Cholesterol's role in the molecular architecture of polyunsaturated model membranes

The role of the native cellular lipid, cholesterol, in modulating the structural/organizational properties of model membranes rich in polyunsaturated lipids was explored. Cholesterol affinity has been proposed to produce heterogeneity in the distribution of membrane constituents. Biophysical evidence presented here supports this hypothesis, unequivocally demonstrating that highly polyunsaturated environments are less compatible with insertion of the sterol than saturated environments. We measured cholesterol's solubility in polyunsaturated membranes by solid state 2H NMR spectroscopy and X-ray diffraction methods that detected the presence of cholesterol monohydrate crystals which have been excluded from the bilayer. Whereas 1-stearoyl-2-oleoyl-sn-phosphatidylcholine (18:0-18:1PC), 1-stearoyl-2-arachidonoyl-sn-phosphatidylcholine (18:0-20:4PC) and 1-stearoyl-2-docosahexaenoyl-sn-phosphatidylcholine (18:0-22:6PC) bilayers containing a saturated sn-1 chain can accommodate >50 mol% cholesterol, which is typical for lipid bilayers, 1,2-diarachidonoyl-sn-phosphatidylcholine (20:4-20:4PC) and 1,2-didocosahexaenoyl-sn-phosphatidylcholine (22:6-22:6PC) bilayers containing a polyunsaturated chain at sn-1 and sn-2 positions can only accommodate <20 mol% cholesterol. The reduction in solubility correlates with a reduction in molecular order parameter, corresponding to a larger tilt angle (α_o), for the steroid moiety in dipolyunsaturated PC bilayers (α_o ∼ 25°) vs. saturated-polyunsaturated PC or saturated-monunsaturated PC bilayers (α_o ∼ 16°) as determined by 2H NMR of (3α-2H₁]cholesterol. These findings provide a molecular rationale for the sterol-dependent sorting of membrane phospholipids into polyunsaturated-rich/cholesterol-poor liquid disordered (l_d) and saturatedrich/cholesterol-rich liquid ordered (l _o) microdomains in native tissues containing a diverse mixture of lipids. The role of membrane composition in neural receptor activation is discussed.