Effect of core charge density on the electrophoresis of a soft particle coated with polyelectrolyte layer

The electrophoresis of a charged soft particle with charged rigid core is considered under a weak imposed field condition. The rigid core of the soft particle is considered to have a finite dielectric permittivity and a fixed volume charge density. The electric potential distribution is determined by solving the Poisson-Boltzman equation out side the rigid core and a Poisson equation within the core along with continuity conditions on the core-shell interface. We have extended the analytic expression of Ohshima (Electrophoresis 27:526–533, 2006) for the electrophoretic mobility of a soft particle with a charged shell to include the effect of the volume charge density of the rigid core. Mobility based on the present expression matches exactly with the existing analytical solutions for a soft particle with an uncharged core. We have also made a comparison of our solution for mobility with an uncharged rigid core with the existing experimental results. The impact of the core charge density on the soft particle mobility is analyzed.     

Approximate analytic expressions for the electrophoretic mobility of spherical soft particles

Approximate analytic expressions are derived for the electrophoretic mobility of a weakly charged spherical soft particle consisting of the particle core covered with a surface layer of polymers in an electrolyte solution. The particle core and the surface polymer layer may be charged or uncharged. The obtained electrophoretic mobility expressions, which involve neither numerical integration nor exponential integrals, are found to be in excellent agreement with the exact numerical results. It is also found that the obtained mobility expressions reproduce all the previously derived limiting expressions and approximate analytic expressions for the electrophoretic mobility of a weakly charged spherical soft particle.     

Molecular dynamics simulations of polyelectrolyte-polyampholyte complexes. Effect of solvent quality and salt concentration

Complexation between polyelectrolyte and polyampholyte chains in poor solvent conditions for the polyelectrolyte backbone has been studied by molecular dynamics simulations. In a poor solvent a polyelectrolyte forms a necklace-like structure consisting of polymeric globules (beads) connected by strings of monomers. The simulation results can be explained by assuming the existence of two different mechanisms leading to the necklace formation. In the case of weak electrostatic interactions, the necklace formation is driven by optimization of short-range monomer-monomer attraction and electrostatic repulsion between charged monomers on the polymer backbone. In the case of strong electrostatic interactions, the necklace structure appears as a result of counterion condensation. While the short-range attractions between monomers are still important, the correlation-induced attraction between condensed counterions and charged monomers and electrostatic repulsion between uncompensated charges provide significant contribution to optimization of the necklace structure. Upon forming a complex with both random and diblock polyampholytes, a polyelectrolyte chain changes its necklace conformation by forming one huge bead. The collapse of the polyelectrolyte chain occurs due to the neutralization of the polyelectrolyte charge by polyampholytes. In the case of the random polyampholyte, the more positively charged sections of the chain mix with negatively charged polyelectrolyte forming the globular bead while more negatively charged chain sections form loops surrounding the collapsed core of the aggregate. In the case of diblock polyampholyte, the positively charged block, a part of the negatively charged block, and a polyelectrolyte chain form a core of the aggregate with a substantial section of the negatively charged block sticking out from the collapsed core of the aggregate. In both cases the core of the aggregate has a layered structure that is characterized by the variations in the excess of concentration of monomers belonging to polyampholyte and polyelectrolyte chains throughout the core radius. These structures appear as a result of optimization of the net electrostatic energy of the complex and short-range attractive interactions between monomers of the polyelectrolyte chain.     

Oil-in-water microemulsions stabilized by charged diblock copolymers

We present here oil-in-water microemulsions stabilized by charged diblock copolymers alone, along with their structural characterization by small-angle neutron scattering measurements. They consist of swollen spherical micelles containing small amounts of oil in their core, which is surrounded by a corona of stretched polyelectrolyte chains. Structural changes, including core size variations, are evidenced when using a cosurfactant, or upon addition of salt, through a contraction of the charged corona. Attempts to relate the micellar structure to the individual copolymer characteristics are also presented, and show that the size of the hydrophobic block mainly determines that of the micelles.     

Structure and phase transformations of DPPC lipid bilayers in the presence of nanoparticles: insights from coarse-grained molecular dynamics simulations

In this article, we investigate fluid-gel transformations of a DPPC lipid bilayer in the presence of nanoparticles, using coarse-grained molecular dynamics. Two types of nanoparticles are considered, specifically a 3 nm hydrophobic nanoparticle located in the core of the bilayer and a 6 nm charged nanoparticle located at the interface between the bilayer and water phase. Both negatively and positively charged nanoparticles at the bilayer interface are investigated. We demonstrate that the presence of all types of nanoparticles induces disorder effects in the structure of the lipid bilayer. These effects are characterized using computer visualization of the gel phase in the presence of nanoparticles, radial distribution functions, and order parameters. The 3 nm hydrophobic nanoparticle immersed in the bilayer core and the positively charged nanoparticle at the bilayer surface have no effect on the temperature of the fluid-gel transformation, compared to the bulk case. Interestingly, a negatively charged hydrophobic nanoparticle located at the surface of the bilayer causes slight shift of the fluid-gel transformation to a lower temperature, compared to the bulk bilayer case.     

Electrophoresis of soft particles: analytic approximations

An approximate analytic expression is derived for the electrophoretic mobility of a weakly charged spherical soft particle (i.e., a hard particle covered with a weakly charged polyelectrolyte layer) on the basis of the general mobility expression for soft particles (Ohshima, H., J. Colloid Interface Sci. 2000, 228, 190-193). The obtained mobility expression, which reproduces various approximate results so far derived and gives some new mobility formulas, covers all types of weakly charged soft particles with arbitrary values of the thickness of polymer layer, the radius of the particle core, the electrophoretic softness, and the Debye length, including spherical polyelectrolytes with no particle core as well as spherical hard particles with no polyelectrolyte layer.     

Design and characterization of randomly speckled spheres

We use the "particle lithography" technique to fabricate randomly speckled spheres. Parts of positively charged 3.3 microm polystyrene microspheres were masked off with negatively charged 0.9 microm silica particles, and the remaining portion was covered with negatively charged 60 nm polystyrene nanoparticles. The masking particles were then removed to leave speckles on the larger core particle. Images from electron microscope and confocal microscope show that the diameter of the circular speckles is predictable and reliable, following an estimate from simple geometry, and that the number of speckles formed on a particle can be altered by changing the concentration of silica particle masks. The process described in this paper can be adapted to a wide variety of materials, opening the door for applications where size-controlled patches of one chemistry can appear on core particles of another chemistry and where the precise placement of patches is of little importance.     

Voltage gated carbon nanotube membranes

Membranes composed of an array of aligned carbon nanotubes, functionalized with charged molecular tethers, show voltage gated control of ionic transport through the cores of carbon nanotubes. The functional density of tethered charge molecules is substantially increased by the use of electrochemical grafting of diazonium salts. Functionality can be forced to occur at the CNT tip entrances by fast fluid flow of an inert solvent through the core during electrochemical functionalization. The selectivity between Ru(bi-pyridine)(3)2+ and methyl viologen2+ flux is found to be as high as 23 with -130 mV bias applied to the membrane as the working electrode. Changes in the flux and selectivity support a model where charged tethered molecules at the tips are drawn into the CNT core at positive bias. For molecules grafted along the CNT core, negative bias extends the tethered molecules into the core. Electrostatically actuated tethers induce steric hindrance in the CNT core to mimic voltage gated ion channels in a robust large area platform.     

Charge transfer and electron backdonation in metallofullerenes encapsulating NSc3

Orbital interaction analysis is employed to understand the complex charge transfer mechanism operative in endohedral metallofullerenes of composition NSc₃@C_n (n = 68, 78). This phenomenon combines substantial electron transfer from the core to the cage with electron backdonation, involving the interaction between the occupied orbitals of the negatively charged cage and the unoccupied d orbitals of the positively charged core. This electron backdonation differs fundamentally from conventional orbital hybridization, which takes place primarily between the HOMO of the metal core and the LUMO of the fullerene cage. These findings imply the pronounced stability of NSc₃@C_n (n = 68, 78), especially for NSc₃ encapsulated in the non-IPR C₆₈ enclosure, as experimentally established.     

Evidence of Molecular Fragmentation inside the Charged Droplets Produced by Electrospray Process

The behavior of the analyte molecules inside the neutral core of the charged droplet produced by the electrospray (ES) process is not unambiguously known to date. We have identified interesting molecular transformations of two suitably chosen analytes inside the ES droplets. The highly stable Ni(II) complex of 1,8-dimethyl-1,3,6,8,10,13-hexaazacyclotetradecane (1) that consists of a positive charge at the metal center, and the allyl pendant armed tertiary amine containing macrocycle 3,4,5:12,13,14-dipyridine-2,6,11,15-tetramethyl-1,7,10,16-tetraallyl-1,4,7,10,13,16-hexaazacyclooctadeca-3,13-diene (M _4p ) have been studied by ESI mass spectrometry as the model analytes. We have shown that these two molecules are not representatively transferred from solution to gas phase by ESI; rather, they undergo fragmentation inside the charged droplets. The results indicated that a charged analyte such as 1 was possibly unstable inside the neutral core of the ES droplet and undergoes fragmentation due to the Coulombic repulsion imparted by the surface protons. Brownian motion of the neutral analyte such as M _4p inside the droplet, on the other hand, may lead to proton attachment on interaction with the charged surface causing destabilization that leads to fragmentation of M _4p and release of resonance stabilized allyl cations from the core of the droplet. Detailed solvent dependence and collision-induced dissociation (CID) studies provided compelling evidences that the fragmentation of the analytes indeed occurs inside the charged ES droplets. A viable model of molecular transformations inside the ES droplet was proposed based on these results to rationalize the behavior of the analyte molecules inside the charged ES droplets.     

Cadmium and metallothionein turnover in different tissues of the gastropod Littorina littorea

This paper attempts to link the kinetics of Cd and metallothionein turnover in the intertidal marine snail Littorina littorea. The results demonstrate that the turnover of metallothionein is tissue dependent. Metallothionein has an estimated half-life of 69 and 160 days in the gills and kidney, respectively. The half-life could not be calculated for metallothionein in the digestive gland and is probably much longer than the other two tissues. Cadmium elimination from the gill and kidney is considerably slower than the respective metallothioneins (half-life in excess of 300 days) indicating closed cycling of the metal in these tissues. In contrast, cadmium levels in the digestive gland continue to increase during the detoxification period reflecting some remobilization from other tissues. Metallothionein turnover is extremely slow in Littorina when compared with mammals and other bivalve molluscs: even though metallothionein degradation is measurable in some gastropod tissues, the released cadmium may induce de novo metallothionein synthesis to which cadmium becomes resequestered. The slow metallothionein turnover rates and the lack of significant cadmium excretion testify to the relatively stable nature of the cadmium-metallothionein complex in this invertebrate.     

Comparison of capillary electrophoretic techniques for analysis and characterization of metallothioneins

To explore and understand the significance of individual metallothionein isoforms, the methods of their identification are needed. Separation of these isoforms requires a high resolution technique which can exploit very small differences in mass, charge, and hydrophobicity. In this report, three different techniques of CE were analyzed and used for metallothionein separation: detection using capillary gel electrophoresis, capillary zone electrophoresis, and capillary isoelectric focusing. Also, three different metallothionein samples were used from horse kidney, rabbit liver, and human liver. We identified metallothionein isoforms based on the determination of their relative molecular masses, on the charge differences in different pH buffers, and based on the pI value. Methods used in this report allow metallothionein identification, permit to quantify the purity and content of its isoforms, and allow studying its polymerization. This report supports and endorses the increased application of CE methodology in proteomics.     

Chromatographic Properties of Metallothioneins on a Gel Permeation Column: Changes Induced by Replacement of Cadmium with Cuprous and Cupric Ions

Abstract

In vitro experiments were conducted to find whether or not a similar elution profile to rat kidney metallothionein with high copper content was obtained on a gel permeation column by replacement of cadmium and/or zinc in rat liver metallothionein with copper. Stepwise replacement of cadmium in rat liver metallothionein with cuprous ion did not cause any shifts of retention times from those of the original proteins on a gel permeation column (SW 3000 column). In contrast to cuprous ion, stepwise replacement of cadmium with cupric ion induced shifts of retention times to larger values than the original ones for the two isometallothioneins on a SW column. Replacement of zinc in zinc-thionein with cupric ion but not with cuprous ion caused a retardation of elution volume on a Sephadex G-75 column. The decreases of cadmium peaks were accompanied by the increases of copper peaks in the case of replacement of cadmium in metallothionein with cuprous ion. Although stepwise decreases of cadmium peaks were observed by the replacement of cadmium in metallothionein with cupric ion, concomitant increases of copper peaks were not observed. Although the relative peak heights of isometallothionein peaks were different from those of rat kidney metallothionein, the third peak with the same retention time as that of rat kidney metallothionein was observed for the replacement of cadmium with cupric but not with cuprous ion.     

Isomerization dynamics and thermodynamics of ionic argon clusters

The dynamics and thermodynamics of small Ar(n) (+) clusters, n=3, 6, and 9, are investigated using molecular dynamics (MD) and exchange Monte Carlo (MC) simulations. A diatomic-in-molecule Hamiltonian provides an accurate model for the electronic ground state potential energy surface. The microcanonical caloric curves calculated from MD and MC methods are shown to agree with each other, provided that the rigorous conservation of angular momentum is accounted for in the phase space density of the MC simulations. The previously proposed projective partition of the kinetic energy is used to assist MD simulations in interpreting the cluster dynamics in terms of inertial, internal, and external modes. The thermal behavior is correlated with the nature of the charged core in the cluster by computing a dedicated charge localization order parameter. We also perform systematic quenches to establish a connection with the various isomers. We find that the Ar(3) (+) cluster is very stable in its linear ground state geometry up to about 300 K, and then isomerizes to a T-shaped isomer in which a quasineutral atom lies around a charged dimer. In Ar(6) (+) and Ar(9) (+), the covalent trimer core is solvated by neutral atoms, and the weakly bound solvent shell melts at much lower energies, occasionally leading to a tetramer or pentamer core with weakly charged extremities. At high energies the core itself becomes metastable and the cluster transforms into Ar(2) (+) solvated by a fluid of neutral argon atoms.     

Synthesis of Highly Charged C3-Symmetrical Organic Molecule with a Fused Planar Core Structure

A new C3-symmetric molecule integrated with a number of interesting features, including three highly charged and flexible pyrrolidinium arms extending from a fused planar core structure, is synthesized.     

金属硫蛋白溶液聚合状态的研究

通过动态光散射实验首次证明了兔肝金属硫蛋白亚型 和 在不同的缓冲体系中不仅仅是以二聚体的形式 ,而且是以多种聚合形式存在[1] .深入考察金属硫蛋白在溶液中的聚合形式对于研究金属硫蛋白的结构和功能都具有非常重要的意义 .我们通过计算机模拟 [1] 比较系统地研究了各种因素对金属硫蛋白聚合的影响 .结果表明 ,在溶液聚合的分子识别过程中可由多种因素共同调控金属硫蛋白 ,这些因素主要包括静电相互作用、疏水性相互作用和溶液中阴阳离子等 .通过这些因素的综合分析 ,首次对兔肝金属硫蛋白在溶液中多种聚合形式的形成机制及相应的分子…     

Direct deposition and assembly of gold colloidal particles using a nanofountain probe

We report the direct delivery and assembly of negatively charged gold colloidal particles atop positively charged amino-terminated silicon oxide surfaces using a nanofountain atomic force microscopy probe. The experimental results and fluid simulations indicate that the flow of nanoparticles is confined to the core tip region of the probe. This leads to the assembly of high-resolution submicron patterns (200 nm) on the substrate with feature sizes dependent on the tip-substrate contact time. A diffusion mechanism for the patterning is proposed and discussed.     

两亲性阴离子HBPN与阳离子表面活性剂CTAB络合机理的研究

用动态光散射研究了两亲性阴离子HBPN(高度枝化的聚酯纳米微粒)和阳离子表面活性剂CTAB(十六烷基三甲基溴化铵)在缓冲液中的相互作用及HBPN/CTAB络合物的形成.结果表明:在静电作用下,于碱性溶剂中,HBPN与由CTAB分子所构成的胶束形成了核壳结构络合物HBPN/CTAB.络合物粒径的大小和稳定性可以通过溶液的pH值和CTAB浓度来控制.     

Photoinduced electron storage and surface plasmon modulation in Ag@TiO2 clusters

The reversible charging and discharging effects associated with photoexcitation of a TiO2 shell in a Ag@TiO2 composite are described. The photoinduced charge separation in the TiO2 shell is followed by electron injection into the silver core. Interestingly, the charging of the silver core is associated with the shift in the surface plasmon band from 460 to 430 nm. The stored electrons are discharged upon exposure of the charged Ag@/TiO2 composite to an electron acceptor. As the electrons from the silver core are discharged, the original surface plasmon absorption of the Ag core is restored.     

Neutral structures with a planar tetracoordinated carbon based on spiropentadiene analogues

Neutral hydrocarbon structures containing a planar tetracoordinated carbon atom are proposed on the basis of quantum chemical calculations. The planarity at the central carbon atom is achieved by using aromaticity for stabilizing a positively charged core moiety that contains the planar atom. This charge is compensated by negatively charged cyclopentadienyl rings fused on the structure, leading to neutral structures. These are found to be stable from a dynamic point of view and are potentially synthesizable through carbene chemistry. These structures can lead to new breakthroughs in the chemical structure theory. A family of species derived from this model is also presented.