阴离子表面活性剂与聚丙烯酰胺间的相互作用

聚集体;阴离子表面活性剂与聚丙烯酰胺间的相互作用     

DNA melting in aggregates: impeded or facilitated?

How does DNA melt in columnar aggregate relative to its melting in diluted solution? Is the melting temperature increased or decreased with the aggregate density? Have DNA-DNA interactions, predominantly of electrostatic nature, an effect on the character of the melting transition? In attempt to answer these questions, we have incorporated the theory of electrostatic interactions between DNA duplexes into the simplest model of DNA melting. The analysis shows that the effect of aggregate density is very different for aggregates built of homologous (or identical) DNA fragments relative to the case of DNA with random base pair sequences. The putative attraction between homologous DNA helices hampers their melting and increases the melting temperature and can even dramatically change the character of the transition. In the aggregate of nonhomologous DNAs, the pattern of electrostatic interactions is more complicated, and their effect could be opposite; in some cases we may even expect electrostatically induced melting. These findings define new directions for melting experiments in dense DNA assemblies.     

Polymorphic Phosphorescence from Separable Aggregates with Unique Photophysical Properties

Platinum complexes aggregate into polymorphs with different intermolecular interactions leading to different photophysical properties. Strong intermolecular interactions stabilize the aggregate to such an extent that the polymorphs can be separated directly by column chromatography. Solid-state structures as well as quantum-chemical calculations confirmed the effect of the interactions on the photophysical properties.     

DNA-based approach for interparticle interaction control

Micron-sized latex particles with single-stranded DNA grafted to the surface have been used as a model system to study DNA mediated interactions. A new approach of tuning the interactions between particles is proposed, which allows for a gradual change of the assembly rate for fixed physical conditions of a solution by combining hybridizing "linker" DNA with nonhybridizing "neutral" DNA. The effect of linker/neutral DNA ratios on particle assembly kinetics and aggregate morphology has been experimentally investigated for a range of ionic strengths. The conditions for controlling various assembly morphologies have been identified, and the involved attractive and repulsive interactions have been described and explained for the proposed approach. The calculated attractive-repulsive behavior is in good agreement with experimental results. The described approach provides general perspectives for further fine-tuning DNA-mediated assembly systems.     

Sharp melting transitions in DNA hybrids without aggregate dissolution: proof of neighboring-duplex cooperativity

Similar to DNA-modified gold nanoparticles, comb polymer-DNA hybrids exhibit very sharp melting transitions that can be utilized in highly selective DNA detection systems. Current theories suggest that such sharp melting results from either a phase transition caused by the macroscopic dissolution of the aggregate or neighboring-duplex interactions in the close-packed environment between adjacent DNA duplexes. To delineate the contributions of each of these effects, an aggregate system based on polymer-DNA hybrids was designed to include both polymer-linked and partially untethered duplexes. When this hybridized system was subjected to thermal analysis, both types of duplexes exhibited sharp melting transitions. The very sharp melting transition displayed by the partially untethered DNA duplexes offers proof that neighboring-duplex interactions can indeed induce cooperativity. Contributions of this neighboring-duplex effect, as well as the enhanced stabilization observed in polymer-DNA:polymer-DNA aggregates, can be quantitatively assessed using a simple thermodynamic model. While neighboring-duplex interactions alone can lead to cooperative melting, the enhanced stabilization observed in polymer-DNA aggregates is a function of both neighboring-duplex interactions and multivalent or aggregate properties.     

铁氧簇合物[Fe11O6(OH)6(O2CCH3)15](C5H5N)6的结构和磁性质

通过三核铁盐[Fe₃O(O₂CCH₃)₆(H₂O)₃]C1在吡啶溶液中水解聚合得到铁氧簇合物[Fe₁₁O₆(OH)₆(O₂CCH₃)₁₅](C₅H₅N)₆。晶体结构表明11个铁离子(Ⅲ)中6个位于扭曲的三棱柱的顶点上，其余5个分别位于三棱柱的每个面之外。铁离子(Ⅲ)之间以氧桥或者羟基氧桥相连。变温磁化率证实铁离子(Ⅲ)之间是反铁磁耦合的。     

Unique spatial heterogeneity in ionic liquids

A multiscale coarse-graining model for ionic liquids has been extended to investigate the unique aggregation of cations in ionic liquids through computer simulation. It has been found that, with sufficiently long side chains, the tail groups of cations aggregate to form spatially heterogeneous domains, while headgroups of the cations and the anions distribute as uniformly as possible. This is understood as the result of competition between the charged electrostatic interactions between headgroups and anions and the collective short-range interactions between the neutral tail groups. This aggregation can help to explain a number of experimentally observed physical phenomena in ionic liquids.     

Breakage rate of colloidal aggregates in shear flow through stokesian dynamics

We study the first breakage event of colloidal aggregates exposed to shear flow by detailed numerical analysis of the process. We have formulated a model, which uses stokesian dynamics to estimate the hydrodynamic interactions among the particles in a cluster, van der Waals interactions and Born repulsion to describe the normal interparticle interactions, and the tangential interactions through discrete element method to account for contact forces. Fractal clusters composed of monodisperse spherical particles were generated using different Monte Carlo methods, covering a wide range of cluster masses (N(sphere) = 30-215) and fractal dimensions (d(f) = 1.8-3.0). The breakup process of these clusters was quantified for various flow magnitudes (γ), under both simple shear and extensional flow conditions, in terms of breakage rate constant (K(B)), mass distribution of the produced fragments (FMD, f(m,k)), and critical stable aggregate mass (N(c)), defined as the largest cluster mass that does not break under defined flow conditions. The breakage rate K(B) showed a power law dependence on the product of the aggregate size and the applied stress, with values of the corresponding exponents depending only on the aggregate fractal dimension and the type of flow field, whereas the prefactor of the power law relation also depends on the size of the primary particles comprising a cluster. The FMD was fitted by Schultz-Zimm distribution, and the parameter values showed an analogous dependence on the product of the aggregate size and the applied stress similar to the rate constant. Finally, a power law relation between the applied stress and corresponding largest stable aggregate mass was found, with an exponent value depending on the aggregate fractal dimension. This unique and detailed analysis of the breakage process can be directly utilized to formulate a breakage kernel used in solving population balance equations.     

Gemini表面活性剂分子结构对其水溶液中聚集行为的影响

Gemini表面活性剂是通过联接基团将两个具有亲水亲油性质的两亲结构单元在其亲水头基上或靠近亲水头基处以共价键方式连接而成的一类表面活性剂。这类表面活性剂由于联接基团的引入具有比传统单链表面活性剂更高的表面活性,同时分子结构中更多的可调控因素使其在水溶液中表现出更为丰富的自聚集行为,而且分子不同部位结构的改变对分子内或分子间相互作用产生不同的影响,可实现通过分子结构的设计有效调控其自聚集能力和聚集体结构。本综述将从联接基团、烷基链、亲水头基、反离子和其它功能性基团这五个方面概述近些年Gemini表面活性剂水溶液中聚集行为方面的研究进展,总结人们对Gemini表面活性剂分子间相互作用规律的认识,期望对于进一步发展这类高效的表面活性剂体系提供有益的帮助。     

Effects of surface interactions on peptide aggregate morphology

The formation of peptide aggregates mediated by an attractive surface is investigated using replica exchange molecular dynamics simulations with a coarse-grained peptide representation. In the absence of a surface, the peptides exhibit a range of aggregate morphologies, including amorphous aggregates, β-barrels and multi-layered fibrils, depending on the chiral stiffness of the chain (a measure of its β-sheet propensity). In contrast, aggregate morphology in the presence of an attractive surface depends more on surface attraction than on peptide chain stiffness, with the surface favoring fibrillar structures. Peptide-peptide interactions couple to peptide-surface interactions cooperatively to affect the assembly process both qualitatively (in terms of aggregate morphology) and quantitatively (in terms of transition temperature and transition sharpness). The frequency of ordered fibrillar aggregates, the surface binding transition temperature, and the sharpness of the binding transition all increase with both surface attraction and chain stiffness.     

卟啉类L-B膜分子间相互作用的光谱研究

以无取代的meso-四-(4-N)-吡啶基卟啉及其过渡族金属(主要Cu~(2+)、Zn~(2+))络合物制备L-B膜,以近紫外-可见吸收光谱和荧光光谱为手段,研究叶啉类分子在氯仿溶液中,L-B膜状态下以及固态状态下的相互作用。探讨分子聚集体的存在对光谱性质的影响。 为了研究叶咻类分子间的相互作用及其对光谱性质的影响,我们首先分析了叶啉在CHCl_3溶液中及固态状态下的近紫外-可见吸收光谱和荧光光谱。并将其与叶啉类分子的L-B膜作比较。结果表明,卟啉类的Soret吸收带带宽及峰位置在三种状态下均不相同,L-B膜的情况介于溶液中的和固体下的情况之间,说明了在L-B膜中,卟啉分子存在着某种形式的聚集体,且在这种聚集体中分子间的相互作用程度比固体弱,可以认为L-B膜上的分子呈准晶体状态。     

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.     

The stability of suspensions of multiwalled carbon nanotubes in organic solvents in the presence of triton X-165

The stability of suspensions of multiwalled carbon nanotubes in isopropanol, toluene, and p-xylene has been studied. Suspensions of multiwalled carbon nanotubes in isopropanol (unlike those in toluene and p-xylene) display high aggregation stability, which is explained by the formation of repulsive solvation layers as a result of interactions between the polar solvent and nanotube surface. The addition of a nonionic surfactant, Triton X-165, noticeably increases aggregate sizes and accelerates sedimentation processes in nanotube suspensions in isopropanol, while, in toluene and p-xylene, the aggregate sizes decrease and the nanotube suspensions are stabilized.     

3D Monte Carlo simulations of a magnetic disk-like particle dispersion

We have investigated the aggregate structures of a colloidal dispersion composed of ferromagnetic disk-like particles with a magnetic moment normal to the particle axis at the particle center, by means of 3D Monte Carlo simulations. Such disk-like particles have been modeled as a circular disk-like particle with the side section shape of spherocylinder. We have attempted to clarify the influences of the magnetic field strength, magnetic interactions between particles and volumetric fraction of particles. In order to discuss quantitatively the aggregate structures of clusters, we have focused on the radial distribution and orientational pair correlation functions, etc. For no applied magnetic field cases, long column-like clusters come to be formed with increasing magnetic particle–particle interactions. The internal structures of these clusters clearly show that the particles incline in a certain direction and their magnetic moments alternate in direction between the neighboring particles in the clusters. For applied magnetic field cases, the magnetic moment of each particle inclines in the magnetic field direction and therefore the column-like clusters are not formed straightforwardly. If the magnetic field is much stronger than magnetic particle–particle interactions, the particles do not have a tendency to form the clusters. As the influence of magnetic particle–particle interactions is significantly strong, thick chain-like clusters or column-like clusters or brick-wall-like clusters come to be formed along the magnetic field direction.     

Polymeric porphyrin and metalloporphyrin assemblies in bulk solution

Some water-soluble porphyrins aggregate in aqueous media to form fibrous assemblies. When polynucleotides are used as matrices, porphyrin monomers are aligned at the periphery or intercalated. Porphyrin-porphyrin interactions hardly occur. Metal-metal or metal-ligand-metal interactions have been observed in oxo-oligomers, pyrazine-bridged dimers and radical dimers. β-Substituted metallo-porphyrins, however, form no polymers. The only linear polymerization, which has so far been observed in molecular asemblies, occurs with amphiphilic porphyrin amides, amines and carboxylates and their metal complexes in water and of bacteriochlorophyll in DMF/water. These compounds form long-lived high molecular weight micellar, inverse micellar and vesicular fibers spontaneously in aqueous media. The porphyrin fibers in water are about 4-6 nm thick and up to several of μm long.     

Aggregation and solubility behavior of asphaltenes and their subfractions

Asphaltenes from four different crude oils (Arab Heavy, B6, Canadon Seco, and Hondo) were fractionated in mixtures of heptane and toluene and analyzed chemically, by vapor pressure osmometry (VPO), and by small angle neutron scattering (SANS). Solubility profiles of the asphaltenes and their subfractions indicated strong cooperative asphaltene interactions of a particular subfraction that is polar and hydrogen bonding. This subfraction had lower H/C ratios and modestly higher N, V, Ni, and Fe contents than the less polar and more soluble subfraction of asphaltenes. VPO and SANS studies indicated that the less soluble subfractions formed aggregates that were considerably larger than the more soluble subfractions. In general, asphaltene aggregate size increased with decreasing solvent aromaticity up to the solubility limit, beyond which the aggregate size decreased with heptane addition. The presence of a low wavevector Q feature in the scattering curves at 25 degrees C indicated that the individual aggregates were flocculating; however, the intensity of the feature was diminished upon heating of the samples to 80 degrees C. The solubility mechanism for Canadon Seco asphaltenes, the largest aggregate formers, appears to be dominated by aromatic pi-bonding interactions due to their low H/C ratio and low nitrogen content. B6 and Hondo asphaltenes formed similar-sized aggregates in heptol and the solubility mechanism is most likely driven by polar interactions due to their relatively high H/C ratios and high nitrogen contents. Arab Heavy, the least polar asphaltene, had a H/C ratio similar to Canadon Seco but formed the smallest aggregates in heptol. The enhancement in polar and pi-bonding interactions for the less soluble subfraction indicated by elemental analysis is reflected by the aggregate size from SANS. The less soluble asphaltenes contribute the majority of species responsible for aggregation and likely cause many petroleum production problems such as pipeline deposition and water-in-oil emulsion stabilization.     

Fluorescence spectra of hematoporphyrin and hematoporphyrin-diacetate aggregates in buffer solution

Different fluorescence spectra are observed on preferential excitation of monomer and aggregate species of hematoporphyrin and hematoporphyrin-diacetate, respectively. From these measurements we obtain the fluorescence spectrum of aggregate species in buffer solution, which shows two maxima, at 628 nm and around 700 nm. The fluorescence intensity in the region between these maxima is very sensitive to environmental changes. It is suggested that the changes in the fluorescence of hematoporphyrin in vivo as compared with that in buffer solution is associated with both hematoporphyrin-substrate interactions and an enhanced contribution from aggregate species.     

Colloidal Interaction at the Air-Liquid Interface

In this paper, we propose a model to analyze the stability of colloidal particles at the air-liquid interface. The proposed model for the colloidal particle interaction considers DLVO interactions and capillary, hydrophobic, and dipolar interactions between the particles. Typical values from the literature were assigned to most parameters included in the model. Numerical computation revealed the most important parameter in determining the total interaction is the density of dipoles at the external surface of the particles. We have found significant differences for the pair potential between hydrophobic and hydrophilic particles. Hydrophobic particles must aggregate in a principal minimum of the interaction potential curve while hydrophilic particles aggregate in a secondary minimum. Copyright 2000 Academic Press.     

Circularly Polarized Luminescence of Achiral Carbon Dots in Bi-Solvent Systems Triggered by Supramolecular Self-Assembly

An innovative strategy for circularly polarized luminescence (CPL) of carbon dots (CDs) has been developed: The achiral CDs displayed contrary supramolecular chirality and opposite CPL in two different bi-solvent systems, which are due to the formation of self-assembled helical nanostructures with two diverse assembly modes (P and M) in aggregate state via intermolecular π-π interactions and differential hydrogen bonding (H-bonding) without the need of any additional element of chirality.     

Multivalent interaction of cyclodextrin vesicles, carbohydrate guests, and lectins: a kinetic investigation

An artificial glycocalix self-assembles when unilamellar bilayer vesicles of amphiphilic β-cyclodextrins are decorated with maltose- and lactose-adamantane conjugates by host-guest interactions. The maltose-decorated vesicles aggregate in the presence of lectin concanavalin A whereas the lactose-decorated vesicles aggregate in the presence of lectin peanut agglutinin. The kinetics of the orthogonal multivalent interfacial interactions present in this ternary system of vesicles, carbohydrates, and lectins were studied by time-dependent measurements of the optical density at 400 nm. The average vesicle and vesicle aggregate sizes were monitored by dynamic light scattering. The aggregation process was evaluated as a function of lectin concentration, vesicle concentration, and surface coverage of the vesicles by the carbohydrate-adamantane conjugates. The initial rate of vesicle aggregation scales linearly with the lectin as well as the cyclodextrin vesicle concentration. Furthermore, each lectin requires a characteristic critical density of carbohydrates at the vesicle surface. These observations allow a prediction of the response of the ternary supramolecular system at different concentrations of its components. Also, the effective binding site separation in a multivalent receptor such as a multiple binding site protein can be accurately determined. This methodology can be extended to multivalent noncovalent interactions in other ligand-receptor systems at interfaces.