Oxidative Cross-linking Research Articles

Oxidative crosslinking induced self-reinforcing waterborne polyurethane with tunable structure as multi-synergistic antibacterial biomimetic composite coating

Waterborne polyurethane (WPU) coatings with antibacterial properties have attracted extensive attention. Herein, we proposed a molecular design strategy was used to establish a series of castor oil/tung oil based WPU antibacterial coatings. By introducing conjugated double bonds of tung oil into WPU skeleton, electrostatic interaction by zwitterion and bacterial division inhibition by conjugated double bonds achieved the synergistic antibacterial effects. To our delight, inspired by lotus leaves, the scanning electron microscope test revealed the bionic micro-nano embedded structure was constructed on the surface of WPU using siloxane-modified nanocellulose (CNC@MTES). The destruction of the phosphorus layers and superhydrophobicity grant the material to have quadruple synergistic antibacterial effects. The antibacterial rates of WPU coatings on E. coli and S. aureus were more than 99.6% and played the mildew protection effect. What's more, peroxy bond formed by unique oxidative cross-linking induced self-reinforcing to enhanced the mechanical properties (the stress and Young's modulus of coating enhanced by 122.06% and 2368% respectively). Strong bonding between CNC@MTES and WPU ensured the durability of the coating, which was still effectively antibacterial and hydrophobic after 400 friction cycles. This research offered an innovative and effective method for coating to be widely used in product packaging, medical devices, smart glass and other fields.

Microstructure and mechanical property analysis of HTPB adhesive under thermal aging based on molecular dynamic simulations

The solidified network structure formed by the addition reaction of hydroxyl-terminated polybutadiene and toluene diisocyanate, under the action of long-term internal and external factors, changes over time in its system structure. The resulting mechanical properties have a great impact on the reliability and structural integrity of solid propellants. In this study, an all-atom molecular dynamics method was used to study the sensitivity of the adhesive to temperature and loading rate at different aging stages, as well as the differences in the damage and fracture process under large deformation. The simulation results showed that degradation chain scission weakened the system modulus and strengthened the deformation ability, making nonbonded energy the main storage form of strain energy. Oxidative crosslinking produced an opposite modulus and deformation effects, with the storage form of strain energy changed from nonbonded energy to bond and angular energy during the loading process. In addition, increased temperature increased the distance between molecules and intensified molecular motion, causing the system strain to expand globally, reducing the stress peak, and enhancing deformation ability. The increase in loading rate increased local strain, causing some covalent bonds to break, while viscosity replaced elasticity as the main mechanical property, causing the stress-strain curve to shift to the upper left.

Effects of hydroxyl radicals oxidation on digestion properties of silver carp myofibrillar protein gel

The generation of hydroxyl radicals from fat oxidation during frozen storage of surimi has been confirmed to induce oxidative cross-linking of proteins, but the further effect on their digestive properties and nutritional qualities is still unclear. In this work, a simulated system composed of the silver carp myofibrillar protein (MP) and Fenton system was used to study their digestive process and activities. Affected by oxidative cross-linking, oxidized MP gels were slowly and incompletely hydrolyzed by gastrointestinal process according to free amino content, particle size and peptide amount. However, the essential amino acid proportion, reduced power and hydroxyl radical chelation of digestion products from slight oxidation group (0.1–0.5 mM H2O2) were found to be generally stronger than over-oxidation group (5.0–10.0 mM H2O2). Furthermore, the in vitro biological evaluations confirmed that there were certain antioxidant activities of digestion products without potential cytotoxicity on HepG2 cells. Oxidation treatment on MP was found to negatively maintain SOD activity and generating GSH. Nevertheless, the slight oxidation group showed great protection effects to keep HepG2 cells from cell membrane damage and release LDH. In conclusion, the slight oxidation of MP is beneficial to improve the nutritional quality and in vitro antioxidant activity of its digested products.

Underwater Adhesives from Redox-Responsive Polyplexes of Thiolated Polyamide Polyelectrolytes.

We report the fabrication of optically clear underwater adhesives using polyplexes of oppositely charged partially-thiolated polyamide polyelectrolytes (TPEs). The thiol content of the constituent PEs was varied to assess its influence on the adhesive properties of the resulting glues. These catechol-free, redox-responsive TPE-adhesives were formulated in aquo and exhibited high optical transparency and strong adhesion even on submerged or moist surfaces of diverse polar substrates such as glass, aluminium, wood, and bone pieces. The adhesives could be cured under water through oxidative disulphide crosslinking of the constituent TPEs. The polyamide backbone provided multi-site H-bonding interactions with the substrates while the disulphide crosslinking provided the cohesive strength to the glue. Strong adhesion of mammalian bones (load bearing capacity upto 7 kg/cm2 ) was achieved using the adhesive containing 30 mol % thiol residues. Higher pH and use of oxidants such as povidone-iodine solution enhanced the curing rate of the adhesives, and so did the use of Tris buffer instead of Phosphate buffer. The porous architecture of the adhesive and its progressive degradation in aqueous medium over the course of three weeks bode well for diverse biomedical applications where temporary adhesion of tissues is required.

Photothermal-promoted multi-functional gallic acid grafted chitosan hydrogel containing tannic acid miniaturized particles for peri-implantitis

Peri-implantitis, a leading cause of implant failure, currently lacks effective therapeutic strategies. Given that bacterial infection and reactive oxygen species overabundance serve as primary pathogenic and triggering factors, respectively, an adhesive hydrogel has been created for in-situ injection. The hydrogel is a gallic acid-grafted chitosan (CS-GA) hydrogel containing tannic acid miniaturized particles (TAMP). This provides antibacterial and antioxidant properties. Therefore, this study aims to evaluate the potential role of this hydrogel in preventing and treating peri-implantitis via several experiments. It undergoes rapid formation within a span of over 20 s via an oxidative crosslinking reaction catalyzed by horseradish peroxidase and hydrogen peroxide, demonstrating robust adhesion, superior cell compatibility, and a sealing effect. Furthermore, the incorporation of TAMP offer photothermal properties to the hydrogel, enabling it to enhance the viability, migration, and antioxidant activity of co-cultured human gingival fibroblasts when subjected 0.5 W/cm2 808 nm near-infrared (NIR) irradiation. At higher irradiation power, the hydrogel exhibits progressive improvements in its antibacterial efficacy against Porphyromonas gingivalis and Fusobacterium nucleatum. It attains rates of 83.11 ± 5.42 % and 83.48 ± 6.855 %, respectively, under 1 W/cm2 NIR irradiation. In summary, the NIR-controlled CS-GA/TAMP hydrogel, exhibiting antibacterial and antioxidant properties, represents a promising approach for the prophylaxis and management of peri-implantitis.

Preparation of a high-strength, hydrophobic performance starch-based adhesive with oxidative cross-linking via Fenton's reagent

Starch is a highly attractive carbohydrate in the production for the preparation of adhesives in recent years, due to its widespread availability, renewability, and abundance of reactive hydroxyl groups. However, the mechanical properties, hydrophobicity, self-adhesion, and particularly high energy efficiency are generally unsatisfactory for current starch-based adhesives. On this premise, starch was oxidized using Fenton's reagent in a "“one-pot cooking” process. The prepared oxidized starch was chain expanded by polyvinyl alcohol (PVA) and then cross-linked with a 10 % isocyanate (PM-200) to fabricate a starch-based adhesive (SFA) with a network crosslinked structure. SF12A35%/2.5-55 adhesive shows significantly higher wet shear strength (1.18 MPa), a remarkable 94 % increase compared to SF0A35%/2.5-55. The adhesive film also demonstrates both hydrophobicity (99° contact angle) and exceptional energy efficiency, with a DSC test revealing a notable 10 % elevation in energy efficiency. In addition, the crosslinked structure increases its molecular weight, thereby increasing its self-adhesion (Fig. S1). This study opens up new possibilities for the design and manufacture of multifunctional starch-based adhesives.

Tuning the oxidation level of GO to construct high performance crosslinked lamellar graphene oxide membrane for pervaporation desalination

Lamellar graphene oxide (GO) membranes have received significant attention in desalination and water treatment due to their unimpeded permeation of water. Intercalation cross-linking is an effective way of improving the stability and perm-selectivity of GO membranes. This work reports a novel strategy to tune the nanostructure and morphology of GO membrane by combining highly oxidized GO nanosheets with borate cross-linking for desalination by pervaporation. The cross-linking processes covalent bonding of hydroxyl groups of GO membrane with sodium borate as the cross-linker, leading to formation of selective and stable three-dimensional GO-borate framework. The enrichment of hydroxyl groups on GO arising from high oxidation benefits the targeted cross-linking. Meanwhile, higher oxidation level of GO leads to higher content of carboxyl groups and improves the hydrophilicity of membrane. Consequently, the borate cross-linked GO membrane fabricated with fully oxidized GO simultaneously exhibits high water flux and enhanced structural stability. The results reveal that the interlayer spacing and swelling behavior of GO membranes are heavily influenced by the oxidation level and cross-linking, which are critical factors affecting the performance of the membranes in pervaporation desalination applications. The findings hold promise for the development of high-performance and stable GO membranes to address global water scarcity challenges.

Laccase-triggered one-step fabrication of positively charged phenolic acid-amine networks for nanofiltration

Molecular deposition provides a flexible and versatile platform for preparing positively charged composite nanofiltration membranes. The widely studied building units for molecular deposition are polyphenols such as catechins and tannins, which usually cause the complexity of the ultimate selective layer due to their relatively high antioxidant activity and limited positive potential originated from the large number of hydroxyl groups. In this work, a laccase-triggered one-step process has been proposed to prepare composite nanofiltration membranes. Highly positively charged phenolic acid-amine networks, which are represented by ferulic acid (FA) and polyethyleneimine (PEI), are constructed as the dense selective layer with a thickness of about 49 nm. This process is conducted in aqueous solutions under mild conditions, driven by electrostatic adsorption, catalytic oxidation, and covalent crosslinking processes. The structure of the selective layer can be finely tuned by the molecular weights of PEI, the chemical composition, and the pH values of the solutions. The prepared composite nanofiltration membranes can effectively reject MgCl2 (94.5 %) and organic dyes such as alcian blue (AB) (>98.7 %) and methyl blue (MB) (>97.8 %). This work not only demonstrates a facile and versatile method for preparing positively charged composite nanofiltration membranes with great separation performance towards divalent cations, but also expands the membrane building blocks.

Effects of thermal process conditions on crystallinity and mechanical properties in material extrusion additive manufacturing of discontinuous carbon fibre reinforced polyphenylene sulphide composites

This study investigates the thermal behaviour of discontinuous carbon fibre reinforced polyphenylene sulphide (CF/PPS), additively manufactured by material extrusion, with a focus on the effects of thermal process conditions on the degree of crystallinity, oxidation crosslinking and mechanical properties of CF/PPS from filament fabrication, material extrusion to annealing treatment. The screw extrusion parameters are optimised by performing a thermal analysis of the fabricated filaments. The effect of crosslinking reactions on the crystallinity process in determining the mechanical properties of the printed samples is illustrated by investigating the influence of the printing conditions. Furthermore, the effect of annealing treatment on the semi-crystalline polyphenylene sulphide (PPS) is studied by measuring the degree of crystallinity and viscoelasticity behaviours. Results demonstrate that the flexural properties of the printed CF/PPS composites at elevated processing temperatures are determined by the oxidation crosslinking between PPS chains. These enhance the crystallisation process of semi-crystalline polymers by acting as the nucleating agent first but negatively affect the mechanical properties at higher temperatures because of the detrimental effects of the polymer inter-chain bonding. The maximum flexural strength of printed CF/PPS reached 164.65 MPa when processing at an extrusion temperature of 280°C, a printing temperature of 320°C, and an annealing temperature of 130°C for 6 h. By adjusting the thermal treatment conditions, the degree of the crystallinity and the mechanical properties of the printed CF/PPS composites can be designed, controlled and tailored.

Thermal aging mechanism of PBT energetic elastomer based on the evolution of microstructure

3,3-bis(azidomethyl) oxetane and tetrahydrofuran copolymer (PBT)-based thermosetting polyurethane is an ideal energetic elastic matrix for high-energy, low-smoke, and insensitive solid propellants used in the field of aeronautics and aerospace. Its aging during long-term storage will cause deterioration of the mechanical properties of the propellant, which will bring serious security risks when working. To explore the thermal aging mechanism, the PBT elastomer was prepared and then aged at 40 °C, 60 °C, and 70 °C. Starting from characteristic structures that affect the mechanical properties of elastomer, active groups, hydrogen bonds, chemical crosslinking, free volume, crystallization, microphase separation, and mechanical properties of the PBT elastomer during thermal aging were analyzed by fourier transform-infrared (FT-IR) spectroscopy, swelling method, positron annihilation lifetime spectrum (PALS), X-ray diffraction (XRD), dynamic mechanical analysis (DMA), and uniaxial tensile test, respectively. Here, we found that the thermal aging of the PBT elastomer was a coupling process of several aging effects and showed a segmented feature. Destruction of hydrogen bonds, degradation of urethanes and oxidative crosslinking between polyether backbones successively dominated the thermal oxidation process of the PBT elastomer. This work is of great significance for the development of antiaging technologies for the PBT propellant and lays the foundation for the establishment of a mechanical property deterioration model based on microstructure evolution.

Control of Intermolecular Interactions toward the Production of Free-Standing Interfacial Polydopamine Films.

Aggregation of the polydopamine (PDA) molecular building blocks at the air/water interface leads to obtaining large surface nanometric-thin films. This mechanism follows two possible pathways, namely, covalent or non-covalent self-assembly, which result in a different degree of structure order and, consequently, different structural properties. Control of this mechanism could be vital for applications that require true self-support PDA free-standing films, for example, electrochemical sensing or membrane technology. Here, we are considering the impact of boric acid (BA) and Cu2+ ions on the mentioned mechanism exclusively for the free-standing films from the air/water interface. We have employed and refined our own spectroscopic reflectometry method to achieve an exceptionally high real-time control over the thickness growth. It turned out that BA and Cu2+ ions significantly impact the film growth process. Reduction of the nanoparticles size and their number was examined via UV-vis spectroscopy and transmission electron microscopy, showing a colossal reduction in the mean diameter of nanoparticles in the case of BA and a moderate reduction in the case of Cu2+. This modification is leading to significant enhancement of the process efficiency through moderation of the topological properties of the films, as revealed by atomic force microscopy. Next, applying infrared, Raman, and X-ray photoelectron spectroscopy, we presented small amounts of metal (B or Cu) in the final structure of PDA and simultaneously their vital role in the oxidation mechanism and cross-linking through covalent or non-covalent bonds. Therefore, we revealed the possibility of synthesizing films via the expected self-assembly mechanism which has hitherto been out of control. Moreover, modification of mechanical properties toward exceptionally elastic films through the BA-assisted synthesis pathway was shown by achieving Young's modulus value up to 24.1 ± 5.6 and 18.3 ± 6.4 GPa, using nanoindentation and Brillouin light scattering, respectively.

Cascade enzymatic preparation of carboxymethyl chitosan-based multifunctional hydrogels for promoting cutaneous wound healing

Designing wound dressings with inherent multifunctional therapeutic effects is desirable for clinical applications. Herein, a series of multifunctional carboxymethyl chitosan (CMCS)-based hydrogels were fabricated by the facile urate oxidase (UOX)-horseradish peroxidase (HRP) cascade enzymatic crosslinking system. For the first time, the cascade enzymatic crosslinking system was not only used for preparing hydrogel wound dressings but also for accelerating wound healing due to the activity retention of the self-compartmental enzymes. A CMCS derivative (HCMCS-mF) synthesized by successively grafting 4-hydroxybenzaldehyde (H) and 5-methylfurfural (mF) on CMCS and a quaternary ammonium crosslinker (QMal) with terminal grafting maleimide (Mal) groups were combined with enzymatic system for the facile preparation of hydrogels. The mild Diels-Alder (DA) crosslinking reaction between mF and Mal groups constructed the first network of hydrogels. The cascade UOX-HRP system mediated the oxidative crosslinking of phenols thus forming the second gel network. Self-entrapped UOX maintained its enzymatic activity and could continuously catalyze the oxidation of uric acid, generating therapeutic allantoin. These porous, degradable, mechanically stable hydrogels with excellent antioxidant performance and enhanced antibacterial capacity could effectively accelerate skin wound repair by simultaneously reducing oxidative stress, relieving inflammation, promoting collagen deposition and upregulating the expression level of CD31.

Oxidative cross-linking of biocompatible chitosan injectable hydrogel by perylene-dopamine to boost phototoxicity of perylene on in vitro melanoma and breast cancer therapy

This research focused on improving the photodynamic properties of perylene through a hydrogel formulation in which PBI-DOPA cross-links chitosan chains. Photosensitive CS/PBI-DOPA hydrogel was synthesized through an oxidative cross-linking method and exhibited a biocompatible microporous structure capable of shear thinning behavior and outstanding singlet oxygen quantum efficiency. An in vitro comparative study on the phototoxic and anticancer effects of perylene tetracarboxylic dihydride (PTCDA), free PBI-DOPA and its CS/PBI-DOPA hydrogel formulation on skin cancer (A375), breast cancer (MDA- MB-231) and normal fibroblast cells were performed. The hydrogel formula showed its superiority in reducing the survival of cancer cells while minimizing toxicity in normal cells, compared to both free perylene compounds. Therefore, CS/PBI-DOPA hydrogel could be introduced as a promising biocompatible carrier system with significant efficiency for perylene delivery.

Research on the Aging Properties of HTPB Propellant under Vacuum

The space environment has high vacuum, rapid temperature alternations and other extreme conditions, which can affect the energy, ignition rate and ignition performance of solid propellants in the space environment. In order to obtain the space aging performance of HTPB propellant, this paper investigates the aging performance of HTPB propellant in vacuum environment and compares it with atmospheric pressure, and analyses the oxidative cross-linking, degradation chain scission and “dehumidification” behaviour based on the mechanical properties, cross-sectional state and infrared spectra of propellant before and after aging. The research results show that the anhydrous environment under vacuum prevents the occurrence of the chain scission degradation of HTPB propellant, and HTPB propellant is basically only affected by the oxidative cross-linking reaction, where the reaction rate is slow. The mechanical properties show that the breaking strength increases slightly in the first stage, in the later stage of aging, the oxidative cross-linking reaction was intensified due to the decomposition of AP, and the breaking strength increased obviously compared with the initial value. Under the atmosphere, the HTPB propellant was mainly affected by oxidative cross-linking reaction in the early stage of aging, and the breaking strength increased. In the later stage of aging, due to the chain scission degradation of OA moisture absorption, the breaking strength showed an opposite trend and decreased. In addition, an obvious “dehumidification” phenomenon occurs inside the propellant after aging, which is more obvious under a vacuum.

How to Design Catechol-Containing Hydrogels for Cell Encapsulation Despite Catechol Toxicity.

Catechol (cat) is a highly adhesive diphenol that can be chemically grafted to polymers such as chitosan (CH) to make them adhesive as well. However, catechol-containing materials experimentally show a large variability of toxicity, especially in vitro. While it is unclear how this toxicity emerges, most concerns are directed toward the oxidation of catechol into quinone that releases reactive oxygen species (ROS) which can, in turn, cause cell apoptosis through oxidative stress. To better understand the mechanisms at play, we examined the leaching profiles, hydrogen peroxide (H2O2) production, and in vitro cytotoxicity of several cat-chitosan (cat-CH) hydrogels that were prepared with different oxidation levels and cross-linking methods. To create cat-CH with different propensities toward oxidation, we grafted either hydrocaffeic acid (HCA, more prone to oxidation) or dihydrobenzoic acid (DHBA, less prone to oxidation) to the backbone of CH. Hydrogels were cross-linked either covalently, using sodium periodate (NaIO4) to trigger oxidative cross-linking, or physically, using sodium bicarbonate (SHC). While using NaIO4 as a cross-linker increased the oxidation levels of the hydrogels, it also significantly reduced in vitro cytotoxicity, H2O2 production, and catechol and quinone leaching in the media. For all gels tested, cytotoxicity could be directly related to the release of quinones rather than H2O2 production or catechol release, showing that oxidative stress may not be the main reason for catechol cytotoxicity, as other pathways of quinone toxicity come into play. Results also suggest that the indirect cytotoxicity of cat-CH hydrogels fabricated through carbodiimide chemistry can be reduced if (i) catechol groups are chemically bound to the polymer backbone to prevent leaching or (ii) the chosen cat-bearing molecule has a high resistance to oxidation. Coupled with the use of other cross-linking chemistries or more efficient purification methods, these strategies can be adopted to synthesize various types of cytocompatible cat-containing scaffolds.

Air oxidation and hyperbranched crosslinking approach for a boiling water-resistant sucrose-derived wood adhesive

At present, petroleum-based adhesives, which are widely used in the wood adhesive sector, are facing problems such as environmental pollution and harm to humans. It has therefore become crucial to replace traditional petroleum-based adhesives with new biomass-based adhesives. Sucrose, which is widely available in plants and has the advantages of being sustainable and inexpensive, provides the basis for the preparation of biomass adhesives. In this experiment, the preparation of sucrose based hyperbranched polyamine adhesive was achieved by air oxidation and hyperbranched cross-linking strategies. The structure of the sucrose based hyperbranched polyamine adhesive was investigated by FTIR, 13C NMR, XPS, LC-MS, DSC and GPC, and a tightly cross-linked network structure was formed by sucrose oxidation and internal graft copolymerisation of polyamino compounds. The best performing sucrose-based biomass binder was screened by the controlled variable method by varying the sucrose/hyperbranched polyamine molar ratio, reaction time, hydrolysis time and reaction temperature respectively. While the dry shear strength of the plywood reached 2.40 MPa, the wet shear strength in hot water and boiling water at 63 °C reached 1.16 MPa and 1.10 MPa respectively. The experimental procedure is simple, the material is inexpensive and the sucrose based hyperbranched polyamine adhesive produced has excellent properties. This hyperbranched crosslinking strategy provides a potential synthetic approach for the development of biomass-based wood adhesives.

Evolution of lean meat tenderness stimulated by coordinated variation of water status, protein structure and tissue histology during cooking of braised pork

Tenderness of lean meat in braised pork is of great importance to the consumer palatability and acceptance. The influence of water status, protein structure and histological changes on lean meat tenderness during cooking was investigated. Results indicated that lean meat began to tenderize mainly after 20 min-cooking. In the early period of cooking, the decrease of total sulfhydryl content caused the protein oxidative cross-linking, leading to the gradual unfolding of the protein structure, thus resulting in a decrease of T22 and an increase of centrifugal loss, which decreased the tenderness of lean meat. However, after cooking for 20 min, the β-sheet decreased and random coil increased, thus generating conversion between P21 and P22. The rupture of perimysium structure was observed. Changes in protein structure, water status, and tissue histology could facilitate the initiation and development of lean meat tenderness.

Natural Rubber Composites Using Hydrothermally Carbonized Hardwood Waste Biomass as a Partial Reinforcing Filler-Part II: Mechanical, Thermal and Ageing (Chemical) Properties.

Natural rubber composites were reinforced by the co-fillers 'hydrochar' (HC), obtained by hydrothermal carbonization of hardwood sawdust and commercial carbon black (CB). The content of the combined fillers was kept constant while their ratio was varied. The aim was to test the suitability of HC as a partial filler in natural rubber. Due to its larger particle size and hence smaller specific surface area, large amounts of HC reduced the crosslinking density in the composites. On the other hand, due to its unsaturated organic character, HC was found to display interesting chemical effects: if it was used as the exclusive filler component, it displayed a very strong anti-oxidizing effect, which greatly stabilized the rubber composite against oxidative crosslinking (and hence embrittlement). HC also affected the vulcanization kinetics in different ways, depending on the HC/CB ratio. Composites with HC/CB ratios 20/30 and 10/40 displayed interesting chemical stabilization in combination with fairly good mechanical properties. The performed analyses included vulcanization kinetics, tensile properties, determination of density of permanent and reversible crosslinking in dry and swollen states, chemical stability tests including TGA, thermo-oxidative aging tests in air at 180 °C, simulated weathering in real use conditions ('Florida test'), and thermo-mechanical analyses of degraded samples. Generally, the results indicate that HC could be a promising filler material due to its specific reactivity.

Study on thermal aging properties of PBT energetic elastomer

AbstractAccelerated aging experiments of 3,3‐bis(azidomethyl)oxetane (BAMO) and tetrahydrofuran (THF) copolymers (molar ratio 1 : 1, marked as PBT prepolymer) and PBT elastomer at elevated temperatures were performed to explore the aging mechanism of PBT energetic elastomer during long‐term storage. The evolution of functional groups, plasticizer content, element chemical state and crosslinking density were studied by FT‐IR, HPLC, XPS and swelling method, respectively. Results show that: (1) Oxidation of terminal hydroxyl, oxidative crosslinking between binder macromolecules, chain breaking of polymers and degradation of −CH2N3 occurred during the thermal aging of PBT prepolymer, and antioxidant (BHT) could effectively delay the thermal aging process of PBT prepolymer by absorbing the active oxidation substances. (2) The relative contents of −C=O showed a cubic function with time, and the relative contents of −NO2 and −N3 changed linearly with time during the thermal aging of PBT elastomer. The relative contents of BDNPF and BDNPA declined linearly with time, and the thermal stability of BDNPF was better than that of BDNPA. (3) The thermal aging of PBT elastomer experienced three main reaction stages: degradation of carbamate, the oxidative crosslinking of molecular chain caused by the decomposition products of BDNPF/BDNPA, and chain breaking of the polymer system. In addition, a small amount of −CH2N3 decomposed throughout the whole aging process.

Discovery and Biosynthesis of Cihunamides, Macrocyclic Antibacterial RiPPs with a Unique C-N Linkage Formed by CYP450 Catalysis.

Cihunamides A-D (1-4), novel antibacterial RiPPs, were isolated from volcanic-island-derived Streptomyces sp. The structures of 1-4 were elucidated by 1 H, 13 C, and 15 N NMR, MS, and chemical derivatization; they contain a tetrapeptide core composed of WNIW, cyclized by a unique C-N linkage between two Trp units. Genome mining of the producer strain revealed two biosynthetic genes encoding a cytochrome P450 enzyme and a precursor peptide. Heterologous co-expression of the core genes demonstrated the biosynthesis of cihunamides through P450-mediated oxidative Trp-Trp cross-linking. Further bioinformatic analysis uncovered 252 homologous gene clusters, including that of tryptorubins, which possess a distinct Trp-Trp linkage. Cihunamides do not display the non-canonical atropisomerism shown in tryptorubins, which are the founding members of the "atropitide" family. Therefore, we propose to use a new RiPP family name, "bitryptides", for cihunamides, tryptorubins, and their congeners, wherein the Trp-Trp linkages define the structural class rather than non-canonical atropisomerism.