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* Institute of Biochemistry and Biophysics, University of Jena, Jena, Germany; Institute of Organic Chemistry, University of Karlsruhe, Karlsruhe, Germany; and Forschungszentrum Karlsruhe, Karlsruhe, Germany
Correspondence: Address reprint requests to Anne S. Ulrich, Institute of Organic Chemistry, University of Karlsruhe, Fritz-Haber-Weg 6, 76131 Karlsruhe, Germany. E-mail: anne.ulrich{at}ifia.fzk.de.
The membrane-disruptive antimicrobial peptide PGLa is found to change its orientation in a dimyristoyl-phosphatidylcholine bilayer when its concentration is increased to biologically active levels. The alignment of the 
-helix was determined by highly sensitive solid-state NMR measurements of 19F dipolar couplings on CF3-labeled side chains, and supported by a nonperturbing 15N label. At a low peptide/lipid ratio of 1:200 the amphiphilic peptide resides on the membrane surface in the so-called S-state, as expected. However, at high peptide concentration (
1:50 molar ratio) the helix axis changes its tilt angle from 
90° to 120°, with the C-terminus pointing toward the bilayer interior. This tilted "T-state" represents a novel feature of antimicrobial peptides, which is distinct from a membrane-inserted I-state. At intermediate concentration, PGLa is in exchange between the S- and T-state in the timescale of the NMR experiment. In both states the peptide molecules undergo fast rotation around the membrane normal in liquid crystalline bilayers; hence, large peptide aggregates do not form. Very likely the obliquely tilted T-state represents an antiparallel dimer of PGLa that is formed in the membrane at increasing concentration.
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