Mutual Electrostatic Interactions Research Articles

Spin Exchange Between Charged Paramagnetic Particles in Dilute Solutions

Kinetic equations for the spin density matrix which take into account binary collisions and a method of calculating the spin exchange effective radius have been generalized to the case of dilute solutions of charged paramagnetic particles. The effective radius of the spin exchange and rate constant of the bimolecular spin exchange between charged paramagnetic particles in solutions have been calculated numerically. Calculations have been performed under the assumption that the exchange interaction is isotropic and decays exponentially with the increase in the distance between radicals, and the solution has a given dielectric permittivity and Debye screening radius. Dependences of the spin exchange rate constant on the mutual diffusion coefficient, exchange and electrostatic interactions parameters have been found numerically. The theory has been applied to experimental results taken from the literature. The rate constant of the spin exchange between radicals of like charge found from the experiment and calculated within the developed theory are in good qualitative agreement .

Preparation of heterogeneous cationic metalloporphyrin/heteropolyanion composite catalysts and their high catalytic activity in hydroxylation of cyclohexane with molecular oxygen

A new route to improve the metalloporphyrin catalysts is developed, and it is to constitute heterogeneous composite catalysts with immobilized cationic metalloporphyrins and heteropolyanions. By using the method of synchronously synthesizing and immobilizing porphyrins on cross-linked polystyrene microspheres (CPS microspheres), the immobilized porphyrin TAPP-CPS microspheres, on which ternary amine (TA) group-containing phenyl porphyrin (PP) was immobilized, were first prepared, and then the immobilized cationic porphyrin TMPP-CPS microspheres, in whose structure trimethylammoniophenyl porphyrin (TMPP) was contained, were obtained via quaternization reaction. Finally, three immobilized metalloporphyrins, CoTMPP-CPS (shorten as CoP-CPS), MnTMPP-CPS (MnP-CPS) and FeTMPP-CPS (FeP-CPS), were gained through coordination reactions. These immobilized metalloporphyrins were composited with heteropolyanions by right of the mutual electrostatic interaction with phosphotungstic (PW) acid and phosphomolybdic (PMo) acid as reagents, respectively, resulting in several heterogeneous metalloporphyrin/heteropolyanion composite catalysts such as CoPPW-CPS, CoPPMo-CPS and MnPPW-CPS. The composite catalysts were used in the catalytic hydroxylation reaction of cyclohexane with molecular oxygen as oxidant, and their catalytic performances were investigated. The experimental results show that the heterogeneous composite catalysts have extraordinarily high catalytic activity in the hydroxylation reaction of cyclohexane by molecular oxygen, and the cyclohexanol yield in 8 h can get up to 45.5 %. More importantly, the catalytic activity of the heterogeneous composite catalysts is obviously higher than that of the immobilized cationic metalloporphyrins, and the enhanced catalytic activity is originated from a protection of heteropolyanions against the deactivation of metalloporphyrins.

Gold‐Nanoparticle‐Assisted Self‐Assembly of Chemical Gradients with Tunable Sub‐50 nm Molecular Domains

A simple and efficient principle for nanopatterning with wide applicability in the sub‐50 nanometer regime is chemisorption of nanoparticles; at homogeneous substrates, particles carrying surface charge may spontaneously self‐organize due to the electrostatic repulsion between adjacent particles. Guided by this principle, a method is presented to design, self‐assemble, and chemically functionalize gradient nanopatterns where the size of molecular domains can be tuned to match the level corresponding to single protein binding events. To modulate the binding of negatively charged gold nanoparticles both locally (<100 nm) and globally (>100 μm) onto a single modified gold substrate, ion diffusion is used to achieve spatial control of the particles’ mutual electrostatic interactions. By subsequent tailoring of different molecules to surface‐immobilized particles and the void areas surrounding them, nanopatterns are obtained with variable chemical domains along the gradient surface. Fimbriated Escherichia coli bacteria are bound to gradient nanopatterns with similar molecular composition and macroscopic contact angle, but different sizes of nanoscopic presentation of adhesive (hydrophobic) and repellent poly(ethylene) glycol (PEG) domains. It is shown that small hydrophobic domains, similar in size to the diameter of the bacterial fimbriae, supported firmly attached bacteria resembling catch‐bond binding, whereas a high number of loosely adhered bacteria are observed on larger hydrophobic domains.

Structure-driven homology pairing of chromatin fibers: the role of electrostatics and protein-induced bridging.

Chromatin domains formed in vivo are characterized by different types of 3D organization of interconnected nucleosomes and architectural proteins. Here, we quantitatively test a hypothesis that the similarities in the structure of chromatin fibers (which we call "structural homology") can affect their mutual electrostatic and protein-mediated bridging interactions. For example, highly repetitive DNA sequences in heterochromatic regions can position nucleosomes so that preferred inter-nucleosomal distances are preserved on the surfaces of neighboring fibers. On the contrary, the segments of chromatin fiber formed on unrelated DNA sequences have different geometrical parameters and lack structural complementarity pivotal for stable association and cohesion. Furthermore, specific functional elements such as insulator regions, transcription start and termination sites, and replication origins are characterized by strong nucleosome ordering that might induce structure-driven iterations of chromatin fibers. We propose that shape-specific protein-bridging interactions facilitate long-range pairing of chromatin fragments, while for closely-juxtaposed fibers electrostatic forces can in addition yield fine-tuned structure-specific recognition and pairing. These pairing effects can account for some features observed for mitotic and inter-phase chromatins.

Synthesis of porous Fe3O4 hollow microspheres/graphene oxide composite for Cr(vi) removal

A composite of porous Fe3O4 hollow microspheres/graphene oxide (Fe3O4/GO) has been fabricated through a facile self-assembly approach. Driven by the mutual electrostatic interactions, the amine-functionalized Fe3O4 microspheres prepared by a hydrothermal method and then modified by 3-aminopropyltrimethoxysilane were decorated with negatively-charged GO sheets. The Fe3O4 microspheres were hollow with porous surfaces and the surfaces were successfully modified with the amine, which was confirmed by Fourier transform infrared spectroscopy. The specific saturation magnetization of Fe3O4/GO was 37.8 emu g(-1). The sorption performance of Fe3O4/GO for Cr(vi) was evaluated. The maximum sorption capacity for Cr(vi) on Fe3O4/GO was 32.33 mg g(-1), which was much higher than that of Fe3O4 microspheres. The GO sheets could not only prevent agglomeration of the Fe3O4 microspheres and enable a good dispersion of these oxide microspheres, but also substantially enhance the specific surface area of the composite. The Fe3O4/GO composite may be a promising sorption material for the separation and preconcentration of heavy metal ions from aqueous solutions in environmental pollution cleanup.

Enhanced Li Storage Capacity in 3 nm Diameter SnO2 Nanocrystals Firmly Anchored on Multiwalled Carbon Nanotubes

A simple synthesis route is demonstrated for the preparation of hybrid nanocomposite electrodes with a combination of SnO2 nanoparticles and conducting multiwalled carbon nanotubes (MWCNTs) for Li ion battery applications. The MWCNTs were initially treated using strong acid solutions to generate functional groups with negative charges on their surfaces. For the formation of the nanocomposites, the next process was driven by the mutual electrostatic interactions between the functionalized MWCNTs and the Sn4+ ion species and was followed by spontaneous oxidation during a hydrothermal reaction at 150 °C. The SnO2 nanoparticles with diameters of less than 3 nm were uniformly loaded onto the MWCNT surfaces, providing an extremely large Brunauer–Emmett–Teller (BET) surface area of ≈240 m2 g–1. Furthermore, we demonstrate that the incorporation of multiwalled carbon nanotubes gives rise to an enhanced reversible capacity estimated to be 420 mA h g–1 after 100 cycles, which is at least three times greater than th...

The Effective Fine-Structure Constant of Freestanding Graphene Measured in Graphite

Electrons in graphene behave like Dirac fermions, permitting phenomena from high-energy physics to be studied in a solid-state setting. A key question is whether or not these fermions are critically influenced by Coulomb correlations. We performed inelastic x-ray scattering experiments on crystals of graphite and applied reconstruction algorithms to image the dynamical screening of charge in a freestanding graphene sheet. We found that the polarizability of the Dirac fermions is amplified by excitonic effects, improving screening of interactions between quasiparticles. The strength of interactions is characterized by a scale-dependent, effective fine-structure constant, α(g)* (k,ω), the value of which approaches 0.14 ± 0.092 ~ 1/7 at low energy and large distances. This value is substantially smaller than the nominal α(g) = 2.2, suggesting that, on the whole, graphene is more weakly interacting than previously believed.

Weakly Interacting Graphene

Many unusual properties of graphene are a consequence of the Dirac dispersion of its electrons—a linear relationship between an electron's momentum and energy. Naively, this dispersion leads to the conclusion that electrons in graphene are strongly affected by mutual electrostatic interactions; however, there is little experimental evidence for strong interaction. Reed et al. (p. [805][1]) resolved this discrepancy by using inelastic x-ray scattering spectra of graphite (which consists of loosely bound layers of graphene) to estimate how much the electric field was damped by the presence of mobile charge carriers. In fact, damping was strong at distances in excess of 1 nanometer, suggesting that graphene is more weakly interacting than was assumed. [1]: /lookup/doi/10.1126/science.1190920

2-Methacryloyloxyethyl phosphorylcholine polymer (MPC)-coating improves the transfection activity of GALA-modified lipid nanoparticles by assisting the cellular uptake and intracellular dissociation of plasmid DNA in primary hepatocytes

We previously reported that modification of GALA peptide on the surface of liposomes enhanced fusion with endosomal membrane, and cytoplasmic release of encapsulated macromolecules. We report herein that an additional coating of GALA-modified liposomes with 2-methacryloyloxyethyl phosphorylcholine (MPC) polymer resulted in a two order of magnitude enhancement in the transfection activity of encapsulating plasmid DNA (pDNA). Quantification of the delivered gene copies in whole cells and isolated nuclei revealed that the increase of transfection activity can be attributed to improved efficiencies in cellular uptake and post-nuclear delivery processes. Imaging studies revealed that the intracellular dissociation of pDNA from the lipid envelope is enhanced by GALA modification and further coating with MPC polymer in a stepwise manner. The MPC polymer-coating decreased the ζ-potential of GALA-modified liposomes, suggesting that it assisted in the functional display of negatively charged GALA on the cationic liposomes by providing shielding from mutual electrostatic interactions. Collectively, these data indicate that MPC polymer-coating induced the fusogenic activity of the GALA-modified envelope with endosomes, leading to a more effective cytoplasmic release pDNA. The extensive fusion of the lipid envelope may also reduce electrostatic interactions between mRNA and cationic lipid components, thereby resulting in an enhancement in the translation process.

Molecular Determinants of Major Histocompatibility Complex Class I Complex Stability: SHAPING ANTIGENIC FEATURES THROUGH SHORT AND LONG RANGE ELECTROSTATIC INTERACTIONS

A single amino acid exchange between the major histocompatibility complex molecules HLA-B(*)2705 and HLA-B(*)2709 (Asp-116/His) is responsible for the emergence of distinct HLA-B27-restricted T cell repertoires in individuals harboring either of these two subtypes and could correlate with their differential association with the autoimmune disease ankylosing spondylitis. By using fluorescence depolarization and pK(a) calculations, we investigated to what extent electrostatic interactions contribute to shape antigenic differences between these HLA molecules complexed with viral, self, and non-natural peptide ligands. In addition to the established main anchor of peptides binding to HLA-B27, arginine at position 2 (pArg-2), and the secondary anchors at the peptide termini, at least two further determinants contribute to stable peptide accommodation. 1) The interaction of Asp-116 with arginine at peptide position 5, as found in pLMP2 (RRRWRRLTV; viral) and pVIPR (RRKWRRWHL; self), and with lysine in pOmega, as found in gag (KRWIILGLNK; viral), additionally stabilizes the B(*)2705 complexes by approximately 5 and approximately 27 kJ/mol, respectively, in comparison with B(*)2709. 2) The protonation state of the key residues Glu-45 and Glu-63 in the B-pocket, which accommodates pArg-2, affects peptide binding strength in a peptide- and subtype-dependent manner. In B(*)2705/pLMP2, protonation of Glu-45/Glu-63 reduces the interaction energy of pArg-2 by approximately 24 kJ/mol as compared with B(*)2705/pVIPR. B(*)2705/pVIPR is stabilized by a deprotonated Glu-45/Glu-63 pair, evoked by allosteric interactions with pHis-8. The mutual electrostatic interactions of peptide and HLA molecule, including peptide- and subtype-dependent protonation of key residues, modulate complex stability and antigenic features of the respective HLA-B27 subtype.

Electron screening in nanostructures

Distributions of electrical potential and carrier concentration, the contact capacity, and its voltage dependence are calculated for Schottky contacts to various types of nanostructures, including nanolayers and nanowires of different thickness, as well as their arrays. The results demonstrate a dramatic dependence on the nanostructure geometry. Single nanostructures and planar arrays of nanowires cannot provide effective screening of the contact potential, so that the total stored charge and the structure capacity depend on the separation between external contacts. On the contrary, for nanolayer and two-dimensional nanowire arrays, the mutual electrostatic interaction between different elements provides effective screening with the screening length equal to the interelement distance, which determines the contact capacity and its voltage dependence.

Bis(thiosemicarbazido-κ2N,S)nickel(II)–succinate–succinic acid (1/1/1)

The title ternary complex, [Ni(CH5N3S)2](C4H4O4)·C4H6O4, consists of a thio­semicarbazide-coordinated nickel cation, a succinate anion, and a neutral succinic acid mol­ecule, all of which are centrosymmetric. The Ni atom is four-coordinated in a planar geometry by N and S atoms from two symmetry-related thio­semicarbazide ligands. In the crystal structure, the three components are effectively linked together by mutual electrostatic interactions and hydrogen-bonding interactions of the N—H⋯O, C—H⋯O and O—H⋯O types.

Stimulated percolation of electrical conductivity in heterogenous mixtures

In a composite of liquid dielectric with granular conducting dopant in a form of a micrometre particles of conductor, one observes the structure ordering process due to external electric field, manifested by the increase of observed electrical conductivity. The mechanism of such an ordering is similar to the phenomena occurring in the electrorheological liquids. By application of the conductor particles as an active polar dopant, the observed ordering effects take place at lower values of the electric field, compared with the ordering case of the granular dopants of an insulator. A simple theoretical description of the above mentioned phenomena was developed. They are attributed to the mutual interaction of conducting particles, having the field-induced dipole moments. The measured current increase is due to the power dissipation in the sample, related to the particle motion in viscous medium, as well as to the increase of sample capacitance. The ordering effect of such a medium consists in the mutual electrostatic interaction of the electric dipoles, induced in the dopant particles, as well as in the buildup of a linear or dendritic spatial structures with a symmetry determined by the direction of the external electric field. In the theoretical part of the work, the assumption on the coagulation of conductor particles into linear conducting structures has been used. One can search, however, the theoretical argumentation based on Flory's concept with respect to the described dipole interactions and construct the statistical sum. By the application of the mathematical method, one can discuss topology and percolation effects in such mixtures. Apart from the effects of linear, regular ordering of the composite, one can observe also the nonlinear effects, when the conducting fractal structure is being created.

Metal-to-Ligand Charge Transfer in the Gas-Phase Cluster Limit

The solvent dependence of metal-to-ligand charge transfer (MLCT) in the bis(2,2‘,2‘‘-terpyridyl)iron(II) complex, [Fe(terpy)2]2+, isolated in small gas-phase clusters with one or four molecules of the polar, organic solvents acetone, acetonitrile, dimethyl sulfoxide, N,N-dimethylformamide, methanol, and pyridazine is reported. The shift in the maximum of the MLCT band, Eop, for [Fe(terpy)2·(solvent)1]2+ clusters, measured using laser photofragmentation mass spectrometry, relative to the corresponding values in bulk solution ranges from +601 cm-1 for acetone to +764 cm-1 for pyridazine. The solvent dependence of the outer-sphere reorganization energy predicted by a dielectric continuum model provides a context for comparing Eop values determined for MLCT in [Fe(terpy)2·(solvent)n]2+ clusters (n = 1, 4) with those measured in solution. A model derived from Kirkwood's equation for the mutual electrostatic interaction energy of an ion and a polar medium predicts that the solvent reorganization energy associated with MLCT in [Fe(terpy)2]2+ is a linear function of (1 − Dop)/(2Dop+ 1), where Dop is the optical dielectric constant of the bulk solvent. A linear relationship between Eop and (1 − Dop)/(2Dop+ 1) is observed not only in the bulk solvents, as anticipated, but also in clusters containing as few as four solvent molecules.

NUCLEI BEYOND THE PROTON DRIP-LINE

▪ Abstract The proton drip-line defines the limit at which nuclei become unbound to the emission of a proton from their ground states. Low-Z nuclei lying beyond this limit only exist as short-lived resonances and cannot be detected directly. The location of the drip-line constrains the path of nucleosynthesis in explosive astrophysical scenarios such as novae and X-ray bursters, and consequently controls the rate of energy generation. In higher-Z regions of the drip-line, the potential energy barrier resulting from the mutual electrostatic interaction between the unbound proton and the core can cause nuclei to survive long enough to be detected. This review describes the recent major advances in the study of these exotic nuclei. Particular emphasis is placed on understanding the phenomenon of proton radioactivity and the unique insights it offers into the structure of nuclei lying beyond one of nature's fundamental limits to stability.

Signal charge sharing in multilinear drift detectors: design and experimental characterization

Multilinear silicon drift detectors (MLSDDs) are radiation detectors which combine excellent energy and position resolution with high count-rate capabilities. This is achieved by controlling the diffusion of the signal charges and limiting their lateral spread during the drift. This paper will present a study of the signal electrons' transport in MLSDDs in comparison with the experimental measurements. Three-dimensional simulations were carried out, taking into account diffusion effects and mutual electrostatic interaction between the signal electrons. A representative sample of these simulations will be presented. Signal sharing in a multilinear drift detector and in a "classical" multi-anode drift detector are characterized and discussed. The results are useful to tailor the design of MLSDDs for the detection of different kinds of radiation (e.g., optical photon, X-ray, ionizing particle) and to optimize signal charge sharing and detector performance according to a particular application purpose.

Fluorine adsorption on GaAs(110) surfaces and the onset of etching after XeF 2 exposures

The interaction of XeF 2 molecules with GaAs(110) surfaces was investigated with high-resolution electron energy-loss (HREELS) and Auger electron spectroscopy (AES) in the exposure range between 1 and 1000 langmuir (L). The AES results show that up to XeF 2 doses of approximately 100 L, fluorine atoms are mainly chemisorbed on GaAs(110) surfaces. After XeF 2 exposures above 100 L, etching of the GaAs(110) surface occurs by removing arsenic atoms. Adsorbed xenon atoms were never detected. In the chemisorption range an energy-loss structure which is attributed to AsF valence vibrations is observed in the HREEL spectra. The excitation energy of the AsF vibration shifts from 670 cm −1 (84 meV) at very low fluorine coverages to 720 cm −1 (90 meV) at 0.3 monolayers. This finding may be explained by a mutual electrostatic interaction between AsF surface dipoles and their images. From that and by use of dielectric theory an effective dynamic charge of the vibrating AsF surface molecules is estimated. In the etching regime this AsF loss structure disappears and a broad feature at about 575 cm −1 (71.3 meV) is observed which may be due to vibrations of GaF x groups.

Geometry of chaos in the two-center problem in general relativity.

The now-famous Majumdar-Papapetrou exact solution of the Einstein-Maxwell equations describes, in general, N static, maximally charged black holes balanced under mutual gravitational and electrostatic interaction. When N=2, this solution defines the two-black-hole spacetime, and the relativistic two-center problem is the problem of geodesic motion on this static background. Contopoulos and a number of other workers have recently discovered through numerial experiments that, in contrast with the Newtonian two-center problem, where the dynamics is completely integrable, relativistic null-geodesic motion on the two-black-hole spacetime exhibits chaotic behavior Here I identify the geometric sources of this chaotic dynamics by first reducing the problem to that of geodesic motion on a negatively curved (Riemannian) surface.

The electrostatic spray emitted from an electrified conical meniscus

The electrostatic spray emitted from an electrified almost conical meniscus is analyzed in this work by means of a Lagrangian model using particle dynamics. Instead of solving the classical Poisson problem for the spray, we follow each one of the emitted particles and their mutual electrostatic interaction. The spray is shown to be dilute, allowing for essential simplifications on the particle aerodynamics. Computer simulations yield as many detailed spatial and statistical characterizations of the spray as one needs once the model is validated by actual experiments using laser-doppler anemometry and particle size analyzers also presented here. The experiments show that the theoretical model can predict not only the mean, but also the RMS of desired quantities such as droplet velocity and size at a certain point.

Electron transport networks in multicentre metalloproteins

Electron transfer (ET) in multicentre proteins exhibits cooperativity, i.e. mutual electrostatic, spectral, and structural interactions between the individual ET centres. We investigate here two kinds of cooperativity. The first is “static” and is represented by the electrostatic interaction between two charged centres inside or outside a solvated dielectric globule. Such effects can be the origin of interaction reduction potentials and also provide a basis for calculation of the protein and solvent reorganization Gibbs energy of intramolecular ET. Cooperativity effects in the second class are “dynamic” and originate from partial vibrational relaxation of intermediate states in multistep ET. This leads to coherence in the ET sequence. The concepts and formalism are applied to the ET patterns of the two-heme cytochrome c 4, four-heme cytochrome c 3, and the bacterial photosynthetic reaction centre. It is shown that cooperativity in the approximately thermoneutral ET steps of the cytochromes is likely to be “static” while the activationless nature of the photosynthetic ET steps can also lead to coherence in the overall ET sequence.