Location of Cholesterol in DMPC Membranes. A Comparative Study by Neutron Diffraction and Molecular Mechanics Simulation

The vertical location of 30 mol % cholesterol in a hydrated dimyristoylphosphatidylcholine (DMPC) membrane was determined by neutron diffraction on annealed samples containing deuterated or protonated cholesterol at 10, 20, 25, 30, and 50 °C. The sterol was deuterium-labeled in positions 2, 2, 3, 4, 4, and 6, and proton−deuterium contrast techniques were used to locate the position of the labeled part of the steroid in the membrane. Cholesterol is found well embedded in the membrane, with ring A at 16.3 ± 0.5 A from the bilayer center at 10 °C. This location linearly decreases to 15.1 ± 0.5 A at 50 °C, demonstrating that the sterol is not expelled from the membrane on crossing the former gel-to-fluid phase transition of pure DMPC (24 °C). Molecular dynamics were also performed on well-hydrated membranes in the presence and absence of cholesterol. Neutron scattering 1D profiles were then calculated for comparison with experimental neutron scattering data. The profile obtained from pure fluid-phase lipids i...