Molecular Dynamics Study of Pore Formation by Melittin in a 1,2-Dioleoyl-sn-glycero-3-phosphocholine and 1,2-Di(9Z-octadecenoyl)-sn-glycero-3-phospho-(1′-rac-glycerol) Mixed Lipid Bilayer

Melittin is a naturally occurring antimicrobial peptide that has the ability to kill bacterial cells through cell membrane penetration, leading to pore formation. In this investigation, an all-atom molecular dynamics (MD) simulation has been carried out to describe the interaction of two, four, or six peptides placed on the surface of a 3:1 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC)/1,2-di(9Z-octadecenoyl)-sn-glycero-3-phospho-(1'-rac-glycerol) (DOPG) lipid bilayer (a mimic of the bacterial cell membrane) corresponding to protein/lipid (P/L) ratios of 2:96, 4:162, or 6:166, respectively. MD simulation was also carried out for (i) one to eight transmembrane peptides (corresponding P/L ratios of 1:128 to 8:128) in a 3:1 DOPC/ DOPG mixed membrane for symmetric as well as asymmetric placement of peptides and (ii) four to eight transmembrane peptides (corresponding to P/L ratios of 4:128 to 8:128) for a pure DOPC lipid bilayer (a mimic of the mammalian cell membrane) for the asymmetric placement of peptides. The broadening of phospholipid head regions in the bottom leaflet as well as thinning of the membrane were found to occur for melittin placed on the top leaflet only for a mixed membrane. A critical P/L ratio of 6:166 was observed for the penetration of melittin from the surface, with this critical value being 6:128 for water channel formation in the case of transmembrane peptides. The phospholipid density profile across the bilayer indicated a toroidal pore structure. Asymmetric transmembrane pores were larger for the top leaflet and grew in size with time, whereas symmetric pores were stable. The behavior for mixed and pure membranes was found to be similar for transmembrane peptides, thus implying high toxicity of melittin.