Hubert Stassen

Imidazolium Salt Ion Pairs in Solution

The formation, stabilisation and reactivity of contact ion pairs of non-protic imidazolium ionic liquids (ILs) in solution are conceptualized in light of selected experimental evidence as well theoretical calculations reported mainly in the last ten years. Electric conductivity, NMR, ESI-MS and IR data as well as theoretical calculations support not only the formation of contact ion pairs in solution, but also the presence of larger ionic and neutral aggregates even when dissolved in solvents with relatively high dielectric constants, such as acetonitrile and DMSO. The presence of larger imidazolium supramolecular aggregates is favoured at higher salt concentrations in solvents of low dielectric constant for ILs that contain shorter N-alkyl side chains associated with anions of low coordination ability. The stability and reactivity of neutral contact species are also dependent on the nature of the anion, imidazolium substituents, and are more abundant in ILs containing strong coordinating anions, in particular those that can form charge transfer complexes with the imidazolium cation. Finally, some ILs display reactivities as contact ion pairs rather than solvent-separated ions.

Molecular dynamics simulations of the depolarized light scattering spectra of liquid OCS in comparison with experiment and simulations of CO2 and CS2

Molecular dynamics computer simulation was used to study depolarized light scattering in liquid OCS at temperatures of 170, 243, and 298 K. Applying the first‐order dipole‐induced dipole model for the interaction‐induced polarizabilities, the time correlation functions and spectral line shapes corresponding to the depolarized Rayleigh and nonvibrational contributions to the symmetric stretch (ν1) mode Raman spectra were calculated. The results were compared with experimental findings and simulations on the liquids CO2 and CS2. We find that the OCS simulations reflect the main spectral features in satisfactory agreement with experiment and that an intermediate position concerning all the studied particularities is assigned to liquid OCS between CO2 and CS2. For these three liquids the difference correlation functions between Rayleigh and Raman light scattering were computed and discussed in terms of their component many‐body contributions.

Membrane-disruptive properties of the bioinsecticide Jaburetox-2Ec: Implications to the mechanism of the action of insecticidal peptides derived from ureases

Jaburetox-2Ec, a recombinant peptide derived from an urease isoform (JBURE-II), displays high insecticidal activity against important pests such as Spodoptera frugiperda and Dysdercus peruvianus. Although the molecular mechanism of action of ureases-derived peptides remains unclear, previous ab initio data suggest the presence of structural motifs in Jaburetox-2Ec with characteristics similar to those found in a class of pore-forming peptides. Here, we investigated the molecular aspects of the interaction between Jaburetox-2Ec and large unilamellar vesicles. Jaburetox-2Ec displays membrane-disruptive ability on acidic lipid bilayers and this effect is greatly influenced by peptide aggregation. Corroborating with this finding, molecular modeling studies revealed that Jaburetox-2Ec might adopt a well-defined beta-hairpin conformation similar to those found in antimicrobial peptides with membrane disruption properties. In addition, molecular dynamics simulations suggest that the protein is able to anchor at a polar/non-polar interface. In the light of these findings, for the first time it was possible to point out some evidence that the peptide Jaburetox-2Ec interacting with lipid vesicles promotes membrane permeabilization.

The Formation of Imidazolium Salt Intimate (Contact) Ion Pairs in Solution

1-n-Butyl-2,3-dimethylimidazolium (BMMI) ionic liquids (ILs) associated with different anions undergo H/D exchange preferentially at 2-Me group of the imidazolium in deuterated solvents. This process is mainly related to the existence of ion pairs rather than the anion basicity. The H/D exchange occurs in solvents (CDCl3 and MeCN for instance) in which intimate contact ion pairs are present and the anion possesses a labile H in its structure, such as hydrogen carbonate and prolinate. In D2 O, separated ion pairs are formed and the H/D exchange does not occur. A plausible catalytic cycle is that the IL behaves as a neutral base in the course of all H/D exchange processes. NMR experiments, density functional calculations, and molecular dynamics simulations corroborate these hypotheses.

Molecular Dynamics Investigations of PRODAN in a DLPC Bilayer

Molecular dynamics computer simulations have been performed to identify preferred positions of the fluorescent probe PRODAN in a fully hydrated DLPC bilayer in the fluid phase. In addition to the intramolecular charge-transfer first vertical excited state, we considered different charge distributions for the electronic ground state of the PRODAN molecule by distinct atomic charge models corresponding to the probe molecule in vacuum as well as polarized in a weak and a strong dielectric solvent (cyclohexane and water). Independent on the charge distribution model of PRODAN, we observed a preferential orientation of this molecule in the bilayer with the dimethylamino group pointing toward the membranes center and the carbonyl oxygen toward the membranes interface. However, changing the charge distribution model of PRODAN, independent of its initial position in the equilibrated DLPC membrane, we observed different preferential positions. For the ground state representation without polarization and the in-cyclohexane polarization, the probe maintains its position close to the membranes center. Considering the in-water polarization model, the probe approaches more of the polar headgroup region of the bilayer, with a strong structural correlation with the choline group, exposing its oxygen atom to water molecules. PRODANs representation of the first vertical excited state with the in-water polarization also approaches the polar region of the membrane with the oxygen atom exposed to the bilayers hydration shell. However, this model presents a stronger structural correlation with the phosphate groups than the ground state. Therefore, we conclude that the orientation of the PRODAN molecule inside the DLPC membrane is well-defined, but its position is very sensitive to the effect of the medium polarization included here by different models for the atomic charge distribution of the probe.

Many‐body correlations in the interaction‐induced light scattering from liquid CS2

Additional information concerning the interaction‐induced contribution to the intensities in depolarized light scattering spectra of liquid carbon disulfide is presented here. This was obtained from molecular dynamics computer simulations of relevant time‐correlation functions (TCFs) along the gas‐liquid coexistence curve. The interaction‐induced part of the polarizability was evaluated assuming a first‐order dipole‐induced dipole mechanism. The simulated spectral TCFs for this system show coupled rotation‐translation dynamics that are due in part to the nonspherical molecular shape of CS2 and in part to the orientation dependence of the dipole‐induced polarizability Π. By writing Π in the usual way as the sum of a diagonal orientation‐independent term plus a second‐rank tensor in the polarizability anisotropy, the TCFs can be split into terms with varying degrees of orientation dependence. In addition, the collective TCFs were broken down into their component 2‐, 3‐ and 4‐body parts. The component parts ...

New approach to free energy of solvation applying continuum models to molecular dynamics simulation.

A new approach to the calculation of the free energy of solvation from trajectories obtained by molecular dynamics simulation is presented. The free energy of solvation is computed as the sum of three contributions originated at the cavitation of the solute by the solvent, the solute‐solvent nonpolar (repulsion and dispersion) interactions, and the electrostatic solvation of the solute. The electrostatic term is calculated based on ideas developed for the broadly used continuum models, the cavitational contribution from the excluded volume by the Claverie‐Pierotti model, and the Van der Waals term directly from the molecular dynamics simulation. The proposed model is tested for diluted aqueous solutions of simple molecules containing a variety of chemically important functions: methanol, methylamine, water, methanethiol, and dichloromethane. These solutions were treated by molecular dynamics simulations using SPC/E water and the OPLS force field for the organic molecules. Obtained free energies of solvation are in very good agreement with experimental data.

Free energy of solvation from molecular dynamics simulation applying Voronoi-Delaunay triangulation to the cavity creation

The free energy of solvation for a large number of representative solutes in various solvents has been calculated from the polarizable continuum model coupled to molecular dynamics computer simulation. A new algorithm based on the Voronoi-Delaunay triangulation of atom-atom contact points between the solute and the solvent molecules is presented for the estimation of the solvent-accessible surface surrounding the solute. The volume of the inscribed cavity is used to rescale the cavitational contribution to the solvation free energy for each atom of the solute atom within scaled particle theory. The computation of the electrostatic free energy of solvation is performed using the Voronoi-Delaunay surface around the solute as the boundary for the polarizable continuum model. Additional short-range contributions to the solvation free energy are included directly from the solute-solvent force field for the van der Waals-type interactions. Calculated solvation free energies for neutral molecules dissolved in benzene, water, CCl4, and octanol are compared with experimental data. We found an excellent correlation between the experimental and computed free energies of solvation for all the solvents. In addition, the employed algorithm for the cavity creation by Voronoi-Delaunay triangulation is compared with the GEPOL algorithm and is shown to predict more accurate free energies of solvation, especially in solvents composed by molecules with nonspherical molecular shapes.

Alkyl chain size effects on liquid phase properties of 1-alkyl-3-methylimidazolium tetrachloroaluminate ionic liquids--a microscopic point of view from computational chemistry.

Microscopic insights for effects of the alkyl chain length on macroscopic physical properties of 1-alkyl-3-methylimidazolium ionic liquids are provided from molecular dynamics computer simulation and quantum mechanical calculations. In particular, the liquid densities, internal energies, and hydrogen bonding behavior of the ions were studied. The effect of the alkyl chain size on the hydrogen bonding behavior of these liquids, in terms of both radial distribution functions and pair energetics, is demonstrated. Finally, studies of the ion pair stabilization were performed at the quantum mechanical level by means of the Kitaura-Morokuma energy partition. The fitting of atomic charges to the ion pairs has been performed to illustrate the charge transfer effect between cation and anion.

Density-dependent solvation dynamics in a simple Lennard-Jones fluid

The density dependence of time correlation functions for the solvation energy in a simple Lennard-Jones liquid has been investigated by molecular dynamics computer simulations. Considering argon dissolved in liquid argon, mechanical solvation dynamics has been studied treating interactions between excited solute states and the solvent by changes in the Lennard-Jones well-depth parameter, the Lennard-Jones size parameter, and a combination of the two. Densities have been varied from supercritical to triple point densities at a constant temperature of 151 K. In addition, a thermodynamic state close to the argon triple point has been considered. All the solvation energy time correlation functions have been broken down into their partial two- and three-body contributions giving an insight into the cancellation effect of solvation dynamics. It is found that the well-depth solvation process produces slowly decaying time correlation functions for the solvation dynamics at lower densities. In this case, the solva...