  The effect of the adsorption of a polypeptide on the lateral interaction of
dimyristoylphosphatidylserine molecules in different phase states on the
surface of a 10\,mM KCl aqueous solution has been studied. Changes in the
surface pressure and Volta potential induced by the adsorption of large
poly-D-lysine molecules (about 200 links in a chain) have been determined at
different areas per lipid molecule in a monolayer. The adsorption of
macromolecules noticeably increases the elasticity of the monolayer under
lateral compression in the liquid expanded state of lipid and reduces the
effective dipole moment from 0.48 to 0.38 D. These properties are in
qualitative agreement with X-ray reflectometry data for the lipid monolayer
obtained with synchrotron radiation with a photon energy of 70 keV. The
electron density profiles perpendicular to the surface of the aqueous subphase
have been reconstructed from reflectometry data within a model approach to the
structure of an interface with two and three layers. These profiles indicate
the existence of a wide diffuse polymer layer 150+/-40 Angstrom in width at the
interface of the monolayer in both the liquid expanded and liquid condensed
states. A decrease in the area per molecule in the monolayer by a factor of 2
results in the doubling of the surface density of the macromolecule film. The
adsorption of the polymer also affects the integral density of the layer of
polar phospholipid groups, which decreases by a factor of ~ 2 in the liquid
expanded phase and by ~ 30% in the liquid condensed phase.
