LIQUID-CRYSTALLINE PHASES OF CHOLESTEROL LIPID BILAYERS AS REVEALED BY THE FLUORESCENCE OF TRANS-PARINARIC ACID

The presence of two liquid-crystalline phases, alpha and beta, in mixe d bilayers of dimyristoylphosphatidylcholine/ cholesterol was detected by the changes in the distribution of the fluorescence lifetimes of t -PnA, as analyzed by the Maximum Entropy Method. The formation of the liquid-ordered beta-phase, in the 30-40 degrees C temperature range as a function of cholesterol concentration (0-40 mol%), could be related quantitatively to the relative amplitude of a long lifetime component of the probe (10-14 ns). Based on this evidence, the phase behavior o f mixtures of the unsaturated lipid palmitoyloleoylphosphatidylcholine and cholesterol was determined using the same technique, for choleste rol concentrations in the 0-50 mol% range, between 10 and 40 degrees C . It was found that two liquid-crystalline phases are also formed in t his system, with physical properties reminiscent of the alpha- and bet a-phases formed with saturated lipids. However, in this case it was de termined that, for temperatures in the physiological range, the alpha- and beta-phases coexist up to 40 mol% cholesterol. This finding may b e of significant biological relevance, because it supports the long he ld notion that cholesterol is responsible for the lipid packing hetero geneity of several natural membranes rich in unsaturated lipid compone nts.