Cholesterol Perturbs Lipid Bilayers Non-Universally*

Cholesterol is well known to modulate the physical properties of biomembranes. Using modern x-ray scattering methods, we have studied the effects of cholesterol on the bending modulus K$_{C}$, the thickness D$_{HH}$, and the orientational order parameter S$_{xray}$ of lipid bilayers. We find that the effects are different for at least three classes of phospholipids characterized by different numbers of saturated hydrocarbon chains. Most strikingly, cholesterol strongly increases K$_{C}$ when both chains of the phospholipid are fully saturated but not at all when there are two mono-unsaturated chains.