How Transmembrane Model Peptides Affect Lipid Head Group Orientation: An Application of 14N NMR

Abstract

Studying interactions in lipid bilayers is not an easy task. It usually requires complex labeling strategies to answer specific questions, but sometimes such labels dramatically affect the subtle balance of interactions involved, especially at the membrane interface. One of the facile routes to study lipids in model membranes has been proposed to be 14N NMR. In this study we used fully hydrated lipid vesicles of phosphatidylcholine, mixed with positively charged (DMTAP) and negatively charged (DMPG) amphiphiles, to monitor the effect of surface charge on the orientation of the phosphatidylcholine head group with 14N wide line NMR. The results showed that the method is very sensitive to small variations in surface charge density. We then applied this method to probe the effect, on lipid head groups, of transmembrane model peptides consisting of a poly-(leucine-alanine) stretch flanked with either tryptophans (WALP) or lysines (KALP) and we could observe changes in orientation of the choline group, which were sensitive to peptide/lipid ratio and peptide length. For KALP peptides the results also indicated lipid demixing in the presence of DMPG in the bilayer, which was supported by molecular dynamics simulations. Since many natural membranes contain around 20 % of negatively charged lipids, these results might help to understand lipid interactions of membrane proteins in their native environment.