Control of the Receptor Binding to a Ligand-Containing Monolayer at the Air/Water Interface by the Lateral Pressure:  A Film Balance and Neutron and Infrared Reflection Study

We have studied the structure of a binary monolayer of dipalmitoylphosphatidylcholine (DPPC) and a biotinylated amphiphilic fullerene (BF) as a function of the lateral pressure. The compression isotherm of the mixture exhibited two separate kinks with a remarkably flat plateau after the second kink. Neutron and infrared reflection measurements suggest that upon compression of the mixed monolayer a (reversible) vertical shift of the BF occurs, leading to a (partial) demixing of the two components in the third dimension at high lateral pressure. As a consequence, the biotin group attached to the BF becomes inaccessible to streptavidin binding from the subphase at high lateral pressure, while the binding is clearly observed at low pressure.