Structure and dynamics of POPC bilayers in water solutions of room temperature ionic liquids

Abstract

Molecular dynamics simulations in the NPT ensemble have been carried out to investigate the effect of two room temperature ionic liquids (RTILs), on stacks of phospholipid bilayers in water. We consider RTIL compounds consisting of chloride ([bmim][Cl]) and hexafluorophosphate ([bmim][PF6]) salts of the 1-buthyl-3-methylimidazolium ([bmim](+)) cation, while the phospholipid bilayer is made of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC). Our investigations focus on structural and dynamical properties of phospholipid and water molecules that could be probed by inelastic and quasi-elastic neutron scattering measurements. The results confirm the fast incorporation of [bmim](+) into the lipid phase already observed in previous simulations, driven by the Coulomb attraction of the cation for the most electronegative oxygens in the POPC head group and by sizeable dispersion forces binding the neutral hydrocarbon tails of [bmim](+) and of POPC. The [bmim](+) absorption into the bilayer favours the penetration of water into POPC, causes a slight but systematic thinning of the bilayer, and further stabilises hydrogen bonds at the lipid/water interface that already in pure samples (no RTIL) display a lifetime much longer than in bulk water. On the other hand, the effect of RTILs on the diffusion constant of POPC (DPOPC) does not reveal a clearly identifiable trend, since DPOPC increases upon addition of [bmim][Cl] and decreases in the [bmim][PF6] case. Moreover, because of screening, the electrostatic signature of each bilayer is only moderately affected by the addition of RTIL ions in solution. The analysis of long wavelength fluctuations of the bilayers shows that RTIL sorption causes a general decrease of the lipid/water interfacial tension and bending rigidity, pointing to the destabilizing effect of RTILs on lipid bilayers.

Room-temperature ionic liquids meet bio-membranes: the state-of-the-art

“…3: RTIL cations are absorbed in the lipid region, whereas the anions remain in the water in contact with the bilayers Fig. 5 Schematic view of one of the sample configurations used in our MD simulations (Benedetto et al 2015). POPC domains in gray, water layers in red, [C 4 Shrinkage of the phospholipid bilayer thickness upon interaction with RTILs with water has been also confirmed by X-ray experiments (Bhattacharya et al 2017, Kontro et al 2016).…”

“…Starting from the above experimental results, Benedetto and coworkers carried out classical atomistic molecular dynamics (MD) simulations on the same systems (Benedetto et al 2015). The MD simulations reproduced the density distributions obtained by neutron reflectivity (Fig.…”

“…Fig. 4 Density distribution profiles as a function of height z obtained from our full-atom classical MD trajectories (Benedetto et al 2015) for neat POPC bilayers (a) and bilayers doped with two RTILs (b). The computed profiles agreed with those measured by neutron reflectivity reported in Fig.…”

Overview of the “Ionic Liquids meet Biomolecules” session at the 19th international IUPAB and 11th EBSA congress

“…The results of these investigations suggest that the ILwater-protein interaction is a complex balance of different contributions by the different components, which is not possible to predict a priori. There is sufficient evidence in the literature to lead to the conclusion that the interaction between proteins and ILs depends on the protein as well as on the cations and anions present in the ILs.Antonio Benedetto (University College Dublin, Ireland, and Paul Scherrer Institut, Switzerland) presents results from joint experimental and computational studies on the interaction between ILs and (model) biomembranes (Benedetto 2017;Benedetto et al 2015, Benedetto et al 2014. Combining neutron scattering and computer simulations these studies show how it is possible to describe the mechanisms of interaction at the molecular level: IL cations penetrate the biomembrane, finding a preferential location between the tails and the heads of the phospholipids.…”

“…Antonio Benedetto (University College Dublin, Ireland, and Paul Scherrer Institut, Switzerland) presents results from joint experimental and computational studies on the interaction between ILs and (model) biomembranes (Benedetto 2017;Benedetto et al 2015, Benedetto et al 2014. Combining neutron scattering and computer simulations these studies show how it is possible to describe the mechanisms of interaction at the molecular level: IL cations penetrate the biomembrane, finding a preferential location between the tails and the heads of the phospholipids.…”