Mismatch Dependent Tilt of Amphipathic α-Helical Antimicrobial Peptides Inserted in Positive Spontaneous Curvature Lipid Membranes

Abstract

We have used solid state NMR to study the orientation of membrane-active peptides in model membranes with different properties. We found that the peptide behavior can to a large extent be explained by the simple biophysical concepts of hydrophobic matching and lipid spontaneous curvature. Hydrophobic peptides insert in the membrane in a transmembrane orientation, and it is well established that peptides which have a longer hydrophobic stretch than the bilayer thickness start to tilt to compensate the hydrophobic mismatch. Here we show that the hydrophobic matching principle also applies to cationic amphipathic α-helical antimicrobial peptides. Using a series of peptides called KIAn, based on the repetitive amino acid sequence KIAGKIA, we found that only peptides long enough to span the hydrophobic thickness of the membrane induce leakage in lipid vesicles. Interestingly, a certain minimum length was also needed to induce hemolysis and to kill bacteria, and this minimum length was different for different bacterial strains, so that the series of peptides can be used as a “molecular ruler” to determine the thickness of erythrocyte or bacterial membranes in vivo. Using solid-state NMR we found that the orientation depends strongly on the lipid spontaneous curvature. The KIA peptides are always staying flat on the membrane surface in lipids with a negative spontaneous curvature. On the other hand, in lipids with a positive spontaneous curvature, like lyso-lipids, they can go deeper into the membrane and have a tilted or even inserted orientation. In the inserted state, the tilt of the peptides is found to be mismatch dependent. This is the first time a mismatch effect has been found for amphipathic peptides, and shows that the effect is more generally valid than has been appreciated until now.