Interaction of the third helix of Antennapedia homeodomain with a deposited phospholipid bilayer: A neutron reflectivity structural study

Neutron reflectivity has been used to study the structure of deposited phospholipid bilayers in the gel phase and the modifications induced by the presence of a 16 amino acid peptide, the third helix of the Antennapedia homeodomain,p-Antp(43-58). The phospholipids were dipalmitoyl phosphatidylcholine (DPPC) and mixtures of DPPC with 10% mol/mol of the negatively charged dipalmitoyl phosphatidylserine (DPPS). They were deposited on silicon single crystals by using the Langmuir-Schaeffer technique. By pushing the resolution of neutron reflectivity measurements, the bilayer structure was determined at 1 Angstrom precision. The thickness of the thin water layer between the substrate and the phospholipids was found to be 5 +/- 1 Angstrom, that of the headgroups 9 +/- 1 Angstrom and that of the chains 36 +/- 1 Angstrom. The deposited bilayer had the same roughness, 5 +/- 1 Angstrom, as the substrate itself. After peptide insertion, the thickness and roughness of DPPC bilayers did not change while the peptide appeared uniformly distributed in the interfacial region. In the presence of 10% DPPS the peptide was mainly detected in the headgroup regions and its insertion induced an increase of the bilayer roughness up to 12 +/- 1 Angstrom. No affinity between the peptide and the lipid hydrocarbon region was detected. Neutron reflectivity was shown once more to be a valuable technique for structural determination and with good potential for studies of relevance in biology.