Wj. Sun et al., ORDER AND DISORDER IN FULLY HYDRATED UNORIENTED BILAYERS OF GEL PHASEDIPALMITOYLPHOSPHATIDYLCHOLINE, Physical review. E, Statistical physics, plasmas, fluids, and related interdisciplinary topics, 49(5), 1994, pp. 4665-4676
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.