Myelin basic protein–lipid complex: an atomic force microscopy study

Myelin basic protein (MBP) efficiently bound Dilauroyl-lα-phosphatidic acid (DLPA) monolayers at the air water interface. Langmuir–Blodgett (LB) films were prepared from these monolayers and the hydrophilic surface of the upper layer was inspected by atomic force microscopy (AFM) in a water solution. The topography images of nominally four-layer films of DLPA in the absence of MBP revealed regions of different thickness corresponding to different numbers of lipid bimolecular layers. This morphology is characteristic of those lipid films which reorganise spontaneously when kept under an aqueous solution. The DLPA films containing MBP did not reorganise; their thicknesses were uniform and surfaces were usually covered by disordered clusters of protein molecules. Quasi-ordered arrays of smaller particles were observed in films with small amounts of bound protein. The surface charge densities of DLPA and DLPA-MPA films were investigated performing force-distance measurements by using tips modified with surface carboxyl groups. The curves obtained on pure lipid films showed repulsive forces well described in terms of double layer forces. Repulsive forces were not observed in the presence of a uniform MBP overlayer indicating that MBP neutralised most of the lipid charge.