X-ray diffraction of myelin membrane. II. Determination of the phase angles of the frog sciatic nerve by heavy atom labeling and calculation of the electron density distribution of the membrane.

The phase signs of the five main X-ray reflections from normal frog sciatic nerve have been determined as all positive using a technique of labeling with very small amounts of heavy metal. The changes in intensity of the individual reflections were studied as a function of uptake of metal label by the membrane. The possible localization of the metal label was decided from computer-analogue studies and from Patterson calculations. These phases are different from those determined by previous workers using techniques of trial of the best set of phases, or a step model, to give the best fit of the combined intensity data of normal and swollen myelin membranes. The electron density map has been calculated using eight reflections and their experimentally determined phases. The map shows an inner low electron density region which is different from that shown by earlier calculations. The center of the low electron density region shows a small region of increased electron density. However, without fixing absolute electron density levels in the map, it is not yet possible to allocate regions of low electron density to pure lipid or lipoprotein. The map shows the two sides of the membranes to be different in molecular structure without significant water spaces between the membranes.