Phase determination of x-ray reflections for membrane-type systems with constant fluid density.

A new technique for phase determination of X-ray reflections from symmetric structures is presented. This method, involving comparison of intensity data from structures with variable fluid layer thickness and constant fluid electron density, permits computation of phase angles, scaling factors, and origin reflection values independently. Possible sources of error inherent in other methods of phase determination are thereby eliminated. Results of the application of this method to model structures and to myelin data are reported. Advantages of the technique, which tests all possible phase angle combinations in a rapid fashion, are discussed

Direct observation of the hydrocarbon chain tilt angle in phospholipid bilayers.

By means of X-ray diffraction we have observed the tilting of hydrocarbon chains within a dipalmitoyl phosphatidylcholine multilayer. The tilting produces a distribution of intensity in the (4.2 A)-1 region that provides a quantitative measure of the angle of tilt. Detailed diffraction investigation of multilayers prepared by a recently developed method and mounted to form a "freestanding" sample has made possible the accurate measurement of intensity in this region. We find the intensity distribution to be different from that previously reported and proceed to develop a new model for tilted chains. The model permits calculation of the intensity distribution as a function of tilt angle which is then fit to the data

Amplitude of Rippling in the Pβ Phase of Dipalmitoylphosphatidylcholine Bilayers

We present x-ray diffraction results of dipalmitoylphosphatidylcholine (DPPC) multilayers in three structural phases. Using pure DPPC, precision temperature control, and high angular resolution methods, we have discovered splitting of the first diffraction order due to multilayering in the Pβ phase. This splitting permits us to calculate the amplitude of ripples in this phase. The amplitude is large enough to suggest a structural mechanism for rippling