Dental mesenchymal stem cells (DMSCs) are crucial in tooth development and periodontal health, and their multipotential differentiation and self-renewal ability play a critical role in tissue ...engineering and regenerative medicine. Methylation modifications could promote the appropriate biological behavior by postsynthetic modification of DNA or protein and make the organism adapt to developmental and environmental prompts by regulating gene expression without changing the DNA sequence. Methylation modifications involved in DMSC fate include DNA methylation, RNA methylation, and histone modifications, which have been proven to exert a significant effect on the regulation of the fate of DMSCs, such as proliferation, self-renewal, and differentiation potential. Understanding the regulation of methylation modifications on the behavior and the immunoinflammatory responses involved in DMSCs contributes to further study of the mechanism of methylation on tissue regeneration and inflammation. In this review, we briefly summarize the key functions of histone methylation, RNA methylation, and DNA methylation in the differentiation potential and self-renewal of DMSCs as well as the opportunities and challenges for their application in tissue regeneration and disease therapy.
Carbon-metal oxide (CMO) nanocomposites have seen increasing research due to their extraordinary properties for energy storage materials and photocatalysts. Flame aerosol synthesis provides a ...promising route to producing CMO nanocomposites. Various CMO nanocomposites have been successfully synthesized through flame aerosol techniques in laboratories. However, a detailed understanding of the formation and growth mechanisms of such materials is lacking. Therefore, in this study, the reactive force-field molecular dynamics (ReaxFF MD) was deployed to gain atomic insights into the initial stage of carbon coating on the titania nanoparticle. We performed a large number of simulations of carbon coating with 18 typical hydrocarbon species in flames including aliphatics of C1–C4 species and polycyclic aromatic hydrocarbons (PAHs) at temperatures ranging from 400K to 2500K. We found that the titania nanoparticle can not only serve as a nucleus for physical adsorption of the surrounding hydrocarbons, but also can form CTi/O bonds with them, and abstract H atoms from the surrounding hydrocarbons. The optimal temperature range for carbon coating is T≤1200K, because CTi/O bonds are unstable at higher temperatures. At T≥1500K, hydrocarbons tend to gather to form larger carbonaceous species instead of coating onto the particle surface, as the formation of C–C bonds is promoted at high temperatures. Small aliphatics are favored to be chemically coated on the particle, while PAH molecules tend to be physically absorbed on the nanoparticle surface due to their stable electronic structure and large size. Coating tendencies of aliphatics are closely related to the number of C–C triple bonds.
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•The roles of chemisorption and physical adsorption during C coating are compared.•The optimal temperature range for carbon coating is T≤1200 K.•A carbon coating mechanism for both aliphatic and PAH species is proposed.•The number of C–C triple bonds in an aliphatic species affects its coating capability.
In this paper we derived the interaction energy between two spherical nanoparticles from the pair-wise Lennard-Jones attractive and repulsive potentials of the constituent atoms of the two particles, ...and proposed a coagulation efficiency model based on the average particle kinetic energy and the potential well depth (i.e. the minimum interaction energy) between two colliding particles. To test the performance of this new coagulation efficiency model, we applied it in detailed population balance modelling of soot particle size distributions (PSDs), and found better agreement with the measured PSDs in a benchmark premixed ethylene flame than that using the unit coagulation efficiency, especially in the range of small particles with mobility diameter less than 5 nm. Moreover, the agreement between the computed and the measured primary particle size distribution (PPSD) was also improved with the new coagulation efficiency model.
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•The interaction energy between two spherical particles was derived from the L-J potentials of the constituent atoms.•A coagulation efficiency model for soot was proposed based on the interaction energy and kinetic energy of colliding partners.•Better agreement between the computed PSDs and PPSDs with measured ones was obtained with this new model.
In this work, we studied the coagulation process of two PAH clusters with diameter ∼2nm using reactive molecular dynamics (MD) simulations. To describe the coagulation process quantitatively, the ...distance between the center of mass (COM) of the two PAH clusters, as well as the inter-cluster potential energy and kinetic energy of the COM of the clusters were calculated. Head-on coagulation efficiencies (η) of two PAH clusters at typical flame temperatures where soot inception is most likely to occur, i.e., 1500K—2000K, were determined based on hundreds of MD simulated trajectories. Our simulation results showed that η decreases with increasing temperature, which is mainly due to the increased kinetic energy of atoms within the PAH clusters at higher temperature. In addition, introduction of surface σ-radical site fraction in the range of 0.01 to 0.1 can only moderately improve η by ∼10% by forming carbon–carbon bonds between the two coagulating clusters, which suggests η of incipient soot nanoparticles with surface σ-radicals in high temperature flame regions is very low even if with reactive coagulation taken into consideration.
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•Reactive MD simulations have been performed to study the coagulation efficiency η of incipient soot nanoparticles at T≥ 1500K.•Head-on coagulation efficiency η has been determined based on hundreds of MD simulated trajectories.•σ -radicals on soot surface have been found to mildly enhance η by ∼10% through C-C bond formation between the nanoclusters.
Numerical simulation of soot formation in a laminar premixed burner-stabilised benchmark ethylene stagnation flame was performed with a new detailed population balance model employing a two-step ...simulation methodology. In this model, soot particles are represented as aggregates composed of overlapping primary particles, where each primary particle is composed of a number of polycyclic aromatic hydrocarbons (PAHs). Coordinates of primary particles are tracked, which enables simulation of particle morphology and provides more quantities that are directly comparable to experimental observations. Parametric sensitivity study on the computed particle size distributions (PSDs) shows that the rate of production of pyrene and the collision efficiency have the most significant effect on the computed PSDs. Sensitivity of aggregate morphology to the sintering rate is studied by analysing the simulated primary particle size distributions (PPSDs) and transmission electron microscopy (TEM) images. The capability of the new model to predict PSDs in a premixed stagnation flame is investigated. Excellent agreement between the computed and measured PSDs is obtained for large burner-stagnation plate separation ( ≥ 0.7 cm) and for particles with mobility diameter larger than 6 nm, demonstrating the ability of this new model to describe the coagulation process of aggregate particles.
The impact of localized π-radicals on soot formation is explored by considering their electronic structure and computing their relative concentrations in flame conditions. Electronic structure ...calculations reveal that the presence of localized π-radicals on rim-based pentagonal rings is due to aromaticity. We further calculated a complete mechanism for the formation and elimination of the site from hydrogen additions and abstractions. A batch reactor with flame concentrations of H• and H2 was used to determine the time-dependent concentration of localized π-radicals. Low temperatures ( < 1000 K) favored the fully saturated rim-based pentagonal ring. Soot nucleation temperatures (1000–1500 K) give way to unsaturated rim-based pentagons being favored. Localized π-radicals on rim-based pentagonal rings are found to be in higher concentration than the aryl-type σ-radical on the rim-based pentagon (mole fractions of 10−6−10−7) in below < 1500 K, consistent with recent experimental observations. Higher temperatures favor the σ-radical and the concentration of the localized π-radical on rim-based pentagons becomes negligible. A kinetic Monte Carlo treatment of multiple sites indicates that multiple localized π-radicals are possible on a single molecule. These results reveal the importance of localized π-radicals on rim-based pentagonal rings for PAH chemistry leading to formation of soot nanoparticles in flames involving aromatic rim-linked hydrocarbons (ARLH).
A new crosslinking reaction between two pentagonal rings around the periphery of planar pericondensed aromatic molecules is proposed and its impact on soot nanoparticle formation explored. The ...reaction mechanism was computed, using density functional theory, between an aryl-type σ-radical on a rim-based pentagonal ring attacking another rim-based pentagonal ring. A hydrogen migration allowed for the formation of a double bond forming a planar aromatic penta-linked hydrocarbon (APLH) complex, recently experimentally observed. The clustering of this planar species is compared with a pericondensed polyaromatic hydrocarbon (PCAH) and an aromatic aryl-linked hydrocarbon (AALH) using molecular dynamics and metadynamics. Similar clustering is found for the investigated species compared with a pericondensed structure of similar mass indicating enhanced physical interactions after forming the crosslink. Finally, a further crosslink is possible between the unsaturated pentagonal ring sites forming an aromatic rim-linked hydrocarbon (ARLH) complex of considerable stability. This was confirmed by simulating the stable molecular dynamics of such a complex with on-the-fly quantum forces from a quantum semi-empirical method, revealing possible reactions under flame conditions that might play a role in soot nucleation.
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