The structure and membrane topology of the antimicrobial peptide temporin L (FVQWFSKFLGRIL-NH 2 ) were studied using liposomes as model bilayers. Circular dichroic spectra revealed temporin L to adopt an ␣-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 (Trp 4 ) and also changed its quantum yield. The observed changes in the characteristics of the Trp 4 fluorescence are in keeping

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... ith the insertion of this residue into the hydrophobic region of the liposomal bilayers. Access of the aqueous quencher acrylamide to Trp 4 decreased in the sequence 1-stearoyl-2-oleoyl-sn-glycero-3-phosphocholine (SOPC)/cholesterol (X chol ‫؍‬ 0.1) > SOPC > SOPC/1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol (POPG, X POPG ‫؍‬ 0.1) > SOPC/POPG (X POPG ‫؍‬ 0.2) Ϸ SOPC/POPG (X POPG ‫؍‬ 0.4), where X represents molar fraction of the indicated lipid. Whereas quenching of Trp 4 by brominated phospholipids was significant in SOPC liposomes, the quenching efficiency was enhanced when the vesicles contained POPG. The depth of insertion of Trp 4 into lipid bilayers was calculated by both the parallax method and distribution analysis and revealed this residue to reside at an average distance of d Ϸ 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 Trp 4 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.