STRUCTURAL-PROPERTIES OF PHOSPHATIDYLCHOLINE IN A MONOLAYER AT THE AIR-WATER-INTERFACE - NEUTRON REFLECTION STUDY AND REEXAMINATION OF X-RAY REFLECTION MEASUREMENTS

Neutron reflectivities of phosphatidylcholine monolayers in the liquid condensed (LC) phase on ultrapure H2O and D2O subphases have been measured on a Langmuir film balance. Using a dedicated liquid surface reflectometer, reflectivities down to R = 10(-6) in the momentum transfer range Q(z) = 0-0.4 Angstrom-1 were accessed. In a new approach, by refining neutron reflectivity data from chain-perdeuterated DPPC-d62 in combination with x-ray measurements on the same monolayer under similar conditions it is shown that the two techniques mutually complement one another. This analysis leads to a detailed conception of the interface structure. It is found that in the LC phase (which is analogous to the L-beta phase in vesicle dispersions) the head group is interpenetrated with subphase water (4 +/- 2.5 molecules per lipid) and the average tilt angle of the hydrophobic chains from the surface normal is 33 +/- 3 degrees.