The structural changes in the polar head group region of unsonicated bilayer membranes of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine and 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine produced by addition of cholesterol have been determined using deuterium and phosphorus-31 NMR. Incorporation of up to 50 mol % cholesterol produces little change in the phosphorus-31 chemical shielding anisotropics, compared with the values in pure bilayers above the phase transition temperatures, while some of the deuterium quadrupole splittings are reduced by almost a factor of two. Adjustment of the head group torsion angles by only a few degrees accounts for the observed spectral changes. Addition of cholesterol therefore has opposite effects on the hydrocarbon and polar regions of membranes: although cholesterol makes the hydrocarbon region gel-like, with an increased probability of trans conformations, the conformation of the polar head groups is very similar to that found in the liquid crystalline phase of pure phospholipid bilayers.
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