Myelin: A One-Dimensional Biological “Crystal” for X-Ray and Neutron Scattering - Molecules: Nucleation, Aggregation and Crystallization - page 86

Positions of the molecules were determined in the following manner.
The origin of the membrane is taken at the center of the cytoplasmic appo-
sition. The absolute electron density profile from swollen mouse sciatic
nerve (Fig. 3) shows the first density peak at 16 Å from the center of the
cytoplasmic space, and the second at 62 Å. The phosphate group of the
phosphatidylethanolamine (PE) (Elder et al ., 1977) is assigned to those
peaks (Fig. 4(A)). The cholesterol molecule is positioned so that the oxy-
gen of cholesterol is 4 Å away from the phosphate (Franks, Lieb 1979).
The two cholesterol oxygens at the cytoplasmic and extracellular leaflets,
therefore, are localized 20 Å and 58 Å from the origin (Fig. 4(B)). The
water distribution is derived from the neutron diffraction according to the
above method, and the electron density is shown in Fig. 4(C).
The chain length of the reported crystallographic data for PE is 12 car-
bon atoms (Elder et al ., 1977), while 18 is the mean chain length of lipid
hydrocarbon in myelin (O'Brien, Sampson, 1965). The electron densities
for PE with different lengths of hydrocarbon chain were considered. The
chains in all trans configurations is extended along the long axis of the
fatty acids. To account for the electron density, the 18-carbon hydrocarbon
chains should interdigitate at the center of the lipid bilayer. Chains having
fewer than 16 carbons do not interdigitate, and give significantly lower
electron density than observed (Fig. 4(D)).
The disposition of cholesterol in the leaflets of the PNS myelin is not
certain. Cholesterol can be asymmetrically placed at a 1:2 molar ratio
between the cytoplasmic and extracellular leaflets (Fig. 5(A)) (Caspar,
Kirschner, 1971), or can be placed at 1:1 (Fig. 5(B)) (Scott et al ., 1980; Allt
et al ., 1985). The net electron density profiles are asymmetric largely due
to an asymmetric distribution of water, i.e. more water molecules are pres-
ent in the cytoplasmic half of the leaflet than in the extracellular half. The
position of protein is estimated from the difference profile between the
observed and calculated electron densities (Fig. 5(C)). The two prominent
peaks at the cytoplasmic and extracellular space likely come from myelin
basic protein (MBP) (Inouye et al ., 1985) and P0-glycoprotein. The elec-
tron density at the center of the membrane bilayer arises from the trans-
membrane domain of P0. The net average electron density of the myelin