DETECTION OF STRUCTURAL DEFECTS IN PHOSPHATIDYLCHOLINE MEMBRANES BY SMALL-ANGLE NEUTRON-SCATTERING - THE CLUSTER MODEL OF A LIPID BILAYER

The oriented DPPC multilayers hydrated by D2O have been studied by a small-angle neutron scattering method in the Guinier range, and the gyration radius of the structural inhomogeneities has been estimated at about 29 angstrom. They are interpreted as the annular defects between adjacent clusters uniting the all-trans chain 'segments' adjacent to the polar head group regions. The angle of the 'segment' tilt is determined by the hydrated polar group area (59.2 angstrom2 for DPPC bilayers) and has been estimated to be about 44-degrees under the given experimental conditions. The hydrocarbon interior of a bilayer can be suggested as a 'sandwich' that is formed by two clustered layers (approx. 7 angstrom of the thickness) and the central disordered (liquid) layer. The average cluster size along the bilayer surface is estimated to be approx. 24 angstrom which correlates with the estimations of the short order region dimensions from the halfwidth of the X-ray 'packing' reflex (4.6 angstrom)-1. The average interchain separation of approx. 5 angstrom and the average cross-section area of a chain in a cluster (21.4 angstrom2) were estimated from the reflex position and the chain cross-section geometry. The total volume of defects and the fraction of a bilayer surface occupied by them were estimated too.