Myelin membrane structure as revealed by x-ray diffraction.

The present work consists of a new interpretation of the data presented in the article entitled "X-Ray Diffraction of Myelin Membrane. II" by C. K. Akers and D. F. Parsons (1970, Biophys. J.10:116). It will be shown that the projection of the electron density onto the normal to the myelin multilayer derived by these authors is no more consistent with their data than another electron density function, or, perhaps, its negative. (A density function and its negative are related as follows: one of them is a certain density distribution, the other is the same function subtracted from a constant uniform density. Two density functions so related produce identical diffracted intensities.) The Fourier series for the projection of the electron density onto the normal to the myelin multilayer has coefficients +/-[hI(h)](1/2) where I(h) are the intensities of the five orders of reflection; data from which these can be estimated are presented by Akers and Parsons. The sequence of signs found here is + - - + + for the positive density (or - + + - - for the negative one). Quantitative agreement exists between the five X-ray diffraction data of Akers and Parsons and the same intensities calculated from the new model of the myelin structure described here. In this model the myelin double layer, 171 A thick, consists of a central lipid layer 72.4 A thick covered on both surfaces by protein layers 6.9 A thick; these protein layers are covered, in turn, by other lipid layers 42.4 A thick. Minor modifications of this model will no doubt be required to produce agreement between the observed and calculated intensities of the higher order reflections.