Interaction of small amounts of bovine serum albumin with phospholipid monolayers investigated by surface pressure and atomic force microscopy

The influence of small amounts of bovine serum albumin (BSA) (nM concentration) on the lateral organization of phospholipid monolayers at the air–water interface and transferred onto solid substrates as one-layer Langmuir–Blodgett (LB) films was investigated. The kinetics of adsorption of BSA onto the phospholipid monolayers was monitored with surface pressure isotherms in a Langmuir trough, for the zwitterionic dipalmitoylphosphatidyl ethanolamine (N,NN,N-dimethyl-PE) and the anionic dimyristoylphosphatidic acid (DMPA). A monolayer of N,NN,N-dimethyl-PE or DMPA incorporating BSA was transferred onto a solid substrate using the Langmuir–Blodgett technique. Atomic force microscopy (AFM) images of one-layer LB films displayed protein–phospholipid domains, whose morphology was characterized using dynamic scaling theories to calculate roughness exponents. For DMPA–BSA films the surface is characteristic of self-affine fractals, which may be described with the Kardar–Parisi–Zhang (KPZ) equation. On the other hand, for N,NN,N-dimethyl-PE–BSA films, the results indicate a relatively flat surface within the globule. The height profile and the number and size of globules varied with the type of phospholipid. The overall results, from kinetics of adsorption on Langmuir monolayers and surface morphology in LB films, could be interpreted in terms of the higher affinity of BSA to the anionic DMPA than to the zwitterionic N,NN,N-dimethyl-PE. Furthermore, the effects from such small amounts of BSA in the monolayer point to a cooperative response of DMPA and N,NN,N-dimethyl-PE monolayers to the protein.

The scheme (a) suggests that the most of the squeezed-out BSA/N,NN,N-dimethyl-PE complexes remain beneath, but close to the monolayer during the compression, being adsorbed back to the surface as the lateral pressure decreases. The scheme (b) suggests the BSA molecules are squeezed out from the DMPA monolayer, without any significant effect on phospholipid migration to the bulk phase. Again, when the monolayer is decompressed, the protein adsorbs back to DMPA monolayer.Figure optionsDownload as PowerPoint slide