Department of Physics and Department of Biological Sciences,
Carnegie Mellon University , Pittsburgh, PA 15213

X-ray Structure Determination of Fully Hydrated Phase DPPC Bilayers

John F. Nagle, Ruitian Zhang, Stephanie Tristram-Nagle, Wenjun Sun,
Horia I. Petrache, and Robert M. Suter

Running Title: Structure of Phase DPPC Bilayers

Keywords: X-Ray Scattering, Phospholipids, Biomembranes and Lipid Bilayers

Correspondence should be addressed to: Prof. J. F. Nagle, Department of Physics, Carnegie Mellon University, Pittsburgh, PA 15213. Tel.No.: 412-268-2768, Fax: 412-681-0648, e-mail address: jn12+@andrew.cmu.edu

Abstract:

Bilayer form factors obtained from X-ray scattering data taken with high instrumental resolution are reported for multilamellar vesicles of phase lipid bilayers of DPPC at 50C 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 increases systematically with increasing 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 Å (fully hydrated) to 53.9 Å. The distance between headgroup peaks is obtained from Fourier reconstruction of samples with four orders of diffraction and also from electron density models that use 38 independent form factors. By combining these results with previous gel phase results, area per lipid molecule and other structural quantities are obtained for the fluid phase. Comparison with results that we derive from previous neutron diffraction data is excellent, and we conclude that Ų from diffraction studies, in excellent agreement with a previous estimate from NMR data.

• INTRODUCTION
• EXPERIMENTAL
• THEORY
• RESULTS
  • Basic Data: Form Factors and Fluctuation Parameters
  • Continuous Transforms and Test of Dehydration Threshold Hypothesis
  • Fourier reconstruction of electon density profiles
  • Method for Determining A from
  • Hybrid Models
  • from Neutron Diffraction Studies
  • and Other Quantities
• DISCUSSION
• References