Interaction of dipyridamole with phospholipid monolayers at the air-water interface: Surface pressure and grazing incidence X-ray diffraction studies

Incorporation of dipyridamole (DIP) in Langmuir monolayers of the zwitterionic phospholipid dipalmitoylphosphatidylcholine (DPPC) has been investigated using surface pressure isotherm and grazing incidence X-ray diffraction (GIXD) measurements. An expansion in the pressure-area isotherms of the DPPC monolayer was observed, which was maximum in the liquid-expanded (LE) to liquid-condensed (LC) transition region and at a DIP concentration of 2 mol%. Accordingly, the GIXD measurements indicated an expansion of the 2D lattice in the LC phase up to a surface pressure of ca. 30 mN/m. A condensation effect on the DPPC packing for high DIP concentrations, as previously suggested, can be ruled out.