THE INFLUENCE OF CHOLESTEROL ON PHOSPHOLIPID MEMBRANE CURVATURE AND BENDING ELASTICITY

The behavior of dioleoylphosphatidylethanolamine (DOPE)/cholesterol/te tradecane and dioleoylphosphatidylcholine (DOPC)/cholesterol/tetradeca ne were examined using x-ray diffraction and the osmotic stress method . DOPE/ tetradecane, with or without cholesterol, forms inverted hexag onal (H-II) phases in excess water, DOPC/tetradecane forms lamellar ph ases without cholesterol at lower temperatures, With tetradecane, as l ittle as 5 mol% cholesterol in DOPC induced the formation of H-II phas es of very large dimension, Increasing levels of cholesterol result in a systematic decrease in the H-II lattice dimension for both DOPE and DOPC in excess water. Using osmotic pressure to control hydration, we applied a recent prescription to estimate the intrinsic curvature and bending modulus of the H-II monolayers, The radii of the intrinsic cu rvature, R-p(o), at a pivotal plane of constant area within the monola yer were determined to be 29.4 Angstrom for DOPE/tetradecane at 22 deg rees C, decreasing to 27 Angstrom at 30 mol% cholesterol. For DOPC/tet radecane at 32 degrees C, R-p(o) decreased from 62.5 Angstrom to 40 An gstrom as its cholesterol content increased from 30 to 50 mol%, These data yielded an estimate of the intrinsic radius of curvature for pure DOPC of 87.3 Angstrom. The bending moduli k(c) of DOPE/tetradecane an d DOPC/tetradecane, each with 30 mol% cholesterol, are 15 and 9 kT, re spectively, Tetradecane itself was shown to have little effect on the bending modulus in the cases of DOPE and cholesterol/DOPE. Surprisingl y, cholesterol effected only a modest increase in the k(c) of these mo nolayers, which is much smaller than estimated from its effect on the area compressibility modulus in bilayers. We discuss possible reasons for this difference.