Fluid phase structure of EPC and DMPC bilayers

X-ray diffraction data taken at high instrumental resolution were obtained for EPC and DMPC under various osmotic pressures, primarily at T=30°C. The headgroup thickness D HH was obtained from relative electron density profiles. By using volumetric results and by comparing to gel phase DPPC we obtain areas A EPC F =69.4±1.1 Å 2 and A DMPC F =59.7±0.2 Å 2. The analysis also gives estimates for the areal compressibility K A . The A F results lead to other structural results regarding membrane thickness and associated waters. Using the recently determined absolute electrons density profile of DPPC, the A F results also lead to absolute electron density profiles and absolute continuous transforms ∣ F( q)∣ for EPC and DMPC. Limited measurements of temperature dependence show directly that fluctuations increase with increasing temperature and that a small decrease in bending modulus K c accounts for the increased water spacing reported by Simon et al. (1995) Biophys. J. 69, 1473–1483.