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 H(2)O and D(2)O 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 A(-1) were accessed.In a new approach, by refining neutron reflectivity data from chain-perdeuterated DPPC-d(62) 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.