Cholesterol is an important component of all animal membranes, both cellular and intracellular. This ubiquitous amphiphilic sterol may constitute up to 50% of the total lipid in some biological membranes. However, its exact role in the structure and function of membranes is unknown, in spite of intensive study of cholesterol's effects on model and natural membrane systems in the last decade. Previous investigations have examined the often rather indirect effects of cholesterol on overall membrane properties, such as permeability. This work attempts to determine the local influence of cholesterol molecules on the dynamical properties of those lipids in their immediate environment. Electron spin resonance (ESR) spectroscopy was employed to monitor the behavior of 3-nitroxide cholestane (CSL), a cholesterol-like spin label molecule, incorporated into various phospholipid-cholesterol multi-bilayer membranes. Being more cholesterol-like than lipid-like, the cholestane spin label may provide information about the cholesterol-rich regions of the bilayer, which phospholipid spin probes may not even enter. This is the first thorough, systematic study of the effects of cholesterol concentration on the thermal behavior of phospholipid multilayers dispersed in excess water (the state most closely simulating the natural membrane environment) by ESR spectroscopy. Differential scanning calorimetry (DSC) was also used to examine phospholipid multilayers with incorporated CSL. Plots of 2A⊥ versus temperature yielded S-shaped transition curves centered around 20°c for all the lipids studied. The lipid gel-liquid crystal transition was also present at the appropriate temperature. Addition of cholesterol results in gradual increase of temperature of the CSL-induced transition, but addition of 25 mole% cholesterol or more resulted in disappearance of the main gel-liquid crystal transition. The spin label-induced 20-degree transition was also observed by DSC, corroborating the ESR results. Peaks were broad, with a small enthalpy change. It is suggested, from evidence accumulated in this work and reported in the literature, that cholesterol-lipid domains with a 1:2 stoichiometry begin coalescing with addition of even small amounts of sterol. These regions are composed of cholesterol molecules, each complexed to a single lipid molecule and loosely associated with a second lipid. Lipid which is complexed to cholesterol does not melt, whereas lipid associated 0 with CSL does melt - around 20° C. Boundary lipid around the complex domains and the lipid loosely associated with 1:1 complex undergo a broad, uncooperative transition slightly above the free lipid melting temperature. At about 20 mole% cholesterol content, there is no more free bulk lipid in the system; only complexed lipid and boundary lipid remain and the main gel-liquid crystal transition disappears. When 33 1/3 mole% cholesterol is present, boundary lipid around the complex domains also disappears. As more cholesterol is added, the loosely associated lipid molecules still left around the complex pairs decrease in number as more 1:1 complex units are formed. At 50 mole% cholesterol, the last uncomplexed lipid molecules are gone, and further addition of the sterol results in separation of cholesterol crystals from the bilayer.
Florence L. Trentacosti Presti (Thu,) studied this question.