ACTH binding to phospholipid bilayers: A 1H-NMR study of the 5-14, 1-14, and 1-24 derivatives

The interaction of the 5-14, 1-14, and 1-24 fragments of ACTH with sonicated phospholipid bilayers containing egg yolk phosphatidylcholine (EPC) either pure or mixed with 10 mole % phosphatidic acid (EPA), was investigated by proton nuclear magnetic resonance (¹H-nmr). The effects observed with zwitterionic EPC vesicles were small, indicating a low binding of the ACTH derivatives. The N-terminal aromatic resonances of the ACTH peptides were markedly broadened in the presence of negatively charged vesicles (EPC/EPA 9:1 M/M), while those of the C-terminal end were barely affected, showing that ACTH interacts with its N-terminal fragment. The choline resonance of the EPC molecules of the outer monolayer was shifted and broadened upon ACTH binding to the lipid vesicles, while that of the inner layer was not affected, suggesting that the peptide molecules interact only with the external leaflet of the lipid bilayer. The C²H and C⁴H resonances of the histidine-6 side chain were both shifted downfield upon peptide binding to the negatively charged lipid interface. In the case of the 1–24 derivative, these resonances were also split into two signals reflecting two different species of membrane-bound ACTH 1–24. Analysis of the line width and chemical shift variations of the ACTH and lipid resonances observed upon peptide binding shows that the membrane-binding potency of the shorter 5–14⁺¹ fragment, which presents a +1 net charge, is roughly similar to that of the highly cationic 1–24⁺⁶ (net charge +6) derivative, implying that the 15–24⁺⁵ segment is not essential for membrane binding. The nmr measurements at a fixed lipid-to-peptide ratio in the presence of increasing amounts of spin-labeled lipids demonstrate that the N-terminal fragment of ACTH does not penetrate the hydrophobic core of the bilayer, and should lie parallel to the membrane surface. © 1997 John Wiley & Sons, Inc. Biopoly 42: 731–744, 1997