Molecular Dynamics Study of Ionic Liquids Complexation within β-Cyclodextrins

Inclusion Complexation of Organic Micropollutants with β-Cyclodextrin

Recently, β-cyclodextrin (βCD)-based polymers with enhanced adsorption kinetics and high removal capacity of organic micropollutants (OMPs) and uptake rates have been synthesized and tested experimentally. Although the exact physical–chemical mechanisms via which these polymers capture the various types of OMPs are not yet fully understood, it is suggested that the inclusion complex formation of OMPs with βCD is very important. In this study, the inclusion complex formation of OMPs with βCD in an aqueous solution is investigated by using the well-established attach–pull–release method in force field-based molecular dynamics simulations. A representative set of OMPs is selected based on the measured occurrences in surface and ground waters and the directives published by the European Union. To characterize the formation of the inclusion complex, the binding free energies, enthalpies, and entropies are computed and compared to experimental values. It is shown that computations using the q4md-CD/GAFF/Bind3P force field combination yield binding free energies that are in reasonable agreement with the experimental results for all OMPs studied. The binding enthalpies are decomposed into the main contributing interaction types. It is shown that, for all studied OMPs, the van der Waals interactions are favorable for the inclusion complexion and the hydrogen bond formation of the guest with the solvent and βCD plays a crucial role in the binding mechanism. Our findings show that MD simulations can adequately describe the inclusion complex formation of βCD with OMPs, which is the first step toward understanding the underlying mechanisms via which the βCD-based polymers capture OMPs.

Highly viscoelastic films at the water/air interface: α-Cyclodextrin with anionic surfactants

This work showcases the remarkable viscoelasticity of films consisting of α-cyclodextrin (α-CD) and anionic surfactants (S) at the water/air interface, the magnitude of which has not been observed in similar systems. The anionic surfactants employed are sodium salts of a homologous series of n-alkylsulfates (n = 8-14) and of dodecylsulfonate. Our hypothesis was that the very high viscoelasticity can be systematically related to the bulk and interfacial properties of the system. Through resolution of the bulk distribution of species using isothermal titration calorimetry, the high dilatational modulus is related to (α-CD)₂:S₁ inclusion complexes in the bulk with respect to both the bulk composition and temperature. Direct interfacial characterization of α-CD and sodium dodecylsulfate films at 283.15 K using ellipsometry and neutron reflectometry reveals that the most viscoelastic films consist of a highly ordered monolayer of 2:1 complexes with a minimum amount of any other component. The orientation of the complexes in the films and their driving force for adsorption are discussed in the context of results from molecular dynamics simulations. These findings open up clear potential for the design of new functional materials or molecular sensors based on films with specific mechanical, electrical, thermal, chemical, optical or even magnetic properties.

Validation and Comparison of Force Fields for Native Cyclodextrins in Aqueous Solution

Molecular dynamics simulations of native α-, β-, and γ-cyclodextrin in aqueous solution have been conducted with the goal to investigate the performance of the CHARMM36 force field, the AMBER-compatible q4md-CD force field, and five variants of the GROMOS force field. The properties analyzed are structural parameters derived from X-ray diffraction and NMR experiments as well as hydrogen bonds and hydration patterns, including hydration free enthalpies. Recent revisions of the torsional-angle parameters for carbohydrate systems within the GROMOS family of force fields lead to a significant improvement of the agreement between simulated and experimental NMR data. Therefore, we recommend using the variant 53A6_GLYC instead of 53A6 and 56A6_{CARBO_R} or 2016H66 instead of 56A6_CARBO to simulate cyclodextrins in solution. The CHARMM36 and q4md-CD force fields show a similar performance as the three recommended GROMOS parameter sets. A significant difference is the more flexible nature of the cyclodextrins modeled with the CHARMM36 and q4md-CD force fields compared to the three recommended GROMOS parameter sets.

Molecular dynamics investigations of an indicator displacement assay mechanism in a liquid crystal sensor

Molecular dynamic simulations were applied to address the binding competition mechanism in an IDA based LC sensor system.

Binding Selectivity of Macrocycle Ionophores in Ionic Liquids versus Aqueous Solution and Solvent-free Conditions

The understanding of supramolecular recognition in room-temperature ionic liquids (RTILs) is key to develop the full potential of these materials. In this work, we provide insights into the selectivity of the binding of alkali metal cations by standard cyclodextrin and calixarene macrocycles in RTILs. A direct laser desorption/ionization mass spectrometry approach is employed to determine the relative abundances of the inclusion complexes formed through competitive binding in RTIL solutions. The results are compared with the binding selectivities measured under solvent-free conditions and in water/methanol solutions. Cyclodextrins and calixarenes in which the peripheral OH groups are substituted by bulkier side groups preferentially bind to Cs(+) . Such specific ionophoric behavior is substantially enhanced by solvation effects in the RTIL. This finding is rationalized with the aid of quantum mechanical calculations, in terms of the conformational features and steric interactions that drive the solvation of the inclusion complexes by the bulky RTIL counterions.