Membrane Topology and Oligomerization of Peptaibols by Solid-State NMR Spectroscopy

Here, we review solid-state NMR investigations of a variety of peptaibols in phospholipid bilayer environments. The peptides were prepared using different approaches such as microbial expression, solid-phase peptide synthesis or the chemical synthesis in solution and a modular fragment condensation approach. Importantly, the techniques allowed the preparation of peptaibols labelled with ¹⁹F as well as with ¹⁵N either uniformly or specifically. In a first approach, ¹⁵N-labelled peptides were reconstituted into uniaxially oriented bilayers to determine membrane alignment, structure and tilt angles. Whereas alamethicin (20 residues) tends to adopt transmembrane orientations, shorter peptaibols such as zervamicin IIB, ampullosporin A and trichogin show increasing preference for orientations parallel to the membrane surface. In a second set of experiments, magic angle spinning solid-state NMR spectroscopy experiments were performed. The CODEX experiments show the formation of oligomers in the membrane that increase in a concentration-dependent manner. Whereas for alamethicin transmembrane dimers (P/L 1/30) and pentamers (1/13) were observed, trichogin occurs as in-plane oriented tetramers (1/8). Interestingly, even the long alamethicin is oriented parallel to the membrane surface in a lipid composition mimicking bacterial membranes. These findings suggest that alternative mechanisms exist for the antimicrobial action of peptaibols.