Structural Properties of POPC Monolayers under Lateral Compression: Computer Simulations Analysis

Abstract : 1-Palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC),a lipid comprising a saturated and an unsaturated acyl chain, belongs to theclass of glycerophosphatidylcholines, major lipids in eukaryotic cellmembranes. To get insight into the structural properties of this lipidwithin monolayers as membrane models, we performed moleculardynamics (MD) simulations of POPC monolayers under compression atthe air/water interface. MD simulations were carried out at 300 K and atdifferent surface pressures using the all-atom general Amber forcefield(GAFF). A good agreement was found between the simulated data andexperimental isotherms. At surface pressures greater than 15 mN/m, twoorientations of the head groups clearly appear: one nearly parallel to themonolayer interface and another one pointing toward the water. On the basis of the analysis of headgroup dihedral angles, wepropose that the conformational variations around the bonds connecting the phosphorus atom to the adjacent oxygens areinvolved in these two orientations of the headgroup. The glycerol group orientation is characterized by a large distributioncentered around 50°with respect to the monolayer normal. The acyl chains are predominantly in trans configuration from 7.5 to43 mN/m surface pressures. Moreover, the calculated order parameter profiles of both chains suggest an independent behavior ofthe saturated and unsaturated chains that could be correlated with the formation of chain-type clusters observed along thesimulated trajectories.