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Lipid Chain-Length Dependence for Incorporation of Alamethicin in Membranes: Electron Paramagnetic Resonance Studies on TOAC-Spin Labeled Analogs

Derek Marsh ^*, Micha Jost , Cristina Peggion  and Claudio Toniolo 

^* Max-Planck-Institut für biophysikalische Chemie, Abteilung Spektroskopie, 37070 Göttingen, Germany; and Institute of Biomolecular Chemistry, CNR Padua Section, Department of Chemistry, University of Padova, 35131 Padova, Italy

Correspondence: Address reprint requests to Derek Marsh, E-mail: dmarsh{at}gwdg.de.

Alamethicin is a 19-residue hydrophobic peptide, which is extended by a C-terminal phenylalaninol but lacks residues that might anchor the ends of the peptide at the lipid-water interface. Voltage-dependent ion channels formed by alamethicin depend strongly in their characteristics on chain length of the host lipid membranes. EPR spectroscopy is used to investigate the dependence on lipid chain length of the incorporation of spin-labeled alamethicin in phosphatidylcholine bilayer membranes. The spin-label amino acid TOAC is substituted at residue positions n = 1, 8, or 16 in the sequence of alamethicin F50/5 [TOACⁿ, Glu(OMe)^7,18,19]. Polarity-dependent isotropic hyperfine couplings of the three TOAC derivatives indicate that alamethicin assumes approximately the same location, relative to the membrane midplane, in fluid diC_NPtdCho bilayers with chain lengths ranging from N = 10–18. Residue TOAC⁸ is situated closest to the bilayer midplane, whereas TOAC¹⁶ is located farther from the midplane in the hydrophobic core of the opposing lipid leaflet, and TOAC¹ remains in the lipid polar headgroup region. Orientational order parameters indicate that the tilt of alamethicin relative to the membrane normal is relatively small, even at high temperatures in the fluid phase, and increases rather slowly with decreasing chain length (from 13° to 23° for N = 18 and 10, respectively, at 75°C). This is insufficient for alamethicin to achieve hydrophobic matching. Alamethicin differs in its mode of incorporation from other helical peptides for which transmembrane orientation has been determined as a function of lipid chain length.

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