Spontaneous Stepwise Self‐Assembly of a Polyoxometalate–Organic Hybrid into Catalytically Active One‐Dimensional Anisotropic Structures

An inorganic–organic hybrid surfactant with a hexavanadate cluster as the polar head group was designed and observed to assemble into micelle structures, which further spontaneously coagulate into a 1D anisotropic structure in aqueous solutions. Such a hierarchical self‐assembly process is driven by the cooperation of varied noncovalent interactions, including hydrophobic, electrostatic, and hydrogen‐bonding interactions. The hydrophobic interaction drives the quick formation of the micelle structure; electrostatic interactions involving counterions leads to the further coagulation of the micelles into larger assemblies. This process is similar to the crystallization process, but the specific counterions and the directional hydrogen bonding lead to the 1D growth of the final assemblies. Since most of the hexavanadates are exposed to the surface, the 1D assembly with nanoscale thickness is a highly efficient heterogeneous catalyst for the oxidation of organic sulfides with appreciable recyclability.     

Supramolecular Assembly of Poly(propyleneimine) Dendrimers Driven By Simple Monovalent Counterions

The self‐assembly of semiglobular, positively charged poly(propyleneimine) (PPI) dendrimers with small monovalent counterions (e.g., Cl^?) in water/acetone mixtures was investigated. We showed that PPI dendrimers can assemble into hollow, spherical, single‐layered blackberry‐type structures mediated by the presence of monovalent counterions. The effects on the assembly of changing the solvent polarity and adjusting the pH were further investigated to confirm the presence of electrostatic interactions and hydrogen bonding as the driving forces. Results showed that PPI dendrimers form stable, hollow spheres in 5–20 % v/v acetone/water and that the size of the spheres decreases monotonically as the solvent polarity and/or the charge on the dendrimers (i.e., lower solution pH) increases. This is the first example to show that small monovalent counterions can trigger attraction among PPI dendrimers (or broadly defined polyelectrolytes) that is strong enough to bring them together to form large, stable supramolecular assemblies, which indicates that these organic macroions have similar solution behavior to more‐well‐defined inorganic molecular macroions.     

Controlling electrostatic co-assembly using ion-containing copolymers: from surfactants to nanoparticles

In this review, we address the issue of the electrostatic complexation between charged-neutral diblock copolymers and oppositely charged nanocolloids. We show that nanocolloids such as surfactant micelles and iron oxide magnetic nanoparticles share similar properties when mixed with charged-neutral diblocks. Above a critical charge ratio, core-shell hierarchical structures form spontaneously under direct mixing conditions. The core-shell structures are identified by a combination of small-angle scattering techniques and transmission electron microscopy. The formation of multi-level objects is driven by the electrostatic attraction between opposite charges and by the release of the condensed counterions. Alternative mixing processes inspired from molecular biology are also described. The protocols applied here consist in screening the electrostatic interactions of the mixed dispersions, and then removing the salt progressively as an example by dialysis. With these techniques, the oppositely charged species are intimately mixed before they can interact, and their association is monitored by the desalting kinetics. As a result, sphere- and wire-like aggregates with remarkable superparamagnetic and stability properties are obtained. These findings are discussed in the light of a new paradigm which deals with the possibility to use inorganic nanoparticles as building blocks for the design and fabrication of supracolloidal assemblies with enhanced functionalities.     

Interaction of similarly charged surfaces mediated by nanoparticles

We study the interaction between two like charged surfaces embedded in a solution of oppositely charged multivalent rod-like counterions.The counterions consist of two rigidly bonded point charges,each of valency Z.The strength of the electrostatic coupling increases with increasing surface charge density or valency of the charges.The system is analyzed by employing a self-consistent field theory,which treats the short and long range interactions of the counterions within different approximations.We find that in the weak coupling limit,the interactions are only repulsive.In the intermediate coupling regime,the multivalent rod-like counterions can mediate attractive interactions between the surfaces. For sufficiently long rods,bridging contributes to the attractive interaction.In the strong coupling limit,the charge correlations can contribute to the attractive interactions at short separations between the charged surfaces.Two minima can then appear in the force curve between surfaces.     

The driving force for solute retention in electron donor-acceptor chromatography: Electrostatic versus charge-transfer interactions

Summary Charge-transfer interactions are often assumed to be dominant among the noncovalent interactions that govern the solute retention in electron donor-acceptor chromatography. This popular view, however, has been called into question by recent studies that suggest an important role for electrostatic interactions in the formation of donor-acceptor complexes. We reported here an experimental investigation concerning the question as to whether charge-transfer or electrostatic interactions are the driving force for solute retention in donor-acceptor chromatography. Using three chromatographic systems composed of a dinitrobenzene derived stationary phase and a hexane based mobile phase, we determined retention factors for a range of aliphatic and aromatic hydrocarbons and correlated them with molecular properties that describe the solute's dispersion, charge-transfer, and electrostatic characteristics. It was found that the molecular polarizability and ionization potential give either very poor or no correlation with solute retention whereas the molecular quadrupole moment is a linear function of the logarithmic retention factor. These results were interpreted as showing that electrostatic, rather than charge-transfer or dispersion, interactions play a major role in determining solute retention. The dominance of the electrostatic interactions over the other noncovalent interactions was discussed in terms of distance dependency of the interaction energy. Dedicated to Professor John H. Knox on the occasion of his 70th birthday.     

Crystallography Without a Lattice

Molecular structure determination in crystals depends on the presence of the crystal lattice. In liquids, there is no underlying lattice, so phase relationships between scattered X-rays or neutrons are not preserved. Hence, only pair-distance distributions can be obtained from simple diffraction experiments. Advances in the past 30 years in radiation sources, instrumentation, and computing have enabled us to go beyond this apparent limitation. We can now obtain detailed structural information on relatively complex liquids and thus see clearly for the first time how molecules actually interact in solution, and how the solvent is perturbed by the presence of the solute. Using as an example recent work on aqueous solutions of tertiary butanol, the ways in which we can obtain high quality structural information in the absence of a lattice are summarized. Not only can we see how intermolecular interactions are influenced by changes in temperature and concentration, we can also begin to see where the structural source of the entropic driving force for the hydrophobic interaction may be found.     

细菌化学趋向性受体聚集体的相互作用

细菌化学趋向性受体的最小结构单元为二聚体,在细胞膜上这些二聚体会聚集成大团簇.X射线晶体结构和低分辨电镜结构测定表明,这些团簇有两类不同的形式,一种是在晶体结构中观察到的倒金字塔式二聚体的三聚体重复形成的聚集,另一种为由二聚体尾部相互盘绕形成的拉链状聚集.有关拉链状聚集的详细分子模型目前尚不清楚.本文使用蛋白质-蛋白质对接的方法研究了大肠杆菌丝氨酸化学趋向性受体Tsr二聚体之间的相互作用.分子对接计算表明,倒金字塔式聚集和拉链状聚集的基本复合物都是可以出现的,相应复合物的分子动力学模拟表明这些结构都具有一定的稳定性.对于所获得的拉链状聚集体的基本复合物结构模型进行了详细的二聚体作用界面分析,发现二聚体间主要通过静电和疏水作用形成复合物,其中Arg388、Phe373和Ile377是形成拉链状聚集的关键作用残基.所建立的Tsr拉链状聚集的结构模型有助于揭示细菌化学趋向性受体在细胞膜上聚集的分子机制,为进一步的聚集理论及模拟研究提供了基础.     

细菌化学趋向性受体聚集体的相互作用

细菌化学趋向性受体的最小结构单元为二聚体,在细胞膜上这些二聚体会聚集成大团簇. X射线晶体结构和低分辨电镜结构测定表明,这些团簇有两类不同的形式,一种是在晶体结构中观察到的倒金字塔式二聚体的三聚体重复形成的聚集,另一种为由二聚体尾部相互盘绕形成的拉链状聚集. 有关拉链状聚集的详细分子模型目前尚不清楚. 本文使用蛋白质-蛋白质对接的方法研究了大肠杆菌丝氨酸化学趋向性受体Tsr 二聚体之间的相互作用. 分子对接计算表明,倒金字塔式聚集和拉链状聚集的基本复合物都是可以出现的,相应复合物的分子动力学模拟表明这些结构都具有一定的稳定性. 对于所获得的拉链状聚集体的基本复合物结构模型进行了详细的二聚体作用界面分析,发现二聚体间主要通过静电和疏水作用形成复合物,其中Arg388、Phe373 和Ile377是形成拉链状聚集的关键作用残基. 所建立的Tsr 拉链状聚集的结构模型有助于揭示细菌化学趋向性受体在细胞膜上聚集的分子机制,为进一步的聚集理论及模拟研究提供了基础.     

DNA cholesteric phases: the role of DNA molecular chirality and DNA-DNA electrostatic interactions

DNA molecules form dense liquid-crystalline twisted phases both in vivo and in vitro. How the microscopic DNA chirality is transferred into intermolecular twist in these mesophases and what is the role of chiral DNA-DNA electrostatic interactions is still not completely clear. In this paper, we first give an extended overview of experimental observations on DNA cholesteric phases and discuss the factors affecting their stability. Then, we consider the effects of steric and electrostatic interactions of grooved helical molecules on the sign of cholesteric twist. We present some theoretical results on the strength of DNA-DNA chiral electrostatic interactions, on DNA-DNA azimuthal correlations in cholesteric phases, on the value of DNA cholesteric pitch, and on the regions of existence of DNA chiral phases stabilized by electrostatic interactions. We suggest for instance that 146 bp long DNA fragments with stronger affinities for the nucleosome formation can form less chiral cholesteric phases, with a larger left-handed cholesteric pitch. Also, the value of left-handed pitch formed in assemblies of homologous DNA fragments is predicted to be smaller than that of randomly sequenced DNAs. We expect also the cholesteric assemblies of several-kbp-long DNAs to require higher external osmotic pressures for their stability than twisted phases of short nucleosomal DNA fragments at the same DNA lattice density.     

Competition between Monovalent and Divalent Counterions in Surfactant Systems

Competition between mono- and divalent ions in the association of counterions to the headgroups of amphiphiles was studied in one surfactant system with organic counterions (piperidine⁺/piperazine²⁺octanesulfonate) and one with inorganic counterions (Na⁺/Ca²⁺octyl sulfate). By conductivity and¹³C NMR chemical shift measurements the critical micelle concentration (CMC) was found to decrease drastically when small amounts of divalent counterions were present in the system. Self-diffusion coefficients of surfactant ions and organic counterions were measured in the micellar phase by the Fourier transform pulsed-gradient spin-echo (FT-PGSE) NMR method. The degree of counterion binding in the micellar system with piperidine⁺/piperazine²⁺counterions was obtained from FT-PGSE NMR measurements. It was observed that the divalent counterions were more strongly bound than the monovalent counterions. The experimental results were compared with theoretical Poisson–Boltzmann calculations. The cell model was used to study the electrostatic effects. Good agreement between electrostatic theory and experiment was observed; however, an attractive force exists between the monovalent piperidine counterions and the micelle, probably because of hydrophobic interactions.     

Quantitative Protein Corona Composition and Dynamics on Carbon Nanotubes in Biological Environments

When nanoparticles enter biological environments, proteins adsorb to form the “protein corona” which alters nanoparticle biodistribution and toxicity. Herein, we measure protein corona formation on DNA-functionalized single-walled carbon nanotubes (ssDNA-SWCNTs), a nanoparticle used widely for sensing and delivery, in blood plasma and cerebrospinal fluid. We characterize corona composition by mass spectrometry, revealing high-abundance corona proteins involved in lipid binding, complement activation, and coagulation. We investigate roles of electrostatic and entropic interactions driving selective corona formation. Lastly, we study real-time protein binding on ssDNA-SWCNTs, obtaining agreement between enriched proteins binding strongly and depleted proteins binding marginally, while highlighting cooperative adsorption mechanisms. Knowledge of protein corona composition, formation mechanisms, and dynamics informs nanoparticle translation from in vitro design to in vivo application.     

Hydration of ions and formation of crystal hydrates

Effects of electrostatic and entropic factors on the dependence between the type of hydration of ions and their ability to form crystal hydrates from water solutions are discussed. It is shown that in the case of the single-charged ions because of the insignificant electrostatic interaction the crystal hydrates are formed mainly by the salts containing ions with different type of hydration. In the case of poly-charged ions the increase in the electrostatic interaction leads to formation of crystal hydrates mainly by the salts containing ions with the positive type of hydration. It was established also that the formation of salt crystal hydrates is influenced not only by the electrostatic and entropic factors, but also by the spatial arrangement and the electronic density distribution of ions.     

Structural role of counterions adsorbed on self-assembled peptide nanotubes

Among noncovalent forces, electrostatic ones are the strongest and possess a rather long-range action. For these reasons, charges and counterions play a prominent role in self-assembly processes in water and therefore in many biological systems. However, the complexity of the biological media often hinders a detailed understanding of all the electrostatic-related events. In this context, we have studied the role of charges and counterions in the self-assembly of lanreotide, a cationic octapeptide. This peptide spontaneously forms monodisperse nanotubes (NTs) above a critical concentration when solubilized in pure water. Free from any screening buffer, we assessed the interactions between the different peptide oligomers and counterions in solutions, above and below the critical assembly concentration. Our results provide explanations for the selection of a dimeric building block instead of a monomeric one. Indeed, the apparent charge of the dimers is lower than that of the monomers because of strong chemisorption. This phenomenon has two consequences: (i) the dimer-dimer interaction is less repulsive than the monomer-monomer one and (ii) the lowered charge of the dimeric building block weakens the electrostatic repulsion from the positively charged NT walls. Moreover, additional counterion condensation (physisorption) occurs on the NT wall. We furthermore show that the counterions interacting with the NTs play a structural role as they tune the NTs diameter. We demonstrate by a simple model that counterions adsorption sites located on the inner face of the NT walls are responsible for this size control.     

Eluotropic strength in non-aqueous liquid chromatography with porous graphitic carbon

Protein adsorption can be either endothermic or exothermic depending upon the protein, the sorbent and process conditions. In the case of protein adsorption onto ion-exchange surfaces exothermic adsorption heats are usually characterized as representing the electrostatic interaction between two oppositely charged surfaces. Endothermic adsorption heats are typically characterized as representing protein reconfiguration and/or repulsive interactions between adsorbed molecules. In certain segments of the literature surface dehydration and solution non-idealities have been suggested as possible sources of endothermic heats of adsorption. Each of these phenomena was investigated during studies concerning the adsorption of bovine serum albumin and ovalbumin onto an anion-exchange sorbent. The results demonstrated that electrostatic repulsive interactions between adsorbed molecules appears to be a larger contributor to endothermic heats of adsorption than surface dehydration or solution non-idealities. The presence of mobile phase cations can reduce the magnitude of endothermic adsorption heats by screening repulsive interactions between adsorbed molecules. Although water release was not found to be a major contributor to endothermic adsorption heats, it is likely to be a contributor to the entropic driving force associated with the adsorption of bovine serum albumin.     

The influence of discrete surface charges on the force between charged surfaces

The force between two parallel charged flat surfaces, with discrete surface charges, has been calculated with Monte Carlo simulations for different values of the electrostatic coupling. For low electrostatic coupling (small counterion valence, small surface charge, high dielectric constant, and high temperature) the total force is dominated by the entropic contribution and can be described by mean field theory, independent of the character of the surface charges. For moderate electrostatic coupling, counterion correlation effects lead to a smaller repulsion than predicted by mean field theory. This correlation effect is strengthened by discrete surface charges and the repulsive force is further reduced. For large electrostatic coupling the total force for smeared out surface charges is known to be attractive due to counterion correlations. If discrete surface charges are considered the attractive force is weakened and can even be turned into a repulsive force. This is due to the counterions being strongly correlated to the discrete surface charges forming effective, oppositely directed, dipoles on the two walls.     

Specific ion effects in solutions of globular proteins: comparison between analytical models and simulation

Monte Carlo simulations have been performed for ion distributions outside a single globular macroion and for a pair of macroions, in different salt solutions. The model that we use includes both electrostatic and van der Waals interactions between ions and between ions and macroions. Simulation results are compared with the predictions of the Ornstein-Zernike equation with the hypernetted chain closure approximation and the nonlinear Poisson-Boltzmann equation, both augmented by pertinent van der Waals terms. Ion distributions from analytical approximations are generally very close to the simulation results. This demonstrates that properties that are related to ion distributions in the double layer outside a single interface can to a good approximation be obtained from the Poisson-Boltzmann equation. We also present simulation and integral equation results for the mean force between two globular macroions (with properties corresponding to those of hen-egg-white lysozyme protein at pH 4.3) in different salt solutions. The mean force and potential of mean force between the macroions become more attractive upon increasing the polarizability of the counterions (anions), in qualitative agreement with experiments. We finally show that the deduced second virial coefficients agree quite well with experimental results.     

Molecular dynamics simulations of multilayer polyelectrolyte films: effect of electrostatic and short-range interactions

The effect of the strength of electrostatic and short-range interactions on the multilayer assembly of oppositely charged polyelectrolytes at a charged substrate was studied by molecular dynamics simulations. The multilayer buildup was achieved through sequential adsorption of charged polymers in a layer-by-layer fashion from dilute polyelectrolyte solutions. The strong electrostatic attraction between oppositely charged polyelectrolytes at each deposition step is a driving force behind the multilayer growth. Our simulations have shown that a charge reversal after each deposition step is critical for steady multilayer growth and that there is a linear increase in polymer surface coverage after the first few deposition steps. Furthermore, there is substantial intermixing between chains adsorbed during different deposition steps. We show that the polymer surface coverage and multilayer structure are each strongly influenced by the strength of electrostatic and short-range interactions.     

Cation Translocation around Single Polyoxometalate–Organic Hybrid Cluster Regulated by Electrostatic and Cation–π Interactions

We report herein an interesting dynamic translocation process of countercations around one polyoxometalate(POM)–organic hybrid anionic cluster at various concentrations and temperatures. It was found that both electrostatic interactions and cation–π interactions regulate the position of small countercations around single clusters. The dynamic geometry and the symmetry of the hybrid macroions are largely affected by the type of counterions, as shown by nuclear magnetic resonance (NMR) spectroscopy studies and all‐atom molecular dynamics simulation. It is also shown that electrostatic interactions dominate over cation–π interactions in determining the locations of the counterions in the current system.     

Ejection dynamics of semiflexible polymers out of a nanochannel

Using theoretical analysis and three-dimensional Langevin dynamics simulations, we investigate the influence of chain rigidity on the ejection dynamics of polymers from a nanochannel. We find that there exist two distinct dynamical regimes divided by a critical chain length for both flexible and semiflexible chains. At the short chain regime, semiflexible chains eject faster than flexible chains of the same chain length due to the longer occupying length. In contrast, at the long chain regime, semiflexible chains eject slower than flexible ones as the effective entropic driving force decreases. Based on these results, we propose that the nanochannels could be used to separate flexible and semiflexible chains effectively.     

C-H...F hydrogen bonding: the origin of the self-assemblies of bis(2,2'-difluoro-1,3,2-dioxaborine)

The effect of the molecular structure on the self-assembly of specially designed two-core 1,3,2-dioxaborines has been studied with various techniques. It was found that the molecules spontaneously adsorbed on HOPG surfaces and self-organized into well-ordered two-dimensional (2D) monolayers. The structural details of the 2D assemblies were investigated by scanning tunneling microscopy (STM). From X-ray analysis of the corresponding three-dimensional (3D) crystal and from theoretical calculation, we were able to reveal the driving force behind the specific self-assembly. The C-H...F hydrogen bonding between the ortho carbon of the phenyl ring and the fluorine of the BF2 group plays an important role in the formation of the adlayers. The different electron affinities and geometries of the molecules affect the intermolecular interactions which further lead to different properties in the bulk materials.