Probing multivalent interactions in a synthetic host-guest complex by dynamic force spectroscopy

Multivalency is present in many biological and synthetic systems. Successful application of multivalency depends on a correct understanding of the thermodynamics and kinetics of this phenomenon. In this Article, we address the stability and strength of multivalent bonds with force spectroscopy techniques employing a synthetic adamantane/β-cyclodextrin model system. Comparing the experimental findings to theoretical predictions for the rupture force and the kinetic off-rate, we find that when the valency of the complex is increased from mono- to di- to trivalent, there is a transition from quasi-equilibrium, with a constant rupture force of 99 pN, to a kinetically dependent state, with loading-rate-dependent rupture forces from 140 to 184 pN (divalent) and 175 to 210 pN (trivalent). Additional binding geometries, parallel monovalent ruptures, single-bound divalent ruptures, and single- and double-bound trivalent ruptures are identified. The experimental kinetic off-rates of the multivalent complexes show that the stability of the complexes is significantly enhanced with the number of bonds, in agreement with the predictions of a noncooperative multivalent model.     

Colloidal interactions at fluid interfaces

We discuss qualitative and quantitative aspects of the effective interactions between micrometer-sized colloids of different types trapped at fluid interfaces, with a particular emphasis on the relation between experimental and theoretical results. For colloids of that size, the interactions can broadly be classified into "direct" ones such as electrostatic, magnetic, or elastic ones. Such interactions appear also for colloids in bulk systems, but they are modified at interfaces. On the other hand, the presence of a fluid interface generates in addition interface-mediated (capillary) interactions which are either induced by nonspherical colloid shapes or by the "direct" interactions.     

A model for investigating the behaviour of non-spherical particles at interfaces

This paper introduces a simple method for modelling non-spherical particles with a fixed contact angle at an interface whilst also providing a method to fix the particles orientation. It is shown how a wide variety of particle shapes (spherical, ellipsoidal, disc) can be created from a simple initial geometry containing only six vertices. The shapes are made from one continuous surface with edges and corners treated as smooth curves not discontinuities. As such, particles approaching cylindrical and orthorhombic shapes can be simulated but the contact angle crossing the edges will be fixed. Non-spherical particles, when attached to an interface can cause large distortions in the surface which affect the forces acting on the particle. The model presented is capable of resolving this distortion of the surface around the particle at the interface as well as allowing for the particle's orientation to be controlled. It is shown that, when considering orthorhombic particles with rounded edges, the flatter the particle the more energetically stable it is to sit flat at the interface. However, as the particle becomes more cube like, the effects of contact angle have a greater effect on the energetically stable orientations. Results for cylindrical particles with rounded edges are also discussed. The model presented allows the user to define the shape, dimensions, contact angle and orientation of the particle at the interface allowing more in-depth investigation of the complex phenomenon of 3D film distortion around an attached particle and the forces that arise due to it.     

A polarizable continuum model for molecules at diffuse interfaces

In this work we illustrate an extension of the polarizable continuum model to describe solvation effects on molecules at the interface between two fluid phases (liquid/liquid, liquid/vapor). This extension goes beyond the naive picture of the interface as a plane dividing two distinct dielectrics, commonly employed in continuum models. The main feature of the model is the use of a diffuse interface with an electric permittivity depending on the position. This characteristic clearly allows the study of simple interfaces as well as more complex membrane or multilayer structures. Moreover the smooth variation of the permittivity in the diffuse interface, in contrast to the sharp boundary between two regions, overcomes the numerical divergences due to charges placed at the boundary. The implementation of the model relies on the integral equation formalism, which allows one to calculate the reaction field acting on a molecule immersed in a dielectric environment once the proper Green's function is known. In the present case, such a Green's function is obtained numerically, allowing a large flexibility in the choice of the dielectric permittivity profile. The applications have been selected with the aim of illustrating the capabilities of the model; its present limitations are also discussed.     

A noncontinuum model of van der Waals interactions for describing the physicochemical properties of pure molecular liquids

A noncontinuum model based on the use of such molecular characteristics as molecular refraction, dipole moment, and molar volume is suggested for quantitatively describing the physicochemical properties (surface tension, enthalpy of vaporization, boiling temperature, viscosity, etc.) of pure molecular liquids. The ratio between the coefficients of correlation equations relating electrostatic and dispersion contributions to all the properties analyzed was found to be invariant.     

Hybrid host-guest complexes: directing the supramolecular structure through secondary host-guest interactions

A set of four hybrid host-guest complexes based on the inorganic crown ether analogue [H12W36O120]12- ({W36}) have been isolated and characterised. The cluster anion features a central rigid binding site made up of six terminal oxygen ligands and this motif allows the selective binding of a range of alkali and alkali-earth-metal cations. Here, the binding site was utilised to functionalise the metal oxide-based cavity by complexing a range of protonated primary amines within the recognition site. As a result, a set of four hybrid organic-inorganic host-guest complexes were obtained whereby the interactions are highly directed specifically within this cavity. The guest cations in these molecular assemblies range from the aromatic 2-phenethylamine (1) and 4-phenylbutylamine (2) to the bifunctional aromatic p-xylylene diamine (3) and the aliphatic, bifunctional 1,6-diaminohexane (4). Compounds 1-4 were structurally characterised by single-crystal X-ray diffraction, elemental analysis, flame atomic absorption spectroscopy, FTIR and bond valence sum calculations. This comparative study focuses on the supramolecular effects of the amine guest cations and investigates their structure-directing effects on the framework arrangement arising by locking the protonated amines within the cavity of the {W36} cluster. It was shown that parts of the organic guest cation protrude from the central binding cavity and the nature of this protruding organic "tail" directs the solid-state arrangement of compounds 1-4. Guest cations with a hydrophobic phenyl tail result in an antiparallel assembly of {W36} complexes arranged in a series of pillared layers. As a consequence, no direct supramolecular interactions between {W36} clusters are observed. In contrast, bifunctional guest cations with a secondary amino binding site act as molecular connectors and directly link two cluster units thus locking the supramolecular assembly in a tilted arrangement. This direct linking of {W36} anions results in the formation of an infinite supramolecular scaffold.     

A time correlation function theory describing static field enhanced third order optical effects at interfaces

Sum vibrational frequency spectroscopy, a second order optical process, is interface specific in the dipole approximation. At charged interfaces, there exists a static field, and as a direct consequence, the experimentally detected signal is a combination of enhanced second and static field induced third order contributions. There is significant evidence in the literature of the importance/relative magnitude of this third order contribution, but no previous molecularly detailed approach existed to separately calculate the second and third order contributions. Thus, for the first time, a molecularly detailed time correlation function theory is derived here that allows for the second and third order contributions to sum frequency vibrational spectra to be individually determined. Further, a practical, molecular dynamics based, implementation procedure for the derived correlation functions that describe the third order phenomenon is also presented. This approach includes a novel generalization of point atomic polarizability models to calculate the hyperpolarizability of a molecular system. The full system hyperpolarizability appears in the time correlation functions responsible for third order contributions in the presence of a static field.     

A model of competing van der Waals interactions for describing the physicochemical properties of binary solutions of nonelectrolytes

A model of competing van der Waals (universal) interactions was suggested for describing the excess physicochemical properties of binary systems and excess thermodynamic functions of solvation. As distinct from the Redlich-Kister and Hwang polynomials, the coefficients of this model are functionally significant and related to various mechanisms of van der Waals, in particular, electrostatic interaction manifestations as the composition of the binary solvent changes. The suggested model was used to estimate the contributions of electrostatic nonstoichiometric interactions and mutual component destructuring effects to the observed physicochemical properties of binary systems. The empirical Dimroth-Reichardt solvatochromic scale of solvent polarities for binary systems was shown to be related to the electrostatic contribution of the model.     

A two-state solvent model for rigid rods adsorption at charged interfaces

A simple two-state solvent model is developed to describe the adsorption of rigid rods at polarizable interfaces. The adsorption isotherm and the equation of state are determined by means of the lattice theory of strictly regular solutions, assuming that the solvent at the interface exists in the form of monomers with two possible orientations, and the adsorbed rods are oriented normal to the electrode surface. In the case of the mercury/aqueous inert electrolyte interface in the presence of small, polar, aliphatic hydrocarbon derivatives, the model predicts that the variation of the isotherm interaction parameter is determined mainly by the dipole-dipole interactions between the permanent dipoles of the adsorbed molecules. This variation becomes more pronounced as the dipolar interactions between adsorbate-solvent molecules become more intense. The Marshall-Conway treatment for the polarization catastrophe and the approach of Levine et al. in incorporating polarizability effects are also taken into consideration and examined critically.     

Physics of interactions at biological and biomaterial interfaces

Soft interlayers based on membranes and biopolymers define the spatial boundaries between different phases in biological systems. Physical interactions of soft matter under biologically relevant conditions (in aqueous media containing various ions) are governed by complex interplays of generic and specific interfacial interactions, which are clearly different from those acting at the interface between hard matter. This review aims at providing a comprehensive overview on: (a) models of cell–cell and cell–tissue interfaces with aid of defined building blocks, (b) new X-ray and neutron scattering techniques to probe fine structures, electrostatics, and mechanics of soft interfaces, and (c) control of dynamic cell morphology and migration of cells using tailor-made, soft interfaces.     

A lattice-gas model for specific ion adsorption at liquid | liquid interfaces

A lattice-gas model is used to investigate the specific adsorption of ions at the interface between two immiscible electrolyte solutions. From Monte Carlo simulations, the profiles of particle densities and of the electrostatic potential are obtained. Specific adsorption is shown to affect the potential distribution markedly. In some cases an overshoot of the potential can be observed, an effect that is well known from specific adsorption at metal electrodes. This redistribution of charge and potential can increase the interfacial capacity, shift the potential of zero charge, and influence the rate of electron-transfer reactions.     

A molecular model for cohesive slip at polymer melt/solid interfaces

A molecular model is proposed which predicts wall slip by disentanglement of polymer chains adsorbed on a wall from those in the polymer bulk. The dynamics of the near-wall boundary layer is found to be governed by a nonlinear equation of motion, which accounts for such mechanisms on surface chains as convection, retraction, constraint release, and thermal fluctuations. This equation is valid over a wide range of grafting regimes, including those in which interactions between neighboring adsorbed molecules become essential. It is not closed since the dynamics of adsorbed chains is shown to be coupled to that of polymer chains in the bulk via constraint release. The constitutive equations for the layer and bulk, together with continuity of stress and velocity, are found to form a closed system of equations which governs the dynamics of the whole "bulk+boundary layer" ensemble. Its solution provides a stick-slip law in terms of the molecular parameters and extruder geometry. The model is quantitative and contains only those parameters that can be measured directly, or extracted from independent rheological measurements. The model predictions show a good agreement with available experimental data.     

Molecular modelling of host-guest interactions for mass sensitive chemical sensors

Paracyclophanes are effective coatings for mass sensitive chemical sensors. The enzyme analogue recognition can preferably be used to detect aromatic and halogenated hydrocarbons. Molecular modelling by the MM3 force field allows the prediction of an efficient analyte inclusion. Besides the necessary steric complementarity it could be shown that the interaction between the methyl groups of aromatic guests (e.g. toluene) and the aromatic walls of the host is essential for hostguest complexation. These phenomena get more pronounced if bicyclic cyclophanes are applied and these sensitive materials enable the detection of only a few ppm of toluene with SAW devices. Furtheron, the electron-rich diphenylether moieties of these hosts guarantee interactions with electron-deficiency analytes such as chlorinated hydrocarbons with a selectivity superior to that of the monocyclic materials.Dedicated to Professor Dr. Dr. h.c. mult. J.F.K. Huber on the occasion of his 70th birthday     

A BET model of the thermodynamics of aqueous multicomponent solutions at extreme concentration

The statistical mechanical basis of the use of Brunauer-Emmett-Teller isotherms to represent activities and other thermodynamic properties in extremely concentrated solutions was established by Ally and Braunstein (J. Chem. Thermodynamics1998, 30, 49–58) for a two-salt, single-solvent, mixture. Based upon the work of these authors, we have derived equations for solute and solvent activities in liquid mixtures containing a single solvent and indefinite number of solutes. New terms have been added to the model equations to express the effects of ternary ion interactions on the salt adsorption parameters. Solution composition is defined on the basis of salts, rather than ions, as components. As examples, the model is used to represent water activities in concentrated (lithium nitrate  +  potassium nitrate  +  water) and (lithium ion  +  sodium ion  +  chloride ion  +  nitrate ion  +  water) mixtures, and salt solubilities in (calcium chloride  +  calcium nitrate  +  water) mixtures.     

基于柱芳烃主客体作用构筑的超分子纳米载体

近年来,基于主客体相互作用的超分子纳米载体因其独特的自组装特性在癌症治疗领域引起了广泛的关注。柱芳烃作为一种新型的大环分子,因其独特的化学结构和优越的主客体包合能力而成为近年来研究的热点。本文根据不同的治疗机制综述了柱状链纳米载体及其在化疗、光动力治疗、联合治疗等领域的应用。在此基础上,展望了柱芳烃基纳米载体的研究方向和发展趋势。     

Carbohydrate-protein interactions at interfaces: synthesis of thiolactosyl glycolipids and design of a working model for surface plasmon resonance

Thiolactosyl lipids designed for carbohydrate-protein binding studies have been synthesised. One representative was selected for binding studies with a plant lectin RCA120, the agglutinin from Ricinus communis. The interactions were measured quantitatively in real time using a BIAcore surface plasmon resonance instrument. Removal of much of the galactose from the thiolactosyl lipid in situ with beta-galactosidase showed that the lectin binding was highly specific. A dissociation constant KD = 8.77 x 10(-8) M was measured for 1-[2-[2-(2-[beta-D-galactopyranosyl-(1-->4)-1-thio-beta-D -glucopyranosyl]ethoxy)ethoxy]ethoxy]octadecane 30 which is four orders of magnitude greater than that determined for binding to lactose in solution. A concentration of lactose of > 80 mM was required to block the lectin binding to thiolactosyl lipid in a neomembrane.     

Control of the ketone to gem-diol equilibrium by host-guest interactions

In water, N-methyl-4-(p-substituted benzoyl)pyridinium cations, BP-X, exist in equilibrium with their hydrated forms (gem-diols), whose concentrations depend on the para substituent (-X). In the presence of cucurbit[7]uril (CB[7]), the benzoyl group shows a preference for the CB[7] cavity, and the ketone to gem-diol equilibrium is shifted toward the keto form, meaning that the stabilization realized through hydrophobic interactions of the benzoyl group in the CB[7] cavity exceeds the hydrogen-bonding stabilization of the gem-diols in the aqueous environment.