Structure of Fully Hydrated Fluid Phase DMPC and DLPC Lipid Bilayers Using X-Ray Scattering from Oriented Multilamellar Arrays and from Unilamellar Vesicles
Quantitative structures of the fully hydrated fluid phases of dimyristoylphosphatidylcholine (DMPC) and dilauroylphosphatidylcholine (DLPC) were obtained at 30°C. Data for the relative form factors F(qz) for DMPC were obtained using a combination of four methods. 1), Volumetric data provided F(0). 2), Diffuse x-ray scattering from oriented stacks of bilayers provided relative form factors |F(qz)| for high qz, 0.22<qz<0.8Å−1. 3), X-ray scattering from extruded unilamellar vesicles with diameter 600Å provided |F(qz)| for low qz, 0.1<qz<0.3Å−1. 4), Previous measurements using a liquid crystallographic x-ray method provided |F(2πh/D)| for h=1 and 2 for a range of nearly fully hydrated D-spacings. The data from method 4 overlap and validate the new unilamellar vesicles data for DMPC, so method 4 is not required for DLPC or future studies. We used hybrid electron density models to obtain structural results from these form factors. Comparison of the model electron density profiles with that of gel phase DMPC provides areas per lipid A, 60.6±0.5Å2 for DMPC and 63.2±0.5Å2 for DLPC. Constraints on the model provided by volume measurements and component volumes obtained from simulations put the electron density profiles ρ(z) and the corresponding form factors F(qz) on absolute scales. Various thicknesses, such as the hydrophobic thickness and the steric thickness, are obtained and compared to literature values.