Structures, intermolecular interactions, and chemical hardness of binary water-organic solvents: a molecular dynamics study.

The evolution of structural properties, thermodynamics and averaged (dynamic) total hardness values as a function of the composition of binary water-organic solvents, was rationalized in view of the intermolecular interactions. The organic solvents considered were ethanol, acetonitrile, and isopropanol at 0.25, 0.5, 0.75, and 1 mass fractions, and the results were obtained using molecular dynamics simulations. The site-to-site radial distribution functions reveal a well-defined peak for the first coordination shell in all solvents. A characteristic peak of the second coordination shell exists in aqueous mixtures of acetonitrile, whereas in the water-alcohol solvents, a second peak develops with the increase in alcohol content. From the computed coordination numbers, averaged hydrogen bonds and their lifetimes, we found that water mixed with acetonitrile largely preserves its structural features and promotes the acetonitrile structuring. Both the water and alcohol structures in their mixtures are disturbed and form hydrogen bonds between molecules of different kinds. The dynamic hardness values are obtained as the average over the total hardness values of 1200 snapshots per solvent type, extracted from the equilibrium dynamics. The dynamic hardness profile has a non-linear evolution with the liquid compositions, similarly to the thermodynamic properties of these non-ideal solvents. Graphical abstract Computed dynamic total hardness, as a function of the cosolvent mass fraction for water-ethanol (EtOH), water-isopropanol (2PrOH) and water-acetonitrile (AN).

Peptides design based on transmembrane Escherichia coli's OmpA protein through molecular dynamics simulations in water-dodecane interfaces.

Recent research efforts have focused on the production of environmentally nonthreatening products, including identifying biosurfactants that can replace conventional surfactants. In order to utilize biosurfactants in different industries such as cosmetic, food or petroleum, it is necessary to understand the underpinnings behind the interactions that could take place for biosurfactants which display potential for interface activity. This work aimed to use molecular dynamics simulations to understand the interactions of rationally obtained peptide sequences from the original sequence of the OmpA gene in Escherichia coli, based on the free energy change (ΔG) during peptide insertion at the water-dodecane interface. Seventeen OmpA-based peptide sequences were selected and analyzed based on their hydropathy index profiles. We found that free energy change due to Columbic interactions and SASA (ΔGCoul/SASA), total free energy change and MW (ΔG/MW), and free energy change due to Coulombic and van der Waals interactions (ΔGCoul/ΔGvdW) ratios could provide a better understating in the contribution of the free energy decrease at the interface. The results indicated that the peptide sequences GKNHDTGVSPVFA and THENQLGAGAFG display biosurfactant potential based on low ΔG per square nanometer, high ΔGCoul/ΔGvdW ratio, clearly defined moieties along its hydrophobic surface and sequence, and the presence of charged residues in the polar head. Clearly defined moieties and SASA were determinant for electrostatic interactions between oil-water interfaces. Experimental validations exhibited that the emulsions prepared remained stable between 3 and 27h, respectively. Even though the peptide GKNHDTGVSPVFA displays strong interactions at the interface, stabilization times showed that the peptide THENQLGAGAFG exhibited the best performance suggesting that the stability can be better described by kinetic rather than thermodynamic criteria once the emulsion is formed.