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Fig. 6 . Neutron scattering length density (fm/Å 3 ) calculated for phospholipid (A), choles-
terol (B), water (C), and PNS myelin (D). The number of molecules at the cytoplasmic and
extracellular side of the PNS myelin membrane in the surface area S = 3724 Å 2 are given
by Inouye and Kirschner (1988). (A) The neutron scattering length density for PE mole-
cules (hydrocarbon chain length, 18 carbons). The density projection was first calculated at
every 0.2 Å from the atomic coordinates of the molecule. Its Fourier transform was then
sampled by either 50, 20 or 6 orders with a 180 Å period. The density profile at a resolu-
tion of 180 Å/50 (solid line) was derived by reverse Fourier transform of the sampled data.
Similar density distributions are shown at lower resolution of 180 Å/20 (dashed line) and
180 Å/6 (dash-dot-dash). (B) The density projection for symmetrically-distributed choles-
terol molecules at a resolution of 180 Å/50 (solid line), 180 Å /20 (dashed line) and 180 Å/6
(dash-dot-dash). Panel (C) shows 5337 water molecules per S = 3724 Å 2 at a resolution of
180 Å/6 for different D 2 O volume fractions, i.e. H 2 O (solid line), 20% D 2 O (dashed line),
65% D 2 O (dash-dot-dash), and 100% D 2 O (dotted). (D) The modeled myelin without any
protein contribution, i.e. total density of 18:0 PE, symmetrically-distributed cholesterol
molecules, and 100% D 2 O per S = 3724 Å 2 . The density from the model are at resolutions
of 180 Å/6 (smoother solid line) and 180 Å/50 (solid line). The observed neutron scattering
length density (dashed line) was calculated from the reported structure factors for rabbit sci-
atic nerve myelin in D 2 O (Kirschner, 1974; Kirschner et al ., 1976). The absolute scale was
derived by assuming that the density at the center of the lipid bilayer as −0.04 fm/Å 3
and
the density at the center of the extracellular space as 0.636 fm/Å 3 (100% D 2 O).
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