Specificity of the ester bond forming condensation enzyme SgcC5 in C-1027 biosynthesis

The SgcC5 condensation enzyme catalyzes the attachment of SgcC2-tethered (S)-3-chloro-5-hydroxy-β-tyrosine (2) to the enediyne core in C-1027 (1) biosynthesis. It is reported that SgcC5 (i) exhibits high stereospecificity toward the (S)-enantiomers of SgcC2-tethered β-tyrosine and analogues as donors, (ii) prefers the (R)-enantiomers of 1-phenyl-1,2-ethanediol (3) and analogues, mimicking the enediyne core, as acceptors, and (iii) can recognize a variety of donor and acceptor substrates to catalyze their regio- and stereospecific ester bond formations.     

Effect of PPEGMA content on the structure and hydrophilicity of PVDF/PPEGMA blends prepared by in situ polymerization

The in situ polymerization of poly(ethylene glycol) methyl ether methacrylate (PEGMA) in poly(vinylidene fluoride) (PVDF) solution was carried out to prepare hydrophilic PVDF/PPEGMA blends. Based on T _g data, the system displayed some miscibility at low PPEGMA content because there were some interactions between PVDF and PPEGMA, as illustrated by Fourier transform infrared spectroscopy analysis, while the miscibility disappeared at high PPEGMA content. The β phase was the predominant crystalline phase of PVDF in the in situ blends, which was independent on PPEGMA content. However, the addition of PPEGMA could change the unit cell structure in the direction of the a-axis of β phase. The degree of crystallinity of the in situ blends calculated by wide-angle X-ray diffraction and differential scanning calorimetry displayed different tendencies with PPEGMA content. The scanning electron microscopy showed that the “spherulites” size increased with the increase in PPEGMA content, since the “spherulites” were comprised of not only crystalline PVDF but also amorphous PVDF and PPEGMA. In addition, the in situ blend had a great improvement in the hydrophilicity; specifically, its water contact angle was down to 26° when the PPEGMA content only increased to 26.9 wt.%.     

Photodegradation of ethylene–octene copolymers with different octene contents

The influence of the octene content on the photodegradation behaviour of ethylene–octene copolymers (EOCs) was revealed by investigating the photooxidation of low density polyethylene (LDPE) and EOCs with different octene contents through a series of characterisation methods. LDPE was very sensitive to ultraviolet light and the photostabilities of EOCs decreased with increasing octene concentration. The photodegradation of all samples produced hydroxyl, carbonyl and vinyl groups. The ease of chain crosslinking and scission was increased as the octene content rose. Crosslinking predominated in late irradiation period of LDPE while chain scission was dominant in that of EOCs. Annealing and chain scission promoted the secondary crystallisation of the crystallisable chain segments. Chain scission enhanced the crystallisation ability of the irradiated EOCs while it decreased that of the weathered LDPE. The photostabilities of crystals could be ranked as follows: the chain-folded lamellar crystals > the bundled crystals > the fringed micellar crystals. The thermal stabilities and mechanical properties of samples decreased with increasing irradiation time and the decreasing extent was correlated with the comonomer content.     

Effect of carbon blacks with various structures on electrical properties of filled ethylene-propylene-diene rubber

The effect of carbon blacks on the electrical properties of filled EPDM is investigated by comparing five types of carbon blacks. Electrical tests show that EPDM filled with carbon black having a large value of surface area (conductive carbon black, CCB) displays the low volume and surface resistivities, and easily succumbs to dielectric breakdown. EPDM/CCB composites show the lowest dielectric constant and highest dissipation factor. Hence, a CCB with large surface area and high content of sulphur on the surface is suitable for conductive polymers; whereas, EPDM filled with other carbon blacks is useful for insulation applications.     

Crystalline Phase Formation of Poly(vinylidene fluoride) from Tetrahydrofuran/N,N‐dimethylformamide Mixed Solutions

The present work focused on the effect of the interactions between poly(vinylidene fluoride) (PVDF) chains and solvent molecules on the structure and crystallization behavior of PVDF in films obtained by solution casting. In a single solvent system, the film cast from the good solvent of N,N‐dimethylformamide (DMF), showed dominantly β‐phase crystals with the highest PVDF crystallinity (50.6%) and the largest spherulite size, about 4 μm, at the top surface. The samples deposited from good swelling agents, such as tetrahydrofuran (THF) and methyl ethyl ketone (MEK), exhibited mainly the original α phase with some amount of β‐phase crystals; the crystallization behavior and the morphology of the surface were similar to the original PVDF resin, because of the only partially dissolved PVDF chains in these two solvents. In a mixed solvent system (THF/DMF), the β phase formation linearly increased as the DMF component increased, determined by Fourier transform infrared spectroscopy (FTIR) techniques, owing to increased interactions between PVDF chains and DMF molecules. The film surface consisted of β spherulites with average size of about 3 μm, which were smaller than those grown from pure DMF, because of the increased crystallization rate in the mixed solvent.     

RTV Silicone Rubber Filled with Surface Modified Montmorillonite

The surface modification of pristine montmorillonite (MMT) with four kinds of coupling agents for reinforcement of the room temperature vulcanized vinyl-terminated polydimethylsiloxane (RTV PDMS) networks was investigated. The four coupling agents containing γ-glycidoxypropyltrimethoxysilane (KH-560), γ-methacryloxypropyltri methoxysilane (KH-570), isopropyl trioleic titanate (NDZ-105), and stearic acid (SA) without N, P, or S compounds were used because the coupling agents harmless to the platinum catalyst used normally in the curing of RTV vinyl-terminated PDMS. The interaction between MMT particles and coupling agents was confirmed by diffused reflectance infrared spectroscopy (DR-FTIR) tests. By scanning electron microscopy (SEM) characterization, surface modified MMT was dispersed heterogeneously in the PDMS matrix for all four coupling agents. By X-ray diffraction (XRD) characterization, the interlayer distances of the four surface modified MMT were not changed. Dielectric tests showed that the dielectric constant and dielectric loss of PDMS filled with KH-560, KH-570, and SA treated MMT were decreased comparing with those of PDMS filled with pristine MMT. With the increasing content of MMT treated with KH-570, the dielectric constant and dielectric loss value of PDMS composites increased.     

Non-isothermal crystallization behaviors of poly(4-methyl-pentene-1)

Non-isothermal crystallization of isotactic poly(4-methyl-pentene-1) (P4MP1) is studied by differential scanning calorimeter (DSC), and kinetic parameters such as the Avrami exponent and the kinetic crystallization rate (Z _c) are determined. From the cooling and melting curves of P4MP1 at different cooling rates, the crystalline enthalpy increases with the increasing cooling rate, but the degree of crystalline by DSC measurement shows not much variation. Degree of crystalline of P4MP1 calculated by wide angle X-ray diffraction pattern shows the same tendency with crystalline enthalpy, indicating that re-crystallization occurs when samples heated above the second glass transition temperature of P4MP1. By Jeziorny analysis, n ₁ value suggests that mainly spherulites’ growth at 2.5 K min⁻¹ transforms into a mixture mode of three-dimensional and two-dimensional space extensions with further increasing cooling rate. In the secondary crystallization process, n ₂ values indicate that the secondary crystallization is mainly the two-dimensional extension of the lamellar crystals formed during the primary crystallization process. The rates of the crystallization, Z _c and t _1/2 both increase obviously with the increase of cooling rate, especially at the primary crystallization stage. By Mo’s method, higher cooling rate should be required in order to obtain a higher degree of crystallinity at unit crystallization time.     

Effect of crystal form and particle size of titanium dioxide on the photodegradation behaviour of ethylene-vinyl acetate copolymer/low density polyethylene composite

The photodegradation behaviour of ethylene-vinyl acetate copolymer (EVA)/low density polyethylene (LDPE) composite containing four different types of titanium dioxide (TiO₂) was investigated through colour difference, Fourier transform infrared (FTIR) spectroscopy, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and mechanical tests. The results showed that the performance losses of composites were qualitatively correlated with the degradation degree. The vinyl acetate (VA) groups in EVA were sensitive to UV light and the photodegradation mainly occurred in the amorphous region. The chain scission and annealing effect facilitated the secondary crystallization of composites. The heterogeneous nucleation effect of TiO₂ on the crystallization of composites was related to the particle size of TiO₂. The micro rutile TiO₂, micro anatase TiO₂ and their mixture (rutile/anatase = 13/87) exhibited a photo-stabilising effect, while the nano mixed crystals TiO₂ (rutile/anatase = 20/80) had an opposite effect.     

Functional Genome Mining Reveals a Class V Lanthipeptide Containing a d‐Amino Acid Introduced by an F420H2‐Dependent Reductase

Lantibiotics are a type of ribosomally synthesized and post‐translationally modified peptides (termed lanthipeptides) with often potent antimicrobial activity. Herein, we report the discovery of a new lantibiotic, lexapeptide, using the library expression analysis system (LEXAS) approach. Lexapeptide has rare structural modifications, including N‐terminal (N,N)‐dimethyl phenylalanine, C‐terminal (2‐aminovinyl)‐3‐methyl‐cysteine, and d ‐Ala. The characteristic lanthionine moiety in lexapeptide is formed by three proteins (LxmK, LxmX, and LxmY), which are distinct from enzymes known to be involved in lanthipeptide biosynthesis. Furthermore, a novel F₄₂₀H₂‐dependent reductase (LxmJ) from the lexapeptide biosynthetic gene cluster (BGC) is identified to catalyze the reduction of dehydroalanine to install d ‐Ala. Our findings suggest that lexapeptide is the founding member of a new class of lanthipeptides that we designate as class V. We also identified further class V lanthipeptide BGCs in actinomycetes and cyanobacteria genomes, implying that other class V lantibiotics await discovery.