How Lipid Headgroups Sense the Membrane Environment: An Application of 14N NMR
The orientation of lipid headgroups may serve as a powerful sensor of electrostatic interactions in membranes. As shown previously by 2H NMR measurements, the headgroup of phosphatidylcholine (PC) behaves like an electrometer and varies its orientation according to the membrane surface charge. Here, we explored the use of solid-state 14N NMR as a relatively simple and label-free method to study the orientation of the PC headgroup in model membrane systems of varying composition. We found that 14N NMR is sufficiently sensitive to detect small changes in headgroup orientation upon introduction of positively and negatively charged lipids and we developed an approach to directly convert the 14N quadrupolar splittings into an average orientation of the PC polar headgroup. Our results show that inclusion of cholesterol or mixing of lipids with different length acyl chains does not significantly affect the orientation of the PC headgroup. In contrast, measurements with cationic (KALP), neutral (Ac-KALP), and pH-sensitive (HALP) transmembrane peptides show very systematic changes in headgroup orientation, depending on the amount of charge in the peptide side chains and on their precise localization at the interface, as modulated by varying the extent of hydrophobic peptide/lipid mismatch. Finally, our measurements suggest an unexpectedly strong preferential enrichment of the anionic lipid phosphatidylglycerol around the cationic KALP peptide in ternary mixtures with PC. We believe that these results are important for understanding protein/lipid interactions and that they may help parametrization of membrane properties in computational studies.