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.