Concentration-dependent re-alignment of the antimicrobial peptide PGLa in lipid membranes observed by solid state 19F-NMR

¹ Institute of Biochemistry and Biophysics, Friedrich Schiller University of Jena
² Institute of Organic Chemistry, University of Karlsruhe
³ Forschungszentrum Karlsruhe, IFIA
⁴ University of Karlsruhe

^* To whom correspondence should be addressed. E-mail: anne.ulrich{at}ifia.fzk.de.

Submitted on November 17, 2004
Revised on December 3, 2004
Accepted on 27 January 2005

	Abstract

The membrane-disruptive antimicrobial peptide PGLa is found to change its orientation in a DMPC bilayer when its concentration is increased to biologically active levels. The alignment of the alpha-helix was determined by highly sensitive solid state NMR measurements of ¹⁹F dipolar couplings on CF₃-labeled side chains, and supported by a non-perturbing 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 about 95° to approximately 125°, with the C terminus pointing towards 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 time-scale 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.

Key Words: NMR orientational constraints, helix tilt angle, lipid-peptide interactions, macroscopically oriented samples, peptide dimer formation

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