X-RAY STRUCTURE DETERMINATION OF FULLY HYDRATED L(ALPHA) PHASE DIPALMITOYLPHOSPHATIDYLCHOLINE BILAYERS

Bilayer form factors obtained from x-ray scattering data taken with hi gh instrumental resolution are reported for multilamellar vesicles of L(alpha) phase lipid bilayers of dipalmitoylphosphatjdylcholine at 50 degrees C under varying osmotic pressure. Artifacts in the magnitudes of the form factors due to liquid crystalline fluctuations have been e liminated by using modified Caille theory. The Caille fluctuation para meter eta(1) increases systematically with increasing lamellar D spaci ng and this explains why some higher order peaks are unobservable for the larger D spacings. The corrected form factors fall on one smooth c ontinuous transform F(q); this shows that the bilayer does not change shape as D decreases from 67.2 Angstrom (fully hydrated) to 53.9 Angst rom. The distance between headgroup peaks is obtained from Fourier rec onstruction of samples with four orders of diffraction and from electr on density models that use 38 independent form factors. By combining t hese results with previous gel phase results, area AF per lipid molecu le and other structural quantities are obtained for the fluid L, phase . Comparison with results that we derived from previous neutron diffra ction data is excellent, and we conclude from diffraction studies that A(F) = 62.9 +/- 1.3 Angstrom(2), which is in excellent agreement with a previous estimate from NMR data.