Researchers in the Berkowitz Group performed molecular dynamics simulations on systems containing phosphatidycholine headgroups attached to graphene plates (PC−headgroup plates) immersed in water to study the interaction between phosphatidylcholine bilayers in water. The potential of mean force (PMF) between PC−headgroup plates shows that the interaction is repulsive.
As described in The Journal of Physical Chemistry B, the investigators observed three distinct regimes in the PMF depending on the interplate distances: the small distance regime, intermediate distance regime, and large distance regime. The researchers believe that the repulsive interaction in the intermediate interplate distance regime is associated with the hydration force due to the removal of water molecules adjacent to the headgroups.
In collaboration with scientists at the Academy of Sciences of the Czech Republic, the Berkowitz Group presents molecular dynamics simulations of a multicomponent, asymmetric bilayer in mixed aqueous solutions of sodium and potassium chloride. Because of the geometry of the system, there are two aqueous solution regions in these simulations: one mimics the intracellular region, and one mimics the extracellular region. Ion-specific effects are evident at the membrane/aqueous solution interface. Namely, at equal concentrations of sodium and potassium, sodium ions are more strongly adsorbed to carbonyl groups of the lipid headgroups.
A significant concentration excess of potassium is needed for this ion to overwhelm the sodium abundance at the membrane. Ion-membrane interactions also lead to concentration-dependent and cation-specific behavior of the electrostatic potential in the intracellular region because of the negative charge on the inner leaflet. In addition, water permeation across the membrane was observed on a timescale of similar to 100 ns. This study represents a step toward the modeling of realistic biological membranes at physiological conditions in intracellular and extracellular environments.