ORDER AND DISORDER IN FULLY HYDRATED UNORIENTED BILAYERS OF GEL PHASEDIPALMITOYLPHOSPHATIDYLCHOLINE

We have performed x-ray scattering measurements of the wide-angle regi on of the gel phase of fully hydrated unoriented multilamellar vesicle s of dipalmitoylphosphatidylcholine that quantitate two satellite peak s in addition to the usual (20) and (11) peaks. All the peaks in the w ide-angle region are adequately fit using an electron density model co nsisting of straight chains with a terminal methyl gap and a head grou p term. The fit yields chains of length close to 20 angstrom that are tilted by theta(t) = 31.6-degrees toward nearest neighbors, at 24-degr ees-C. The fit also requires that the two monolayers in the bilayer ar e slightly offset rather than collinear. Subtraction of the fitted pea k scattering and the background scattering obtained from samples with no lipid indicates that there is considerable broad diffuse scattering underlying the prominent peaks, thus providing a measure of the disor der in gel phase bilayers. The result that the head group term require d to fit the wide-angle peaks is smaller than the corresponding head g roup term required to fit the low-angle reflections suggests that ther e is less order in the head groups than in the chains. However, the la rge amount of diffuse scattering appears to require disorder in both t he chain and head regions.