Myelin: A One-Dimensional Biological “Crystal” for X-Ray and Neutron Scattering - Molecules: Nucleation, Aggregation and Crystallization - page 87

Fig. 4. Electron density projection calculated for phospholipid (A), cholesterol (B),
water (C), and PNS myelin (D). The number of molecules at the cytoplasmic and extra-
cellular side of the PNS myelin membrane in the surface area S = 3724 Å 2 are given by
Inouye and Kirschner (1988). The number of phospholipid molecules at the cytoplasmic
and extracellular sides are 34 and 45, and for cholesterol molecules 30(20) and 31(41),
where the values in parenthesis are for the 1:2 molar distribution of cholesterol. (A) The
electron density projection of phosphatidylethanolamine (PE) molecules having 18-carbon
chain lengths. The electron density projection was first calculated at every 0.2 Å from the
atomic coordinates of the molecule. Its Fourier transform was then sampled using either
50, 20, or 5 orders for a 180 Å period. The electron density profile at a resolution of 180
Å/50 (solid line) was derived by reverse Fourier transform of the sampled data. Similar
electron density distributions are shown at lower resolution (180 Å/20, dashed line, and
180 Å/5, dash-dot-dash). (B) The electron density projection of symmetrically-distributed
cholesterol molecules at a resolution of 180 Å/50 (solid line), 180 Å/20 (dashed), and
180 Å/5 (dash-dot-dash). Panel (C) shows 5337 water molecules per S = 3724 Å 2 at a res-
olution of 180 Å/6. (D) The modeled myelin without any protein contribution, i.e. total
electron density of PE, cholesterol, and water per S = 3724 Å 2 with PE containing differ-
ent chain lengths (18:0 carbons, solid line; 16:0 carbons, dashed; and 12:0 carbons, dash-
dot-dash). The cholesterol molecules are shown symmetrically disposed at the cytoplasmic
and extracellular sides of the membrane. The resolution is at 180 Å/5, which is compara-
ble to that of the observed electron density.