Orientation of a polyleucine-based peptide in phosphatidylcholine bilayers of different thickness. Solid-state NMR and CD spectroscopy

A study was performed of the orientation and secondary structure of the peptide pLeuD11 (KKGL7DL[15N]WL9KKA) in phosphatidylcholine (PC) bilayers. The lipid bilayer thickness was varied by using PCs with monounsaturated acyl chains of different lengths, ranging from 14 to 24 carbon atoms. The peptide/lipid molar ratio was kept at 1:30 for all systems with water content of 50 wt.%. The secondary structure was determined by circular dichroism (CD) spectroscopy. The peptide adopted a transmembrane orientation in all bilayers, independent on their thickness. The location of the peptide was determined by 15N solid-state magic angle spinning (MAS) NMR spectroscopy, exploiting the effects of paramagnetic lanthanide ions at the membrane surface. From static solid-state 31P NMR spectroscopy measurements it was concluded that all lipid/peptide systems formed a lamellar liquid crystalline phase. As found by CD the distribution of secondary structures in the peptide changed only slightly for the different lipid membranes. The fraction of α-helix was highest (≈60%) in bilayers with lipids, having an acyl chain length of 18 and 22 carbon atoms, while for lipids with 14 and 24 carbon atoms the helical content decreased slightly to about 50%. This reduction was accompanied by an increase in the fraction of β-like structures.