Interaction of the peptide antibiotic alamethicin with bilayer- and non-bilayer-forming lipids: Influence of increasing alamethicin concentration on the lipids supramolecular structures

Incorporation of the helical antimicrobial peptide alamethicin from aqueous phase into hydrated phases of dioleoylphosphatidylethanolamine (DOPE) and dioleoylphosphatidylcholine (DOPC) was investigated within a range of pepti de concentrations and temperatures by time-resolved synchrotron X-ray diffr action, It was found that alamethicin influences the organizations of the n on-bilayer-forming (DOPE) and the bilayer-forming (DOPC) lipids in differen t ways. In DOPC, only the bilayer thickness was affected, while in DOPE new phases were induced. At low peptide concentrations (<1.10(-4) M), an inver ted hexagonal (H-II) phase was observed as with DOPE dispersions in pure bu ffer solution, A coexistence of two cubic structures was found at the criti cal peptide concentration for induction of new lipid/peptide phases. The fi rst one Q(224) (space group Pn3m) was identified within the entire temperat ure region studied (from 1 to 45 degrees C) and was found in coexistence wi th H-II-phase domains. The second lipid/peptide cubic structure was present only at temperatures below 16 degrees C and its X-ray reflections were bet ter fitted by a Q(212) (P4(3)32) space group, rather than by the expected Q (229) (Im3m) space group. At alamethicin concentrations of 1 mM and higher, a nonlamellar phase transition from a Q(224) cubic phase into an H-II phas e was observed. Within the investigated range of peptide concentrations, la mellar structures of two different bilayer periods were established with th e bilayer-forming lipid DOPC, They correspond to lipid domains of associate d and nonassociated helical peptide, The obtained X-ray results suggest tha t the amphiphilic alamethicin molecules adsorb from the aqueous phase at th e lipid head group/water interface of the DOPE and DOPC membranes. At suffi ciently high (>1.10(-4) M) solution concentrations, the peptide is probably accommodated in the head group region of the lipids thus inducing structur al features of mixed lipid/peptide phases. (C) 2000 Academic Press.