The Absorption of Phosphonium Cations and Dications into a Hydrated POPC Phospholipid Bilayer: a Computationa Study

Molecular dynamics (MD) based on an empirical force field is applied to investigate the effect of phosphonium cations ([P 6,6,6,6 ] + ) 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 µs and is virtually irreversible. The neutralising counterions ([Cl] − , in the present simulation) remain dissolved in water, giving origin to charge separation and strong electrostatic double layer at the water/lipid interface. Incorporation of cations and dications changes properties of the lipid bilayer such as diffusion, viscosity and 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, that takes place nearly two times faster in the [P 6,6,6,6 ] + 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.