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 phos pholipid bilayers in the gel phase and the modifications induced by the pre sence of a 16 amino acid peptide, the third helix of the Antennapedia homeo domain,p-Antp(43-58). The phospholipids were dipalmitoyl phosphatidylcholin e (DPPC) and mixtures of DPPC with 10% mol/mol of the negatively charged di palmitoyl phosphatidylserine (DPPS). They were deposited on silicon single crystals by using the Langmuir-Schaeffer technique. By pushing the resoluti on of neutron reflectivity measurements, the bilayer structure was determin ed at 1 Angstrom precision. The thickness of the thin water layer between t he substrate and the phospholipids was found to be 5 +/- 1 Angstrom, that o f 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 sub strate itself. After peptide insertion, the thickness and roughness of DPPC bilayers did not change while the peptide appeared uniformly distributed i n the interfacial region. In the presence of 10% DPPS the peptide was mainl y detected in the headgroup regions and its insertion induced an increase o f the bilayer roughness up to 12 +/- 1 Angstrom. No affinity between the pe ptide and the lipid hydrocarbon region was detected. Neutron reflectivity w as shown once more to be a valuable technique for structural determination and with good potential for studies of relevance in biology.