and scalable solution processing methods, but current devices exhibit poor mechanical stability and degrade under deformation due to cracking and delamination. Recent approaches to improve mechanical durability involve modifying the side-chain or main-chain structures of conjugated polymers in the active layer, but in general it is difficult to simultaneously optimize electronic properties, morphology, and mechanical stability. Here, we present a general approach to improve the mechanical stability of bulk heterojunction active layers through incorporation of an internal elastic network. Network-stabilized bulk heterojunction OPVs are prepared using reactive small molecular additives that are rapidly cross-linked through thiol–ene coupling after processing the active layer. Thiol–ene reactions catalyzed by a base or initiated through short exposure to UV light produce insoluble, elas...

Polyethylene-like macrolactone-based polyesters: Rheological, thermal and barrier properties

Linear aliphatic poly(ester)s are receiving increasing interest due to their properties comparable to that of common polyethylene. In this work, unsaturated macrolactone ω-6-hexadecenlactone (6HDL) is used for the preparation of long-methylene chain aliphatic polyesters by metal-catalyzed ring-opening polymerization (ROP). Block and random copolymer of ε-caprolactone (CL) are then synthetized. The microstructures of these polyesters are characterized by 1H and 13C NMR. Their rheological, thermal and barrier properties are evaluated and discussed. For comparison purposes, the properties of the relative homopolymers are also determined. Results show that the properties of these polymers differ depending on the main chain structure. In particular, the viscoelastic behavior of the copolymer sample shows an order-disorder transition in the temperature range of 75–85 °C. Thermogravimetric analysis and barrier properties show that the copolymers have a resistance to the diffusion of gases and vapors intermediate between that of the parent homopolymers.

Structural Characterization, Antioxidant Activity, and Biomedical Application of Astragalus Polysaccharide Degradation Products

To study the antioxidant capacity of Astragalus polysaccharides (APS) with different molecular weights, we used hydrogen peroxide to degrade original Astragalus polysaccharide (APS0) with an initial molecular weight of 11.03 kDa and obtained three degraded polysaccharides with molecular weights of 8.38 (APS1), 4.72 (APS2), and 2.60 kDa (APS3). The structures of these polysaccharides were characterized by 1H NMR, 13C NMR, FT-IR, and GC/MS. The degradation process did not cause significant changes in the main chain structure of APS. The monosaccharide component of APS before and after degradation was slightly changed. The antioxidant ability in vitro (removing hydroxyl and ABTS radicals and reducing ability) and in cells (superoxide dismutase and malondialdehyde generation) of these polysaccharides is closely related to their molecular weight. If the molecular weight of APS is very high or low, it is not conducive to their activity. Only APS2 with moderate molecular weight showed the greatest antioxidant activity and ability to repair human kidney epithelial (HK-2) cells. Therefore, APS2 can be used as a potential antistone polysaccharide drug.

Pressure-Induced Variations of Aggregation Structures in Colorless and Transparent Polyimide Films Analyzed by Optical Microscopy, UV-Vis Absorption, and Fluorescence Spectroscopy.

Pressure-induced variations in the main chain and aggregation structures of colorless and transparent semialiphatic polyimide (PI) films were investigated by optical microscopy, UV-vis absorption, and fluorescence spectroscopy up to 8 GPa. Upon application of pressures up to 2 GPa, a gradual volumetric compression was clearly observed by microscopy, and definite bathochromic shifts of locally excited (LE) absorption bands were detected, which was attributed to the compression of interchain free volume and enhanced intermolecular interactions. In addition, a significant reduction in fluorescence intensity was observed for PIs with quasilinear structures below 2 GPa due to enhanced energy transfer in the excited states caused by the densification of PI chain packing. In contrast, the volumetric compression of the PI films and bathochromic shifts of the LE absorption bands were gradually reduced at pressures above 2 GPa. The former is closely correlated with the bulkiness and flexibility of the alicyclic diamine structure. The latter reflects the intense compression stress generated around the dianhydride moiety, associated with the deformability and in-plane orientation of the main PI chains. High-pressure experiments on PI films are beneficial to investigate variations in aggregation structures and local electronic structures of PI chains induced by dense molecular packing and enhanced intermolecular interactions.

Efficient Antibacterial Performance and Effect of Structure on Property Based on Cationic Conjugated Polymers

Cationic conjugated polymers (CCPs) have attracted more and more attention in antibacteria and tumor treatment based on photodynamic therapy (PDT). However, the main chain structure–antibacterial activities relationship has been rarely reported. Herein, we designed and synthesized four cationic conjugated polymers: one poly(fluorine phenylene) derivative (PFP) and three poly(fluorene-co-phenylene ethynylene) derivatives (PFE-1, PFE-CN-2, and PFE-NP-3). PFP and PFE-1 have the same side chains but bear different conjugated backbones. PFE-CN-2 (π-A) and PFE-NP-3 (A-π-A) are modified with electron-donating and/or electron-withdrawing groups. Three PFEs can produce reactive oxygen species (ROS) faster than PFP. The order of antibacterial activities of the four CCPs is as follows: PFE-CN-2 > PFE-1 ≫ PFE-NP-3 ≈ PFP, which is coincident with the generation rate of ROS. In addition, CCPs with D-π-A structure is more advantageous than A-π-A in ROS production and in antibacterial performance. These results provide a...

Comparative Studies on Properties of Polymers with Bulky Side Groups Synthesized by Cyclopolymerization of α,ω-Dienes and α,ω-Diynes

Four polymers containing five-membered rings in the main chain, with or without conjugation structure along the backbone and with or without conjugated pendent groups, were designed and synthesized by metathesis cyclopolymerization of functionalized α,ω-diynes, and cyclopolymerization of functionalized α,ω-dienes catalyzed by the α-diimine palladium-based catalyst, respectively. High to moderate monomer conversions were achieved. Chain structure, molecular weight, and molecular weight distribution (MWD) of the cyclopolymerization products were characterized by 1H-, 13C-NMR, FTIR, and GPC. The polymers showed regular main chain structures, moderately high molecular weight, and narrow MWD. Thermal properties and chain stacking behaviors of the polymers were investigated by differential scanning calorimetry (DSC) and X-ray diffraction (XRD) as well as atomic force microscopy (AFM). The polymer with conjugation system in both the backbone and the pendent groups exhibited UV-Vis absorption at a much longer wavelength than those with the conjugation only in the backbone or only in the side groups. The polymers with conjugated backbone need more space for chain stacking, and the conjugated backbone causes enhanced size of polymer particles assembled from solution. The results showed that primary microstructures of the polymer exerted significant influences on the physical properties.

NMR-Observed Atomic Bond Length Stability Supports a Dimensionality Shift in Protein Main Chain 3D Structure Description and Representation

<p class=MsoListParagraph style=margin-left:.25in;mso-add-space:auto; text-indent:-.25in;mso-char-indent-count:0;mso-list:l0 level1 lfo1;mso-mirror-indents: yes> 1. Abstract <p class=MsoListParagraph style=margin-left:.25in;mso-add-space:auto; text-indent:-.25in;mso-char-indent-count:0;mso-list:l0 level1 lfo1;mso-mirror-indents: yes> To date, the Cartesian ( x, y, z ) coordinate system is the default system in the Protein Data Bank to specify atomic positions in protein structures. Presented here is an alternative spherical coordinate system approach for a three-dimensional lossless deconstruction of protein main chain structures experimentally determined by NMR spectroscopy. To the default Cartesian system and a previously reported global spherical approach, this alternative local spherical approach provides a geometric description of the three-dimensional structure of protein main chains, which requires only two parameters ( θ and <span style=font-size:10.0pt;font-family: times= new= roman,serif;mso-fareast-font-family:calibri;mso-fareast-theme-font:= minor-latin;color:red=>φ ) , instead of the default three, i.e., x, y, z . Intrinsically a simpler approach than the default and previously reported approaches, this 2018 one induces a dimensionality shift from three to two, allowing it to find its potential application in significantly increasing the efficiency of protein structure-centered researches. <span style=font-size:11.0pt;font-family: times= new= roman,serif;mso-fareast-font-family:times= roman=>2. Keywords: Atomic Bond Length Stability; Dimensionality Shift; Frequency Distribution; Protein Main Chain Structure; Spherical Coordinate System

The role of ortho-, meta- and para-substitutions in the main-chain structure of poly(etherimide)s and the effects on CO2/CH4 gas separation performance

A homologous series of 12 all-aromatic PEI membranes was investigated with the aim to understand how subtle changes in the PEI main-chain affect the carbon dioxide/methane (CO2/CH4) gas separation performance. The 3-ring diamines selected for this study are either para-, meta- or ortho-aryloxy substituted with respect to the central benzene ring, i.e. 1,4-bis(4-aminophenoxy)benzene (P1), 1,3-bis(4-aminophenoxy)benzene (M1) and 1,2-bis(4-aminophenoxy)benzene (O1). Doing so changes the backbone geometry from a more linear to a more kinked conformation. In addition, four dianhydrides were selected with the aim to tailor the segmental mobility and hence the free volume of the PEIs, i.e. pyromellitic dianhydride (PMDA), 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA), 3,3′,4,4′-benzophenonetetracarboxylic dianhydride (BTDA) and 3,3′,4,4′-oxydiphthalic dianhydride (ODPA). We have investigated how subtle changes in these prototypical PEIs affect membrane critical performance criteria such as CO2 permeability, CO2/CH4 selectivity and ability to withstand high operating pressures. In ODPA-based membranes the CO2 permeability decreases in the order P1 > O1 > M1 and remains steady throughout measurements with mixed feed pressures up to 40 bar, however, the selectivity decreases for ODPA-O1 and ODPA-M1. For high-pressure applications, the OPDA-P1 membrane is a good candidate with a selectivity of 48, permeability of CO2 of 0.74 Barrer and ability to resist plasticization up to 40 bar of total pressure (16 bar of CO2 partial pressure). Alternatively, for applications up to 10 bar of total mixed feed (5 bar of CO2 partial pressure), BPDA-O1 is a promising candidate because this membrane displays a high selectivity of 70 and permeability of 1.3 Barrer.

Properties of a novel carboxymethyl chitosan derived from silkworm pupa.

In this study, a carboxymethyl chitosan derived from silkworm pupa (SP-carboxymethyl chitosan) was prepared. The physical characteristics of the SP chitin, chitosan, and carboxymethyl chitosan were analyzed. The scanning electron microscopy results showed that the surfaces of the samples from SP were more uneven, with more surface fractures compared with those of the reference substance (RS). Thermal analysis, X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy analysis showed that the main molecular chain structures of SP samples and RSs had no substantial differences. However, the crystallinity and thermal decomposition temperature of the SP samples were lower compared with those of the RSs. All of these results provide a theoretical basis for the development of applications for the SP-carboxymethyl chitosan.

Azopolyimides – influence of chemical structure on azochromophore photo-orientation efficiency

This work is a summary of our previous investigations of polyimides containing derivatives of azobenzene or azopyridine. Here we discussed the relationship between the microstructure of azopolyimides and azochromophore photo-orientation efficiency determined in measurements of photoinduced birefringence as well as the relationship between the microstructure ofazopolyimides and their selected physical properties. The designed architecture of azopolymers allowed to trace the effect of such structural elements as the structure of the polymer main chain and chromophore, its content, location including the method of dye assembling with the polymer matrix on thermal, optical and photoinduced properties. We also discussed the possibility of the intermolecular hydrogen bonds formation in functionalized azopolyimides, which hinder isomerization of the azobenzene molecules results the photoinduced birefringence is not observed in the material. Additionally, the possibility of potential applications of azopolyimides as layers for the liquid crystal alignment was presented.

High Proton Conduction of Organized Sulfonated Polyimide Thin Films with Planar and Bent Backbones

Fast proton conduction was achieved in organized lamellar structures with in-plane oriented structure parallel to the substrate surface using a lyotropic liquid-crystalline (LC) property. Alkyl sulfonated polyimides (ASPIs) with bent main chain structure were newly synthesized to investigate relations between the higher order structure and proton transport properties. Proton conductivity of all polyimide thin films was greater than 10–2 S/cm. Grazing-incidence small-angle X-ray scattering (GI-SAXS) revealed that both planar and bent ASPI thin films exhibited humidity-induced lyotropic lamellar structure. Infrared p-polarized multiple-angle incidence resolution (pMAIR) studies revealed that main chain backbones of both planar and bent ASPI thin films show an in-plane orientation parallel to the substrate surface. Results demonstrate that sulfonated alkyl side chains contribute strongly to the lyotropic LC property, which enhances molecular orderings and proton conductivity by water uptake. This study exten...