X-ray structure determination of fully hydrated L alpha phase dipalmitoylphosphatidylcholine bilayers

Abstract

Bilayer form factors obtained from x-ray scattering data taken with high instrumental resolution are reported for multilamellar vesicles of L alpha phase lipid bilayers of dipalmitoylphosphatidylcholine at 50 degrees C under varying osmotic pressure. Artifacts in the magnitudes of the form factors due to liquid crystalline fluctuations have been eliminated by using modified Caillé theory. The Caillé fluctuation parameter eta 1 increases systematically with increasing lamellar D spacing and this explains why some higher order peaks are unobservable for the larger D spacings. The corrected form factors fall on one smooth continuous transform F(q); this shows that the bilayer does not change shape as D decreases from 67.2 A (fully hydrated) to 60.9 A. The distance between headgroup peaks is obtained from Fourier reconstruction of samples with four orders of diffraction and from electron density models that use 38 independent form factors. By combining these results with previous gel phase results, area AF per lipid molecule and other structural quantities are obtained for the fluid L alpha phase. Comparison with results that we derived from previous neutron diffraction data is excellent, and we conclude from diffraction studies that AF = 62.9 +/- 1.3 A2, which is in excellent agreement with a previous estimate from NMR data.

An Improved Empirical Potential Energy Function for Molecular Simulations of Phospholipids

“…In Figure 4, the electron density calculated from the simulation trajectory is seen to be in excellent agreement with experimental density profiles obtained by Nagle and co-workers. 32 For the best resolved feature, the location of the headgroup peaks, the simulation results lie between the two experimental curves (the difference between the experimental curves is related to differences in data analysis). For the width of both the methylene plateau and methyl trough, the simulation results again are found midway between the two experimental estimates.…”

Fluid phase structure of EPC and DMPC bilayers

“…The CHESS beamline monochromator was used to select X-rays with λ = 1.2147 Å. An inplane resolution of 0.002° (FWHM) in 2θ was achieved using a silicon analyzer crystal for selecting the scattered radiation (Zhang et al, 1996). The flux at the sample was 4 × 10 9 photons s −1 in an area of 0.75 mm (vertical) × 1.0 mm (horizontal).…”

“…This theory has been shown to fit lipid bilayer data (Zhang et al, 1996) , is obtained from the result (1) Raw data showing fits for DMPC and experimental results for σ are given in a recent paper . Results for the form factors are presented in this paper.…”

“…The area per molecule is obtained following a procedure initiated by McIntosh and Simon (1986b) and employed by Nagle et al (1996). These studies compared a lipid bilayer in the F phase with the same lipid in the G phase.…”

“…By using high instrumental resolution synchrotron X-rays, we were able to correct for these effects (Zhang et al, 1996). Another key element in our analysis was to determine the difficult F phase structure by making use of measured differences with the structure of the G phase; G phase structure is determined independently because of the extra data from wide angle scattering .…”