X-ray diffraction of myelin membrane. I. Optimal conditions for obtaining unmodified small angle diffraction data from frog sciatic nerve.

The X-ray diffraction pattern of myelin of frog sciatic nerve has been investigated, using a Kratky small angle slit camera to obtain the electron density distribution across the membrane. All major reflections observed were related to a fundamental repeat distance of 171 +/- 2.8 A. There was no further increase in the number of reflections on varying the experimental conditions (varying pH, applying tension, immersion in various isotonic buffer solutions, etc.) or by varying the camera slit arrangement. The degree of disorder within the myelin sheath was examined by comparing the crystallite size to the half-width of the diffraction peak at half-height. The limiting of the diffraction spectra to five major reflections was determined not to be caused by disorder. It is concluded that the observed X-ray diffraction pattern is a consequence of the particular electron density distribution of the membrane. Therefore, the membrane cannot contain sharply distinct step-function regions of electron density, but approaches a modified cosine distribution.