Myelin: A One-Dimensional Biological “Crystal” for X-Ray and Neutron Scattering - Molecules: Nucleation, Aggregation and Crystallization - page 88

Fig. 5. (A) Modeled PNS myelin with the asymmetric cholesterol disposition
(1:2 molar ratio at the cytoplasmic and extracellular sides) at a resolution of 180 Å/50
(solid line), 180 Å/5 (thick solid line), and mouse sciatic nerve from X-ray diffraction
data at 216 Å/6 resolution (dashed line). The number of carbons for the chain length of
PE is 18:0. Note that the water distribution is at a resolution of 180 Å/6. (B) Modeled
myelin with the symmetric cholesterol disposition at the same resolution as in (A).
(C) The difference between the observed and calculated electron densities with the
symmetric cholesterol disposition (solid line) and with the asymmetric cholesterol dis-
position (dashes). Note that because the calculated curve does not include the electron
density of protein, then the difference here is accounted for by the disposition of pro-
tein. The box plot shows the estimated electron density level from the primary
sequences of the mouse P0 (without sugar moiety) and myelin basic protein (14 kDa
and 18 kDa isoforms; shaded box).
membrane calculated from the chemical composition is 0.347 e/Å 3 , which
is similar to the measured value of 0.343 e/Å 3 from the electron density on
an absolute scale.
The neutron scattering lengths (in fm/Å 3 ) are calculated for PE with
18-carbon chains (Fig. 6(A)), symmetrically-distributed cholesterol
(Fig. 6(B)), and water for different D 2 O volume fractions (Fig. 6(C)). The
net density for the mixture of PE, cholesterol and water is compared with