Binding of the Antimicrobial Peptide Temporin L to Liposomes Assessed by Trp Fluorescence

The structure and membrane topology of the antimicrobial peptide temporin L (FVQWFSKFLGRIL- NH2) were studied using liposomes as model bilayers. Circular dichroic spectra revealed temporin L to adopt an α-helical conformation when bound to liposomes. Binding of temporin L to liposomes induced significant blue shifts of the emission spectra of the single Trp residue (Trp4) and also changed its quantum yield. The observed changes in the characteristics of the Trp4 fluorescence are in keeping with the insertion of this residue into the hydrophobic region of the liposomal bilayers. Access of the aqueous quencher acrylamide to Trp4 decreased in the sequence 1-stearoyl-2-oleoyl-sn-glycero-3-phosphocholine (SOPC)/cholesterol (Xchol = 0.1) > SOPC > SOPC/1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol (POPG, XPOPG = 0.1) > SOPC/POPG (XPOPG = 0.2) ≈ SOPC/POPG (XPOPG = 0.4), where X represents molar fraction of the indicated lipid. Whereas quenching of Trp4 by brominated phospholipids was significant in SOPC liposomes, the quenching efficiency was enhanced when the vesicles contained POPG. The depth of insertion of Trp4 into lipid bilayers was calculated by both the parallax method and distribution analysis and revealed this residue to reside at an average distance ofd ≈ 8.0 ± 0.5 Å from the center of both SOPC and SOPC/POPG bilayers. However, in the presence of cholesterol,d was increased to 9.5 ± 0.5 Å, thus revealing Trp4 to become accommodated more superficially in the bilayer. The above data suggest the presence of two populations of temporin L in SOPC- and POPG-containing membranes with parallel and perpendicular orientation with respect to the plane of the membrane surface.