Order and disorder in fully hydrated unoriented bilayers of gel-phase dipalmitoylphosphatidylcholine

We have performed x-ray scattering measurements of the wide-angle region of the gel phase of fully hydrated unoriented multilamellar vesicles of dipalmitoylphosphatidylcholine that quantitate two satellite peaks in addition to the usual (20) and (11) peaks. All the peaks in the wide-angle region are adequately fit using an electron density model consisting of straight chains with a terminal methyl gap and a head group term. The fit yields chains of length close to 20 Å that are tilted by θ_t=31.6° toward nearest neighbors, at 24 °C. The fit also requires that the two monolayers in the bilayer are slightly offset rather than collinear. Subtraction of the fitted peak 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 disorder in gel phase bilayers. The result that the head group term required to fit the wide-angle peaks is smaller than the corresponding head group term required to fit the low-angle reflections suggests that there is less order in the head groups than in the chains. However, the large amount of diffuse scattering appears to require disorder in both the chain and head regions.