Molecular simulation study of structural and dynamic properties of mixed DPPC/DPPE bilayers

Molecular dynamics simulations have been used to study structural and dynamic properties of fully hydrated mixed 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine (DPPE) bilayers at 0, 25, 50, 75, and 100 mol % DPPE. Simulations were performed for 50 ns at 350 K and 1 bar for the liquid-crystalline state of the mixtures. Results show that the average area per headgroup reduces from 0.65 +/- 0.01 nm(2) in pure DPPC to 0.52 +/- 0.01 nm(2) in pure DPPE systems. The lipid tails become more ordered with increasing DPPE concentration, resulting in a slight increase in membrane thickness (3.43 +/- 0.01 nm in pure DPPC to 4.00 +/- 0.01 nm in pure DPPE). The calculated area per headgroup and order parameter for pure DPPE deviates significantly from available experimental measurements, suggesting that the force field employed requires further refinement. In-depth analysis of the hydrogen-bond distribution in DPPE molecules shows that the amine groups strongly interact with the phosphate and carbonyl groups through inter/intramolecular hydrogen bonds. This yields a bilayer structure with DPPE headgroups preferentially located near the lipid phosphate and ester oxygens. It is observed that increasing DPPE concentrations causes competitive hydrogen bonding between the amine groups (hydrogen-donor) and the phosphate/carbonyl groups or water (hydrogen-acceptor). Due to the increasing number of hydrogen-donors from DPPE molecules with increasing concentration, DPPE becomes more hydrated. Trajectory analysis shows that DPPE molecules in the lipid mixtures move laterally and randomly around the membrane surface and the movement becomes more localized with increasing DPPE concentrations. For the conditions and simulation time considered, no aggregation or phase separation was observed between DPPC and DPPE.