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 alpha-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 with 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) approximately 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 approximately 8.0 +/- 0.5 A from the center of both SOPC and SOPC/POPG bilayers. However, in the presence of cholesterol, d was increased to 9.5 +/- 0.5 A, 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.