Absorption of Phosphonium Cations and Dications into a Hydrated POPC Phospholipid Bilayer: A Computational Study

Molecular dynamics (MD) based on an empirical force field is applied to investigate the effect of phosphonium cations ([P6,6,6,6]+) and geminal dications ([DxC10]2+) inserted at T = 300 K into the hydration layer separating planar POPC phospholipid bilayers. Up to high concentration, nearly every added cation and dication becomes absorbed into the lipid phase. Absorption takes place during several microseconds and is virtually irreversible. The neutralizing counterions ([Cl]-, in the present simulation) remain dissolved in water, giving origin to the charge separation and the strong electrostatic double layer at the water/lipid interface. Incorporation of cations and dications changes the properties of the lipid bilayer such as diffusion, viscosity, and the electrostatic pattern. At high ionic concentration, the bilayer acquires a long-wavelength standing undulation, corresponding to a change of phase from fluid planar to ripple. All these changes are potentially able to affect processes relevant in the context of cell biology. The major difference between cations and dications concerns the kinetics of absorption, which takes place nearly two times faster in the [P6,6,6,6]+ case, and for [DxC10]2+ dications displays a marked separation into two-stages, corresponding to the easy absorption of the first phosphonium head of the dication and the somewhat more activated absorption of the second phosphonium head of each dication.

Computational analysis of the effect of [Tea][Ms] and [Tea][H2PO4] ionic liquids on the structure and stability of Aβ(17-42) amyloid fibrils

Experimental studies have reported the possibility of affecting the growth/dissolution of amyloid fibres by the addition of organic salts of the room-temperature ionic-liquid family, raising the tantalizing prospect of controlling these processes under physiological conditions. The effect of [Tea][Ms] and [Tea][H₂PO₄] at various concentrations on the structure and stability of a simple model of Aβ42 fibrils has been investigated by computational means. Free energy computations show that both [Tea][Ms] and [Tea][H₂PO₄] decrease the stability of fibrils with respect to isolated peptides in solution, and the effect is significantly stronger for [Tea][Ms]. The secondary structure of fibrils is not much affected, but single peptides in solution show a marked decrease in their β-strand character and an increase in α-propensity, again especially for [Tea][Ms]. These observations, consistent with the experimental picture, can be traced to two primary effects, i.e., the difference in the ionicity of the [Tea][Ms] and [Tea][H₂PO₄] water solutions and the remarkable affinity of peptides for [Ms]^- anions, due to the multiplicity of H-bonds.

The transition from salt-in-water to water-in-salt nanostructures in water solutions of organic ionic liquids relevant for biological applications

Computer simulations show how nano-structural motifs in organic salts/water solutions change with salt content increasing from dilute to highly concentrated.

Solubility Advantage of Amorphous Ketoprofen. Thermodynamic and Kinetic Aspects by Molecular Dynamics and Free Energy Approaches

Thermodynamic and kinetic aspects of crystalline (c-KTP) and amorphous (a-KTP) ketoprofen dissolution in water have been investigated by molecular dynamics simulation focusing on free energy properties. Absolute free energies of all relevant species and phases have been determined by thermodynamic integration on a novel path, first connecting the harmonic to the anharmonic system Hamiltonian at low T and then extending the result to the temperature of interest. The free energy required to transfer one ketoprofen molecule from the crystal to the solution is in fair agreement with the experimental value. The absolute free energy of the amorphous form is 19.58 kJ/mol higher than for the crystal, greatly enhancing the ketoprofen concentration in water, although as a metastable species in supersaturated solution. The kinetics of the dissolution process has been analyzed by computing the free energy profile along a reaction coordinate bringing one ketoprofen molecule from the crystal or amorphous phase to the solvated state. This computation confirms that, compared to the crystal form, the dissolution rate is nearly 7 orders of magnitude faster for the amorphous form, providing one further advantage to the latter in terms of bioavailability. The problem of drug solubility, of great practical importance, is used here as a test bed for a refined method to compute absolute free energies, which could be of great interest in biophysics and drug discovery in particular.