Biophysical Journal 59: 1325-1332 (1991)
© 1991 the Biophysical Society

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Structural properties of phosphatidylcholine in a monolayer at the air/water interface

Neutron reflection study and reexamination of x-ray reflection measurements

David Vaknin ^*, Kristian Kjaer ^*, Jens Als-Nielsen ^* and Mathias Lösche 

Physics Department, Risø National Laboratory, DK-4000 Roskilde, Denmark
Institute of Physical Chemistry, Johannes-Gutenberg-Universität Mainz, D-6500 Mainz, Germany

ABSTRACT

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 Å^-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_ß, 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.

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