1,721,324 research outputs found
Separating hydrogen isotopologues via kinetic quantum sieving: Understanding important pore characteristics for an efficient separation
Full text linkThe potential performance of porous membranes in separating hydrogen isotopologues has been explored employing model systems and quanto-mechanical calculations including both zero-point energy and a numerically exact description of tunneling effects along the reaction coordinate. Membranes have been modeled as cylindrically pierced impenetrable wall, whereas diatomic molecules are described as dumbells composed of hard-sphere atoms. With the relative energetics of diatomics confined into cylindrical pores suggesting that differences in the adiabatic energy profiles between isotopologues for pore radii lower than 2.1 & Aring; should favor transport of heavier species, we investigated the selectivity for the latter process when membranes are 2.0 & Aring; thick. Chosen a pore radius, the results suggest that non-interacting pores represent the best compromise between selectivity and permeance, the addition of attraction between the membrane walls and molecular projectiles improving permeance while markedly depressing selectivity. A repulsive interaction with the pore inner surface, instead, reduced both properties. Finally, sieving molecules through a double membrane layer was found to marginally impact on the separation properties, which could be improved, at best, by 25% with a careful selection of the inter-membrane distance. Our results appear useful for the process of designing more effective sieving systems to separate di-deuterium molecules from its lighter counterparts
Impact of Chemically Specific Interactions between Anions and Weak Polyacids on Chain Ionization, Conformations, and Solution Energetics
Full text linkThe presence of salts in a solution containing weak polyelectrolytes is known to modify both their titration behavior and conformations due to electrostatic screening. Instead, little is currently known about the changes induced by chemically specific interactions (e.g., charged hydrogen bonds, c-H-bonds). To investigate this aspect, we simulated the titration of weak polyacids with a primitive model and Monte Carlo methods in the presence of monovalent salts whose anions are capable of forming c-H-bonds with associated acid groups. The interaction between anions and weak polyacids (e.g., poly(acrylic acid)) substantially hampers ionization at low pH despite the somewhat limited number of coordinated anions, whereas it has a limited impact once pH > pKa + 2 due to a progressive anion decoordination. Importantly, the suppression of ionization appears extremely local in nature, with different chain segments differing in pKa by up to 1.3 units. As for chain conformations, c-H-bonds reduce the average sizes of polyacids independently of their structure as a consequence of multidentate binding or multiarm coordination in starlike species. Analyzing the length of chain segments with all monomers coordinated or uncoordinated has also evidenced that anion binding is extremely local in nature. The energetic analysis of c-H-bond formation suggests that polyacid chemical potential may be strongly lowered (up to -0.7 kcal/mol per monomer), the impact of such results on a few phenomena relevant for the physical chemistry of polyacid-containing solutions being analyzed in some detail
Synthesis through recombinant DNA of a chimeric protein between diphteric toxin and human ciliar neurotrophic factor capable of killing tumor cells
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Modulation of macrophage suppressive activity and prostaglandin release by lymphokines and interferon: comparison of alveolar, pleural and peritoneal macrophages
No full textIn order to better characterize the mechanisms which regulate the immune response at the pulmonary level, the effects of beta-interferon (IFN-beta) and lymphokines (LK) on prostaglandin E (PGE) release and the suppressive capacity of mouse resident alveolar (AM phi) and pleural macrophages (PlM phi) were investigated in comparison with peritoneal macrophages (PM phi). After in vitro exposure to IFN-beta, PlM phi and PM phi showed a significant decrease of suppressive capacity and PGE release, whereas LK treatment did not affect such activities. In contrast, pre-treatment of AM phi with LK caused a strong impairment of their suppressive capacity. This effect was optimal after an incubation time of 20 h, was evident also at very low doses of LK and was not paralleled by any change of PGE release. Again in contrast with PlM phi and PM phi, suppressive capacity of AM phi was decreased only by very high doses of IFN-beta, whereas lower doses caused either an increase or no change of this activity. Furthermore, PGE release by AM phi was markedly increased after treatment with IFN-beta. Thus, suppressive capacity of AM phi appears to be controlled by different mechanisms from those of PlM phi and PM phi. In addition, a dissociation is evident between suppressive capacity and PGE release by AM phi
Absorbed weak polyelectrolytes: Impact of confinement, topology, and chemically specific interactions on ionization, conformation free energy, counterion condensation, and absorption equilibrium
No full textAbsorption of weak polyelectrolytes impacts on properties such as ionization, conformations, and counterion (CI) condensation that are important in several areas of applied and fundamental science. We used a weak polyelectrolyte model and Monte Carlo simulations to investigate how the mentioned properties depend on pH or the size of a spherical cavity (SC) permeable to CIs but not to polyelectrolytes; the latter have either linear or starlike topologies and may be allowed to form charged hydrogen bonds (c Symbol of the Klingon Empire H-bonds) between ionized and neutral monomers. Average ionization decreases upon increasing the number of arms at a constant number of monomers; it instead increases with the arm length in large SC due to CI screening. The way SC size, c Symbol of the Klingon Empire H-bonds, and pH values interrelate to define ionization is instead more complicate due to arm pairing or clustering when c Symbol of the Klingon Empire H-bonds are possible. These induce oscillations in the arm local ionization and impact on both monomer and CI distributions in the complete simulation cell in a way that also depends on polymer topology. The impact of ionization on the confinement free energy is also estimated; this highlighted that c Symbol of the Klingon Empire H-bonding may enhance absorption compared with neutral chains. (c) 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 491-51
Interaction between surfaces decorated with like-charged pendants: Unravelling the interplay between energy and entropy leading to attraction
No full textHypothesis: The stronger motional coupling between monovalent counterions neutralizing homogeneously like-charged surfaces induced by an increase in charge density is known to foster inter–surface attraction. Compared to a uniformly distributed charge, point-like charges generate locally more intense fields, so that the correlation induced between counterions may be even stronger despite an identical total charge. It should thus be possible to induce surface attraction at lower charge densities than commonly expected. Experiments: Monte Carlo simulations on primitive electrolyte models have been exploited to compute potential of mean force profiles and mobile ion densities for systems composed of two parallel surfaces bearing surface-tethered monovalent like-charged pendants as a function of the surface distance and pendant densities. Findings: Surfaces bearing like-charged pendants are found to attract each other over a wide range of distances despite the presence of very low charge densities. Notwithstanding the attractive contribution to the inter-surface forces provided by electrostatic interactions, the entropic component of the system Helmholtz energy is found to play the key role in defining the overall magnitude. The latter finding appears justified by an increase in the relative delocalization of counterions upon decreasing the surface distance
Impact of Charge Correlation, Chain Rigidity, and Chemical Specific Interactions on the Behavior of Weak Polyelectrolytes in Solution
No full textIn this work, we performed titration simulations of weak linear polyelectrolytes via the Monte Carlo method and the constant pH ensemble aiming to understand how polyelectrolyte concentration, chain rigidity, and the formation of intra- A nd inter-chain charged hydrogen bonds (c-H-bonds) impact on ionization and conformations of polyacidic species, counterions (CIs) distribution, and system Helmholtz energy. Increasing polyelectrolyte concentration resulted in enhanced acidity for all cases investigated due to the increased screening of chain charges by CIs and, when possible, the formation of interchain c-H-bonds. Our simulations also evidenced that polyelectrolytes able to form c-H-bonds can populate simultaneously two conformational states (aggregated and unfolded) in a range of pH, the transition between the two appearing first order-like. To better understand how properties of two polyelectrolytic chains are modified by their relative distance, we performed window sampling (WS) simulations, which highlighted nontrivial features in the ionization and conformational behaviors. As byproducts of WS simulations, we obtained also the potential of mean force between two chains; from this, it emerges that the reversible work needed to reach a specific interchain distance does not always increase with the pH, especially for c-H-bonds forming semirigid chains brought at short distances
Tunable Knot Segregation in Copolyelectrolyte Rings Carrying a Neutral Segment
Full text linkWe use Langevin dynamics simulations to study the knotting properties of copolyelectrolyte rings carrying neutral segments. We show that by solely tuning the relative length of the neutral and charged blocks, one can achieve different combinations of knot contour position and size. Strikingly, the latter is shown to vary nonmonotonically with the length of the neutral segment; at the same time, the knot switches from being pinned at the block's edge to becoming trapped inside it. Model calculations relate both effects to the competition between two adversarial mechanisms: the energy gain of localizing one or more of the knot's essential crossings on the neutral segment and the entropic cost of such localization. Tuning the length of the neutral segment sets the balance between the two mechanisms and hence the number of localized essential crossings, which in turn modulates the knot's size. This general principle ought to be useful in more complex systems, such as multiblock copolyelectrolytes, to achieve a more granular control of topological constraints
Effect of Counterion Size on Knotted Polyelectrolyte Conformations
No full text: Using Langevin dynamics simulations and a coarse-grained primitive model of electrolytes, we show that the behavior of knotted circular strong polyelectrolytes (PEs) in diluted aqueous solution is largely affected by the diameter of the counterions (CIs), σCI. Indeed, we observe that both gyration radius and knot length vary nonmonotonically with σCI, with both small and bulky CIs favoring knot localization, while medium-sized ones promote delocalized knots. We also show that the conformational change from delocalized to tight knots occurs via the progressive coalescence of the knot's essential crossings. The emerging conformers correspond to the minima of the free energy landscape profiled as a function of the knot length or PE size. We demonstrate that different conformational states can coexist, the transition between them appearing first-order-like and controlled by the enthalpic and entropic trade-off of the amount of CIs condensed on the PE. Such balance can be further altered by varying CI concentrations, thus providing an additional and more convenient tuning parameter for the system properties. Our results lay the foundation for achieving broader and more precise external adjustability of knotted PE size and shape by choosing the nature of its CIs. Thus, they offer new intriguing possibilities for designing novel PE-based materials that are capable of responding to changes in ionic solution properties
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