Cross-linking Of Side Chains Research Articles

An n-Doping Cross-Linkable Quinoidal Compound as an Electron Transport Material for Fully Stretchable Inverted Organic Solar Cells.

The photocatalytic activity and inherent brittleness of ZnO, which is commonly used as an electron transport layer (ETL) in inverted organic solar cells (OSCs), have impeded advancements in device stability and the development of fully stretchable OSCs. In this study, an intrinsically stretchable ETL for inverted OSCs through a side-chain cross-linking strategy has been developed. Specifically, cross-linking between bromine atoms on the side chains of a quinoidal compound and the amino groups in polyethylenimine resulted in a film, designated QBr-PEI-50, with high electrical conductivity (0.049 S/m) and excellent stretchability (crack-onset strain>45 %). When used as the ETL in inverted OSCs, QBr-PEI-50 was markedly superior to ZnO in terms of device performance and stability, yielding a power conversion efficiency (PCE) of 18.27 % and a T80 lifetime exceeding 10000 h. Moreover, incorporation of QBr-PEI-50 in fully stretchable inverted OSCs yielded a PCE of 14.01 %, and 80 % of the initial PCE was maintained after 21 % strain, showcasing its potential for wearable electronics.

Pyrolysis mechanism of silicone rubber thermal protection system materials in service environment

The excellent oxidation and ablation resistance of silicone rubber thermal protection system (TPS) materials have made them extensively utilized in large-area thermal protection applications, such as ramjet engines and spacecraft reentry capsules, where air is present in the service environment. The ablation resistance is significantly influenced by the pyrolysis reactions, which serves as the foundation for subsequent ceramic transformation and the development of anti-erosion structures. The majority of previous research has been conducted in inert gas environments. To investigate the pyrolysis mechanism of silicone rubber TPS materials in realistic service environments, thermal analysis tests were performed using a range of analytical instruments including a thermal analyzer, mass spectrometer, infrared spectrometer, and X-ray photoelectron spectrometer from room temperature up to 1300 K in air atmosphere and compared with the results in argon atmosphere. The results demonstrate that silicone rubber TPS materials undergo both pyrolysis and oxidation reactions in air atmosphere. The primary pyrolysis of the matrix is attributed to cyclization reactions and side-chain cross-linking reactions. In comparison to argon atmosphere, the oxidation reaction produces a greater amount of organic gases, thereby diminishing the extent of cross-linking reaction and the thermal stability of the pyrolysis residue. The prolonged exposure to elevated temperatures allows for an extended duration of oxidation reactions, consequently diminishing the long-term thermal stability of TPS materials. The increase in mass of TPS materials at temperatures exceeding 1100 K can be attributed, in part, to the formation of new organic groups through oxidation reactions. The observed morphology of the residue, along with the weight gain from oxidation and SiO2 formation, promotes ceramic transformation of the materials at elevated temperatures.

High-Performance Monovalent Selective Cation Exchange Membranes with Ionically Cross-Linkable Side Chains: Effect of the Acidic Groups.

Inspired by the charge-governed protein channels located in the cell membrane, a series of polyether ether ketone-based polymers with side chains containing ionically cross-linkable quaternary ammonium groups and acidic groups have been designed and synthesized to prepare monovalent cation-selective membranes (MCEMs). Three acidic groups (sulfonic acid, carboxylic acid, and phenolic hydroxyl) with different acid dissociation constant (pKa) were selected to form the ionic cross-linking structure with quaternary ammonium groups in the membranes. The ionic cross-linking induced the nanophase separation and constructed ionic channels, which resulted in excellent mechanical performance and high cation fluxes. Interesting, the cation flux of membranes increased as the ionization of acidic groups increase, but the selectivity of MCEMs did not follow the same trend, which was mainly dependent on the affinity between the functional groups and the cations. Carboxyl group-containing MCEMs exhibited the best selectivity (9.01 for Li+/Mg2+), which was higher than that of the commercial monovalent cation-selective CIMS membrane. Therefore, it is possible to prepare stable MCEMs through a simple process using ionically cross-linkable polymers, and tuning acidic groups in the membranes provided an attractive approach to improving the cation flux and selectivity of MCEMs.

Crosslinker Architectures Impact Viscoelasticity in Dynamic Covalent Hydrogels.

Dynamic covalent crosslinked (DCC) hydrogels represent a significant advance in biomaterials for regenerative medicine and mechanobiology. These gels typically offer viscoelasticity and self-healing properties that more closely mimic in vivo tissue mechanics than traditional, predominantly elastic, covalent crosslinked hydrogels. Despite their promise, the effects of varying crosslinker architecture - side chain versus telechelic crosslinks - on the viscoelastic properties of DCC hydrogels have not been thoroughly investigated. This study introduces hydrazone-based alginate hydrogels and examines how side-chain and telechelic crosslinker architectures impact hydrogel viscoelasticity and stiffness. In hydrogels with side-chain crosslinking (SCX), higher polymer concentrations enhance stiffness and decelerates stress relaxation, while an off-stoichiometric hydrazine-to-aldehyde ratio leads to reduced stiffness and shorter relaxation time. In hydrogels with telechelic crosslinking, maximal stiffness and slowest stress relaxation occurs at intermediate crosslinker concentrations for both linear and star crosslinkers, with higher crosslinker valency further increasing stiffness and relaxation time. Our result suggested different ranges of stiffness and stress relaxation are accessible with the different crosslinker architectures, with SCX hydrogels leading to slower stress relaxation relative to the other architectures, and hydrogels with star crosslinking (SX) providing increased stiffness and slower stress relaxation relative to hydrogels with linear crosslinking (LX). The mechanical properties of SX hydrogels are more robust to changes induced by competing chemical reactions compared to LX hydrogels. Our research underscores the pivotal role of crosslinker architecture in defining hydrogel stiffness and viscoelasticity, providing crucial insights for the design of DCC hydrogels with tailored mechanical properties for specific biomedical applications.

Structural Insights into a Side Chain Cross-Linking Biarylitide P450 from RiPP Biosynthesis

Structural Insights into a Side Chain Cross-Linking Biarylitide P450 from RiPP Biosynthesis

Two-step construction of KPDMS/Al2O3 ultra-barriers for wearable sensors.

Wearable devices hold significant potential in healthcare and medical diagnostics. One major challenge in realizing this potential is the low barrier property of polymer substrates, which fail to withstand surrounding moisture and biofluids. In this work, a two-step strategy involving ALI followed by a UV-curing process is developed to fabricate a K48PDMS/Al2O3 ultra-barrier with high barrier property of 7.82 × 10-5 g m-2 day-1 under stretching strain, representing one of the highest values among current works. Moreover, the K48PDMS/Al2O3 barrier enables Ca-tested devices to exhibit extended operational lifetimes of up to 12 days in simulated rain. It also ensures the high sensitivity of strain sensors for real-time monitoring of health-related physiological signals, even when exposed to aggressive solutions such as PBS, KOH, and glucose. A clear "filling-cross-linking" mechanism is revealed, involving the filling of the void spaces within polymer chains followed by the cross-linking of polymer side chains to enhance the density of the hybrid layer. Adjusting porosity and functional group density ensures complete Al2O3 infiltration into the polymer. The cross-linking increases from 12.31% to 73.79% compared to the UV-curing process alone due to the presence of Al2O3, further enhancing the density of the hybrid layer and its barrier properties. The proposed strategy in our work shows great potential for providing highly reliable encapsulation for wearable electronics.

Molecular Simulation Strategies for Understanding the Degradation Mechanisms of Acrylic Polymers.

Acrylic polymers, commonly used in paints, can degrade over time by several different chemical and physical mechanisms, depending on structure and exposure conditions. While exposure to UV light and temperature results in irreversible chemical damage, acrylic paint surfaces in museums can also accumulate pollutants, such as volatile organic compounds (VOCs) and moisture, that affect their material properties and stability. In this work, we studied the effects of different degradation mechanisms and agents on properties of acrylic polymers found in artists' acrylic paints for the first time using atomistic molecular dynamics simulations. Through the use of enhanced sampling methods, we investigated how pollutants are absorbed into thin acrylic polymer films from the environment around the glass transition temperature. Our simulations suggest that the absorption of VOCs is favorable (-4 to -7 kJ/mol depending on VOCs), and the pollutants can easily diffuse and be emitted back into the environment slightly above glass transition temperature when the polymer is soft. However, typical environmental fluctuations in temperature (<16 °C) can lead for these acrylic polymers to transition to glassy state, in which case the trapped pollutants act as plasticizers and cause a loss of mechanical stability in the material. This type of degradation results in disruption of polymer morphology, which we investigate through calculation of structural and mechanical properties. In addition, we also investigate the effects of chemical damage, such as backbone bond scission and side-chain cross-linking reactions on polymer properties.

Study on the Change Behavior of Fluorosilicone Rubber in RP-3 Kerosene

Fluorosilicone rubber has excellent high temperature oil resistance and has been widely used in high temperature oil media. The performance of fluorosilicone rubber will degrade with the length of oil immersion time. In this paper, through oil resistance test, hardness test, tensile test, infrared analysis, differential scanning calorimetry analysis and thermal weight loss analysis, the performance, structure and composition changes of fluorosilicone rubber FS6265 at 150°C and RP3 kerosene soaking process are studied. The results show that in 150°C, RP3 kerosene, the mass and volume of fluorosilicone rubber FS 6265 first increase and then decrease. The hardness and tensile strength gradually decrease with the increase of immersion time, the elongation at break tends to increase slightly and then decrease. The performance changes are mainly caused by swelling, filler migration, side chain rupture and cross-linking damage. The swelling in the early stage of immersion plays a major role, and the filler migration, side chain and cross-linking failure play a major role in the later stage.

Helical Foldamers and Stapled Peptides as New Modalities in Drug Discovery: Modulators of Protein-Protein Interactions

A “foldamer” is an artificial oligomeric molecule with a regular secondary or tertiary structure consisting of various building blocks. A “stapled peptide” is a peptide with stabilized secondary structures, in particular, helical structures by intramolecular covalent side-chain cross-linking. Helical foldamers and stapled peptides are potential drug candidates that can target protein-protein interactions because they enable multipoint molecular recognition, which is difficult to achieve with low-molecular-weight compounds. This mini-review describes a variety of peptide-based foldamers and stapled peptides with a view to their applications in drug discovery, including our recent progress.

The Effect of Cross-Linking Type on EPDM Elastomer Dynamics and Mechanical Properties: A Molecular Dynamics Simulation Study.

The cross-linking structure of the Ethylene-propylene-diene monomer (EPDM) is made up of a number of cross-linking types, including carbon atoms from the main chain or monomer and ether crosslinks formed during degradation. Through molecular dynamic simulations, the contribution of each type of cross-linked structure to the dynamics and mechanical properties of EPDM, the study’s focus, were investigated. Cross-linking between the tertiary carbons of two main chains, cross-linking at the monomer’s unsaturated position, ether cross-linking after oxidation, and other combinations of target cross-linked carbon atoms from different positions, totaling eight types of cross-linked types, were mixed with EPDM free chains in a 1:1 ratio to form eight types of cross-linked EPDMs. These varieties of cross-linked EPDMs were then compared to an uncross-linked EPDM in terms of density, radius of gyration, free volume, mean square displacement, and uniaxial tensile stress-strain curves. It was found that the cross-linking was always proven to have a favorable influence on mechanical characteristics; however, the relaxation inhibition effect varied. The cross-linking between the diene monomer at the C9 position resulted in a more flexible molecular shape and was more than double the free volume of the uncross-linked EPDM, resulting in an improved diffusion ability. The ether cross-linking produced by the oxidation of the side chain cross-linking improved the positive contribution to stiffness and enhanced the inhibitory impact on diffusion properties, whereas the main chain cross-linking had the opposite effect. The research presented in this study leads to a better knowledge of the microscopic aspects underlying EPDM performance.

Neutralizing SARS-CoV-2 by dimeric side chain-to-side chain cross-linked ACE2 peptide mimetics.

We present the finding of a dimeric ACE2 peptide mimetic designed through side chain cross-linking and covalent dimerization. It has a binding affinity of 16 nM for the SARS-CoV-2 spike RBD, and effectively inhibits the SARS-CoV-2 pseudovirus in Huh7-hACE2 cells with an IC50 of 190 nM and neutralizes the authentic SARS-CoV-2 in Caco2 cells with an IC50 of 2.4 μM. Our study should provide a new insight for the optimization of peptide-based anti-SARS-CoV-2 inhibitors.

Coupled effects of temperature and compressive strain on aging of silicone rubber foam

The coupled effects of temperature and compressive strain on aging behaviors and mechanisms of silicone rubber foam were investigated by conducting the accelerated aging tests. The silicone rubber foams subjected to various levels of temperature and compressive strain were characterized by Attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy, micro hardness, volume fraction, compression set, continuous stress relaxation, mechanical properties, and thermogravimetric analysis (TGA) etc. The ATR-FTIR results indicate that the complex oxidation reactions (crosslinking, chain scission, and oxidation of side chains) occurred simultaneously with the formation of oxidation products. The high temperature facilitated the increase in crosslinking density and enhanced the thermal stability of the foams, implying that the oxidation crosslinking predominated over chain scission leading to the formation of a denser network structure. The physical and mechanical properties of the foams underwent significant changes due to the oxidation reactions and the irreversible collapse of the porous microstructure. The elevated temperature and the compressive stress both have a direct effect on the degradation of the foam. The elevated temperature played a more important role in changes of the physical-mechanical properties and microstructure.

Helical Stabilization of Peptide Macrocycles by Stapled Architectures.

Over the past two decades, significant efforts have invested in the development of strategies for the stabilization of macrocyclic peptides with α-helix structure by stapling their architectures. These strategies can be divided into two categories: side chain to side chain cross-linking and N-terminal helix nucleation. These stable macrocyclic peptides have been applied in PPI inhibitors and self-assembly materials. Compared with unmodified short peptides, stable α-helix macrocyclic polypeptides have better biophysical properties including higher serum stability, cell permeability, and higher target affinity. This chapter will systematically introduce approaches for helical stabilization of peptide macrocycles, such as ring-closing metathesis (RCM), lactamisation, cycloadditions, reversible reactions, thioether formation as well as newly found sulfonium center formation and the common use of helical stabilized macrocyclic peptides.

Effect of oxidative modification by reactive oxygen species (ROS) on the aggregation of whey protein concentrate (WPC)

The effects of reactive oxygen species (ROS) on the oxidative aggregation of whey protein concentrate (WPC) were studied. Increasing the oxidation time resulted in the accumulation of carbonyl groups, a decrease in α-helix structures, an increase in disordered structures and the aggregation of secondary structures. The surface hydrophobicity decreased and the zeta potential increased at the beginning of oxidation, indicating that electrostatic interactions were the cause of the noncovalent aggregation of the whey protein. Covalent aggregates were examined in terms of the sulfhydryl/disulfide content (total SH/SS), molecular weight and carbonyls, with the results implying that covalent interactions included protein-protein cross-linking, carbonyl cross-linking, and aromatic side chain cross-linking but excluded the cross-linking of sulfur-containing amino acid residues. The solubility and stability of initial aggregations were high due to flexible structures, while upon further oxidation, the solubility and stability worsened, consist with the soluble protein content and stability index.

Semi-Natural Superabsorbents Based on Starch-g-poly(acrylic acid): Modification, Synthesis and Application.

Biopolymer-based superabsorbent polymers (SAPs) are being synthesized and investigated as a biodegradable alternative for an entirely synthetic SAPs, particularly those based on acrylic acid and its derivatives. This article focuses on the chemical modification of starch (S), and synthesis of new potentially biodegradable polymers using acrylic acid (AA) as side chain monomer and crosslinking mediator together with N,N’-methylenebisacrylamide (MBA). The graft co-polymerization was initiated by ceric ammonium nitrate (CAN) or potassium persulfate (KPS), leading to different reaction mechanisms. For each of the initiators, three different synthetic routes were applied. The structures of new bio-based SAPs were characterized by means of IR spectroscopy. Thermogravimetric measurements were made to test the thermal stability, and morphology of the samples were examined using scanning electron microscopy (SEM). Physico-chemical measurements were performed to characterize properties of new materials such as swelling characteristics. The water absorption capacity of resulting hydrogels was measured in distilled water and 0.9% NaCl solution.

Effects of oxidative modification on the functional, conformational and gelling properties of myofibrillar proteins from Culter alburnus

Protein oxidation is a critical process in the deterioration and spoilage of fish and related commodities during processing and storage. In this study, the hydroxyl radical generation system (HRGS) was used to simulate the effect of oxidation on the functional, conformational and gelling properties of topmouth culter (Culter alburnus) myofibrillar proteins (MP). Additionally, the effects of oxidation on the gel-forming abilities of MP were also systematically analyzed from the perspective of intermolecular interaction forces. Oxidation was shown to decrease the total sulfhydryl content, increase the surface hydrophobicity, and induce conformational changes in MP. Rheological analysis showed that oxidation reduced the gel strength. Water holding capacity (WHC) and low-field nuclear magnetic resonance (LF-NMR) analyses showed that low oxidation could enhance water binding of protein matrix, while high-degree oxidation could substantially reduce the gelling properties of MP. The selective solubility of MP gel proved that oxidation could reduce the content of ionic and hydrogen bonds and increase hydrophobic interactions. All the results indicate that oxidation could alter the intermolecular interactions between protein-protein and protein-water molecules, due to irregular unfolding and inhibition of the cross-linking of amino acid side chains, leading to reduction in the quality and function of fish and related products.

Pasting properties of hydrothermally treated maize starch with added stearic acid

The effects of stearic acid addition followed by hydrothermal treatment on the functional properties of maize starch were studied. Addition of stearic acid followed by hydrothermal treatment resulted in non-gelling starch. Starch with stearic acid had significantly (P < 0.05) higher viscosity as compared to heat-moisture treated starch. There was no significant difference on the pasting properties of starch with stearic acid alone and in combination with annealing. Stearic acid addition followed by heat-moisture treatment significantly reduced starch susceptibility to acid hydrolysis as compared to stearic acid alone and heat-moisture treatment alone. These changes related well with the increased amylose lipid complexes and relative crystallinity observed by the DSC and XRD, suggesting that heat-moisture treatment promoted amylopectin side chain crosslinking and amylose-stearic acid complex formation. Stearic acid addition followed by hydrothermal treatment produced a ‘clean label’ starch that can potentially substitute chemically cross-linked and non-gelling starch in the food industry.

Ultrahigh oxygen permeability of chemically-modified membranes of novel (co)polyacetylenes having a photodegradative backbone and crosslinkable side chains

Four new partly modified membranes of novel (co)polyacetylenes having a photodegradative backbone and crosslinkable side chains showed excellent performances as oxygen permselective membranes, that is, ultrahigh oxygen permeability (PO2) with relatively high oxygen permselectivity (α = PO2/PN2). In α vs. PO2 plots, two of them showed a performance close to Robeson's upper bound 2008 and the other two exceeded Robeson's upper bound 1991. The combination of the rigid main chain of the (co)polymers, the regular structure of the supramolecular polymer, and the flexible crosslinking structure may have produced these excellent performances.

Trypanothione reductase inhibition and anti-leishmanial activity of all-hydrocarbon stapled α-helical peptides with improved proteolytic stability

Trypanothione reductase (TryR) is a well-established target in the search for novel antitrypanosomal and antileishmanial agents. We have previously identified linear and lactam-bridged 13-residue peptides derived from an α-helical region making up part of the dimeric interface of Leishmania infantum TryR (Li-TryR) which prevent trypanothione reduction by disrupting enzyme dimerization. We now show that i,i + 4 side-chain cross-linking with an all-hydrocarbon staple stabilizes the helical structure of these peptides and significantly improves their resistance to protease cleavage relative to previous linear and cyclic lactam analogues. Interestingly, replacement of the amide bridge by the hydrocarbon staple at the same cyclization positions generates derivatives (2 and 3) that similarly inhibit oxidoreductase activity of the enzyme but unexpectedly stabilize the TryR homodimer. The most proteolytically stable peptide 2 covalently linked to oligoarginines displayed potent in vitro leishmanicidal activity against L. infantum parasites.

Enzymatic Cross-Linking of Side Chains Generates a Modified Peptide with Four Hairpin-like Bicyclic Repeats

Macrocyclization of peptides is often employed to generate novel structures and biological activities in the biosynthesis of natural products and drug discovery. The enzymatic cross-linking of two side chains in a peptide via an ester or amide has a high potential for making topologically diverse cyclic peptides but is found with only a single consensus sequence in the microviridin class of natural products. Here, we report that a peptide with a new sequence pattern can be enzymatically cross-linked to make a novel microviridin-like peptide, plesiocin, which contains four repeats of a distinct hairpin-like bicyclic structure and shows strong inhibition of proteases. A single ATP-grasp enzyme binds to a leader peptide, of which only 13 residues are required for binding, and performs eight esterification reactions on the core peptide. We also demonstrate that the combination of tandem mass spectrometry and an ester-specific reaction greatly facilitates the determination of connectivity. We suggest that the enzymatic cross-linking of peptide side chains can generate more diverse structures in nature or by engineering.