Dissociation Of Chains Research Articles

Small dop of comonomer, giant shift of dynamics: α-methyl-regulated viscoelasticity of poly(methacrylamide) hydrogels

α-Methyl groups play significant roles in the regulation of water molecules within both small molecular systems and bio-macromolecular systems. Systematically studying the influence of α-methyl on the dynamics of water molecules within hydrogel systems is therefore worthwhile. In this study, we prepared a series of hydrogen-bonded (H-bonded) hydrogels with varying densities of α-methyl groups by copolymerizing methacrylamide (MAm) with its α-methyl-absent analogue, acrylamide (Am). Introducing a small amount of Am (≤6 mol%) into the polymer chain resulted in significant shifts in the viscoelasticity of the hydrogels. The hydrogels exhibit a “time-temperature-α-methyl equivalence”, meaning that introduction of α-methyl-absent monomer has effects similar to elevating temperature and prolonging observation time on the dynamic properties. Based on low-field nuclear magnetic resonance spectroscopy and Raman scattering, a “hydrophilic defects-assisted H-bonds dissociation” mechanism is proposed, depicting that the α-methyl-absent monomer can disturb the rearrangement of water molecules surrounding the polymer chain and accelerate chain dissociation. These findings enabled the copolymer hydrogels with functions such as fast self-healing and tunable interface adhesion.

Mechanistic basis of the dynamic response of TWIK1 ionic selectivity to pH

Highly selective for K+ at neutral pH, the TWIK1 channel becomes permeable to Na+ upon acidification. Using molecular dynamics simulations, we identify a network of residues involved in this unique property. Between the open and closed states previously observed by electron microscopy, molecular dynamics simulations show that the channel undergoes conformational changes between pH 7.5–6 involving residues His122, Glu235, Lys246 and Phe109. A complex network of interactions surrounding the selectivity filter at high pH transforms into a simple set of stronger interactions at low pH. In particular, His122 protonated by acidification moves away from Lys246 and engages in a salt bridge with Glu235. In addition, stacking interactions between Phe109 and His122, which stabilize the selectivity filter in its K+-selective state at high pH, disappear upon acidification. This leads to dissociation of the Phe109 aromatic side chain from this network, resulting in the Na+-permeable conformation of the channel.

Gamma Irradiation Effect on Polymeric Chains of Epoxy Adhesive.

The study of materials for space exploration is one of the most interesting targets of international space agencies. An essential tool for realizing light junctions is epoxy adhesive (EA), which provides an elastic and robust material with a complex mesh of polymeric chains and crosslinks. In this work, a study of the structural and chemical modification of a commercial two-part flexible EA (3M™ Scotch-Weld™ EC-2216 B/A Gray), induced by 60Co gamma radiation, is presented. Combining different spectroscopic techniques, such as the spectroscopic Fourier transform infrared spectroscopy (FTIR), the THz time-domain spectroscopy (TDS), and the electron paramagnetic resonance (EPR), a characterization of the EA response in different regions of the electromagnetic spectrum is performed, providing valuable information about the structural and chemical properties of the polymers before and after irradiation. A simultaneous dissociation of polymeric chain and crosslinking formation is observed.The polymer is not subject to structural modification at an absorbed dose of 10 kGy, in which only transient free radicals are observed. Differently, between 100 and 500 kGy, a gradual chemical degradation of the samples is observed together with a broad and long-living EPR signal appearance. This study also provides a microscopic characterization of the material useful for the mechanism evaluation of system degradation.

Establishing superfine nanofibrils for robust polyelectrolyte artificial spider silk and powerful artificial muscles

Spider silk exhibits an excellent combination of high strength and toughness, which originates from the hierarchical self-assembled structure of spidroin during fiber spinning. In this work, superfine nanofibrils are established in polyelectrolyte artificial spider silk by optimizing the flexibility of polymer chains, which exhibits combination of breaking strength and toughness ranging from 1.83 GPa and 238 MJ m−3 to 0.53 GPa and 700 MJ m−3, respectively. This is achieved by introducing ions to control the dissociation of polymer chains and evaporation-induced self-assembly under external stress. In addition, the artificial spider silk possesses thermally-driven supercontraction ability. This work provides inspiration for the design of high-performance fiber materials.

Investigation of lubrication mechanism of phosphate ionic liquid by ReaxFF molecular dynamics simulations

The frictional characteristics of the interface between ionic liquid (IL) and metal can be significantly improved by the formation of a tribo-film, while the lubrication mechanism remains unclear. This study utilized reactive force field molecular dynamics (ReaxFF MD) simulation to investigate the tribological properties, interfacial tribochemical reactions, and formation mechanism of the tributyltetradecyl phosphonium di(2-ethylhexyl) phosphate ([P44414] [DEHP]) tribo-film. The findings indicate that the friction coefficients of ILs decrease as the normal pressure increases. The combination of normal pressure and shear force promotes the dissociation of ionic liquid chains and enhances the binding of free radicals to the end faces. The friction force is primarily influenced by the number of Fe-C bonds at the interface, which demonstrates a positive correlation with the normal pressure.

Effect of the roughness on the photoinduced growth of crystalline tellurium on MoTe2 matrix

In this work, a series of 5 nm thick films of molybdenum oxide (MoO3) were synthesized on SiO2/Si substrates by magnetron sputtering of molybdenum in low vacuum conditions. We used closed-space sublimation (CSS) to tellurize the Mo oxide and to produce pure-phase 1 T′-MoTe2 films by controlling the temperature inside the CSS reactor. Surface analyses showed that tellurium spreads over the surface of the as-grown films, forming localized crystalline precipitates whose distribution depends on the tellularization time. From Raman spectroscopy investigations, we observed that the exposure of these films to visible light leads to a growth of a highly-ordered trigonal phase of tellurium. The intensity evolution of the vibrational modes related to the crystalline phase of tellurium allows us to perform a quantitative analysis of the crystallization. Our results indicate that this process is consistent with the Kolmogorov-Johnson-Mehl-Avrami (KJMA) theory for the overall crystallization process. By the characteristic growth exponent, we introduce a more specific model which takes into account the diffusion and the dissociation of tellurium chains, which is used to probe how the effective growth rate is affected by the roughness of the samples.

Fate of Magnetic Nanoparticles during Stimulated Healing of Thermoplastic Elastomers.

We have investigated the heating mechanism in industrially relevant, multi-block copolymers filled with Fe nanoparticles and subjected to an oscillatory magnetic field that enables polymer healing in a contactless manner. While this procedure aims to extend the lifetime of a wide range of thermoplastic polymers, repeated or prolonged stimulus healing is likely to modify their structure, mechanics, and ability to heat, which must therefore be characterized in depth. In particular, our work sheds light on the physical origin of the secondary heating mechanism detected in soft systems subjected to magnetic hyperthermia and triggered by copolymer chain dissociation. In spite of earlier observations, the origin of this additional heating remained unclear. By using both static and dynamic X-ray scattering methods (small-angle X-ray scattering and X-ray photon correlation spectroscopy, respectively), we demonstrate that beyond magnetic hysteresis losses, the enormous drop of viscosity at the polymer melting temperature enables motion of nanoparticles that generates additional heat through friction. Additionally, we show that applying induction heating for a few minutes is found to magnetize the nanoparticles, which causes them to align in dipolar chains and leads to nonmonotonic translational dynamics. By extrapolating these observations to rotational dynamics and the corresponding amount of heat generated through friction, we not only clarify the origin of the secondary heating mechanism but also rationalize the presence of a possible temperature maximum observed during induction heating.

Experimental study of the effect of temperature on collagen conformational changes in skin tissue welded by femtosecond laser

In this study, in vitro skin tissues were welded using various laser settings at four temperature ranges, 30–40 °C, 40–60 °C, 60–80 °C, and > 80 °C., Raman spectra between 500 and 2000 cm−1 were examined in order to examine the relationship between the changes of collagen and the mechanical properties of skin tissue bonding and the degree of thermal damage of tissue. How collagen in skin tissues changed into disordered and less stable structures due to the thermal damage, and how the structure of protein and amino acid-related bands changed with increasing temperature were analyzed. Analysis of the deconvolution of the amide I band (1580–1720 cm−1) and the secondary structure of collagen showed that the secondary structure of collagen changed from α-like helix-dominated to β-aggregate-dominated structures due to the thermal damage. As the temperature increased, the number of protein molecule chain dissociation and reconnection within the skin tissue increased, which contributed to the improvement of the tensile strength of the skin after welding. However, when the temperature was too high, the collagen structure presented an extremely unstable state, and the degree of protein denaturation increased, causing serious thermal damage. When the skin tissue was welded using the femtosecond laser process parameters of laser power of 18 W, scanning speed of 50 mm/s, and scanning 50 times, the tensile strength of the in vitro skin tissue incision was greater and the degree of protein denaturation was less. The degree of thermal damage characterized by the degree of protein denaturation was consistent with that characterized by the microstructure texture characteristics parameters of the skin tissue.

The exploration of the inverse vulcanization mechanism of tung oil by controlling the oxygen and moisture presence during reactions

Inverse vulcanization is a cost-effective method for producing high sulfur-content copolymers by combining elemental sulfur with organic monomers, which has rapidly gained popularity due to its simplicity of synthesis and wide range of applications. Although numerous examples of sulfur-rich copolymers have been synthesized at different reaction rates and temperatures using various monomers, the precise reaction mechanism remains unclear. In this paper, we used tung oil containing conjugated triene as a monomer to synthesize sulfur-rich copolymers under six different reaction conditions and investigate the effects of oxygen and moisture. Our study, which employed DSC, XRD, 1H NMR, and XPS characterization methods, revealed that oxygen accelerated the reaction rate and decreased the free sulfur content of the products, while moisture shortened the gel times but increased the free sulfur content. These findings confirm that reverse vulcanization involves two simultaneous mechanisms: the free radical mechanism and the anion mechanism. With regard to the radical mechanism, we discuss the source, ease, and reactivity of radicals and show that the creation of radicals depends on the second monomer rather than sulfur. Tung oil not only acts as a comonomer in the reaction but also plays an initiating role in promoting the dissociation of sulfur chains to generate free radicals for addition of non-conjugated double bonds. The effect of the anion mechanism exceeds that of the radical mechanism once certain factors are stimulated, such as the presence of metal ions and sulfide ions. Understanding the detailed mechanisms involved in inverse vulcanization is essential for selecting optimal monomers, which can enhance not only the synthesis process but also the properties of sulfur-rich materials.

The α-Synuclein Monomer May Have Different Misfolding Mechanisms in the Induction of α-Synuclein Fibrils with Different Polymorphs

The aggregation of alpha-synuclein (α-Syn) is closely related to the occurrence of some neurodegenerative diseases such as Parkinson's disease. The misfolding of α-Syn monomer plays a key role in the formation of aggregates and extension of fibril. However, the misfolding mechanism of α-Syn remains elusive. Here, three different α-Syn fibrils (isolated from a diseased human brain, generated by in vitro cofactor-tau induction, and obtained by in vitro cofactor-free induction) were selected for the study. The misfolding mechanisms of α-Syn were uncovered by studying the dissociation of the boundary chains based on the conventional molecular dynamics (MD) and Steered MD simulations. The results showed that the dissociation paths of the boundary chains in the three systems were different. According to the reverse process of dissociation, we concluded that in the human brain system, the binding of the monomer and template starts from the C-terminal and gradually misfolds toward the N-terminal. In the cofactor-tau system, the monomer binding starts from residues 58-66 (contain β3), followed by the C-terminal coil (residues 67-79). Then, the N-terminal coil (residues 36-41) and residues 50-57 (contain β2) bind to the template, followed by residues 42-49 (contain β1). In the cofactor-free system, two misfolding paths were found. One is that the monomer binds to the N/C-terminal (β1/β6) and then binds to the remaining residues. The other one is that the monomer binds sequentially from the C- to N-terminal, similar to the human brain system. Furthermore, in the human brain and cofactor-tau systems, electrostatic interactions (especially from residues 58-66) are the main driving force during the misfolding process, whereas in the cofactor-free system, the contributions of electrostatic and van der Waals interactions are comparable. These results may provide a deeper understanding for the misfolding and aggregation mechanism of α-Syn.

Insights into the Fischer–Tropsch mechanism on χ-Fe5C2(510) based on the hydrogen coverage effect

Investigating the mechanism of the Fischer–Tropsch synthesis is beneficial for modifying catalysts, which would result in an improved catalyst performance. However, theoretical calculations have mainly focused on several key elementary reactions and have thus provided fragmented information. Here, we select χ-Fe5C2(510) as representative facet and consider the elementary reactions on as comprehensively as possible via the density-functional theory (DFT)-based kinetic Monte Carlo (kMC) method to generate the fine structure of the Fischer–Tropsch mechanism. The results reveal that the CO activation mechanism is divided into cyclical pathways and termination pathways. The chain growth process and chain dissociation process coexist in the Fischer-Tropsch synthesis while the former follows the CO insertion mechanism, and the latter follows the carbide mechanism. The product methane and ethylene are produced major from the intermediates which are generated in the chain dissociation process rather than the chain grown process as the article reported before. The deep interpretation of the Fischer–Tropsch mechanism from the fine structure could be the guidance to experimentalists in mechanism verification or catalyst design.

A statistical-chain-based theory for dynamic living polymeric gels with concurrent diffusion, chain remodeling reactions and deformation

Different from natural living systems that constantly evolve to adapt to the changing environment, most engineering materials are static in both form and function. The living characters of natural systems stem from a malleable building block that mainly consists of a dynamic macromolecular network and a liquid solution. To resemble the dynamic features of living materials, researchers started to incorporate various chain reactions such as new chain formation, chain dissociation and association, monomer insertion or extraction into polymer and polymeric gel designs. Through these systems, a range of living functions such as growth, self-healing and degradation have been realized. In this work, building from a statistical chain description, we formulate a non-equilibrium thermodynamic framework that couples various chain reactions, diffusion and deformation for the living gel systems. We first calibrate the model using the experimental data in literature on the growth of a living gel. We next investigate the evolution processes of the living gels in both open and closed systems. Using the model, we then study the structure–property relations of the dynamic living gels, which is useful in material design. At the end, we discuss how the different reaction rate and diffusion rate give rise to different morphology and mechanical properties of the living gels.

Concerted enhanced-sampling simulations to elucidate the helix-fibril transition pathway of intrinsically disordered α-Synuclein

Fibril formation of α-synuclein is linked with Parkinson's disease. The intrinsically disordered nature of α-syn provides extensive conformational plasticity and becomes difficult to characterize its transition pathway from native monomeric to disease-associated fibril form. We implemented different simulation methods such as steered dynamics-umbrella sampling, and replica-exchange and conventional MD simulations to access various conformational states of α-syn. Nineteen distinct intermediate structures were identified by free energy landscape and cluster analysis. They were then sorted based on secondary structure and solvent exposure of fibril-core residues to illustrate the fibril dissociation pathway. The analysis showed that following the initial dissociation of the polypeptide chain from the fibril, α-syn might form either compact-conformations by long-range interactions or extended-conformations stabilized by local interactions. This leads α-syn to adapt two different pathways. The secondary structure, solvation, contact distance, interaction energies and backbone dihedrals of thirty-two selected residues were analyzed for all the 19 intermediates. The results suggested that formation of β-turns, reorganization of salt bridges, and dihedral changes in the hydrophobic regions are the major driving forces for helix-fibril transition. Structural features of the intermediates also correlated with the earlier experimental and computational studies. The study provides critical information on the fibrillation pathway of α-syn.

Molecular-orbital-based machine learning for open-shell and multi-reference systems with kernel addition Gaussian process regression.

We introduce a novel machine learning strategy, kernel addition Gaussian process regression (KA-GPR), in molecular-orbital-based machine learning (MOB-ML) to learn the total correlation energies of general electronic structure theories for closed- and open-shell systems by introducing a machine learning strategy. The learning efficiency of MOB-ML(KA-GPR) is the same as the original MOB-ML method for the smallest criegee molecule, which is a closed-shell molecule with multi-reference characters. In addition, the prediction accuracies of different small free radicals could reach the chemical accuracy of 1 kcal/mol by training on one example structure. Accurate potential energy surfaces for the H10 chain (closed-shell) and water OH bond dissociation (open-shell) could also be generated by MOB-ML(KA-GPR). To explore the breadth of chemical systems that KA-GPR can describe, we further apply MOB-ML to accurately predict the large benchmark datasets for closed- (QM9, QM7b-T, and GDB-13-T) and open-shell (QMSpin) molecules.

Ab-Initio Investigations on Hydrogen Dissociation and Cross-Linking of Hydrocarbon Chains of Self-Assembled Monolayers of Alkanes

First-principles calculations were carried out to study the structural and electronic properties of hydrocarbon chains of self-assembled monolayers with hydrogen dissociation. It was found that the incoming hydrogen could lead to the formation of H2 molecules by stripping the nearby hydrogen atoms in the chains and thereby leave the neighboring carbon atoms to be free radicals. Two parallel hydrocarbon chains with dangling bonds can form a direct C-C bond, i.e., cross-linking happens between the two chains, which is ascribed to a charge accumulation in the cross-linking region. The polymerization of short molecules into long hydrocarbon chains through a different cross-linking mode is also discussed.

Revealing Polymerization Kinetics with Colloidal Dipatch Particles.

Limited-valency colloidal particles can self-assemble into polymeric structures analogous to molecules. While their structural equilibrium properties have attracted wide attention, insight into their dynamics has proven challenging. Here, we investigate the polymerization dynamics of semiflexible polymers in 2D by direct observation of assembling divalent particles, bonded by critical Casimir forces. The reversible critical Casimir force creates living polymerization conditions with tunable chain dissociation, association, and bending rigidity. We find that unlike dilute polymers that show exponential size distributions in excellent agreement with Flory theory, concentrated samples exhibit arrest of rotational and translational diffusion due to a continuous isotropic-to-nematic transition in 2D, slowing down the growth kinetics. These effects are circumvented by the addition of higher-valency particles, cross linking the polymers into networks. Ourresults connecting polymer flexibility, polymer interactions, and the peculiar isotropic-nematic transition in 2D offer insight into the polymerization processes of synthetic two-dimensional polymers and biopolymers at membranes and interfaces.

Modeling the Damage and Self-healing Behaviors of Plasticized PVC Gels

Plasticized polyvinyl chloride (PVC) gel is a type of electroactive polymers, which has potentials to be applied as soft actuators. However, few studies have been performed to investigate the mechanical responses of PVC gels in cyclic loading conditions. In this work, we combined experiments and theory to characterize the mechanical responses of two plasticized PVC gels. The gels were subjected to different types of loading-unloading-reloading cycles. The experimental results showed that the plasticized PVC gels exhibited stress softening response (the Mullins effect) in the cyclic tests. Meanwhile, the gels regained some strength after annealing at room temperature for several hours, indicating the self-healing ability of gels. We further extended the eight-chain model with incorporation of the chain dissociation and reassociation mechanism. The developed constitutive model is able to reproduce the Mullins effect and the recovery of the Mullins effect observed in the experiments. Our results clearly show that understanding the complex mechanical response is crucial for the applications of plasticized PVC gels.

A new insight into pyrolysis mechanism of three typical actual biomass: The influence of structural differences on pyrolysis process

Biomass pyrolysis involves complex structural changes and numerous chemical reactions. At present, most studies on actual biomass pyrolysis focus on the macroscopic thermal degradation process, while the studies on microscopic pyrolysis rules are still very limited. This work provides a new insight into the pyrolysis mechanism of three types of actual biomass, namely hardwood (eucalyptus saligna), softwood (pinus sylvestris) and straw (corn straw), by combining in-situ characterization of the evolution of biomass structures with density functional theory (DFT) calculations of pyrolysis pathways. Firstly, the evolution of various characteristic structures during biomass pyrolysis was monitored by in-situ diffuse reflectance infrared Fourier transform spectroscopy (in-situ DRIFT). It was found that the thermal stability of the same functional group in different biomass samples are not exactly the same, and the structural differences in biomass would lead to different pyrolysis behaviors. Then, considering the great structural difference of hemicellulose in different biomass, the key initial pyrolysis reaction paths including the cleavage of glycosidic bonds, the dissociation of side chains and the opening of sugar ring of hemicellulose were studied by DFT calculations. It was found that the cleavage of glycosidic bonds and the dissociation of O-acetyl side chains of glucomannan in softwood hemicellulose are more advantageous than those of xylan in hardwood and straw hemicellulose. In addition, the cleavage mechanism of hemicellulose-lignin bond (LC), a characteristic connection structure between different components of actual biomass, was investigated. The result showed that the cleavage of Cα-O mainly occurred by concerted mechanism for α-ether bond of LC.

Matrix stiffness changes affect astrocyte phenotype in an in vitro injury model

Injury to the central nervous system (CNS) usually leads to the activation of astrocytes, followed by glial scar formation. The formation of glial scars from active astrocytes in vivo has been found to be dependent on the cell microenvironment. However, how astrocytes respond to different microenvironmental cues during scar formation, such as changes in matrix stiffness, remains elusive. In this work, we established an in vitro model to assess the responses of astrocytes to matrix stiffness changes that may be related to pathophysiology. The investigated hydrogel backbones are composed of collagen type I and alginate. The stiffness of these hybrid hydrogels can be dynamically changed by association or dissociation of alginate chains through adding crosslinkers of calcium chloride or a decrosslinker of sodium citrate, respectively. We found that astrocytes obtain different phenotypes when cultured in hydrogels of different stiffnesses. The obtained phenotypes can be switched in situ when changing matrix stiffness in the presence of cells. Specifically, matrix stiffening reverts astrogliosis, whereas matrix softening initiates astrocytic activation in 3D. Moreover, the effect of matrix stiffness on astrocytic activation is mediated by Yes-associated protein (YAP), where YAP inhibition enhances the upregulation of GFAP and contributes to astrogliosis. To investigate the underlying mechanism of matrix stiffness-dependent GFAP expression, we also developed a mathematical model to describe the time-dependent dynamics of biomolecules involved in the matrix stiffness mechanotransduction process of astrocytes. The modeling results further indicate that the effect of matrix stiffness on cell fate and behavior may be related to changes in the cytoskeleton and subsequent activity of YAP. The results from this study will guide researchers to re-examine the role of matrix stiffness in reactive astrogliosis in vivo and inspire the development of a novel therapeutic approach for controlling glial scar formation following injury, enabling axonal regrowth and improving functional recovery by exploiting the benefits of mechanobiology studies.

β-sheet breakers with consecutive phenylalanines: Insights into mechanism of dissolution of β-amyloid fibrils.

β-sheet breakers (BSB) constitute a class of peptide inhibitors of amyloidogenesis, a process which is a hallmark of many diseases called amyloidoses, including Alzheimer's disease (AD); however, the molecular details of their action are still not fully understood. Here we describe the results of the computational investigation of the three BSBs, iaβ6 (LPFFFD), iaβ5 (LPFFD), and iaβ6_Gly (LPFGFD), in complex with the fibril model of Aβ42 and propose the kinetically probable mechanism of their action. The mechanism involves the binding of BSB to the central hydrophobic core (CHC) region (LVFFA) of Aβ fibril and the π-stacking of its Phe rings both internally and with the Aβ fibril. In the process, the Aβ fibril undergoes distortion accumulating on the side of chain A (located on the odd tip of the fibril). In a single replica of extended molecular dynamics run of one of the iaβ6 poses, the distortion concludes in a dissociation of chain A from the fibril model of Aβ42. Altogether, we postulate that including consecutive Phe residues into BSBs docked around Phe 20 in the CHC region of Aβ42 improve their potency for dissolution of fibrils.