Phase equilibriums in binary mixtures of phosphatidylcholine and cholesterol

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Abstract
The EPR spectra of 2 spin-labels, 2,2,6,6-tetramethylpiperadinyl-1-oxy and a head-group spin-labeled phosphatidylethanolamine (L-.alpha.-dipalmitoylphosphatidyl-N-ethanolamine), were used to study solid-liquid and liquid-liquid phase separations in binary mixtures of dimyristoylphosphatidylcholine (DMPC) and cholesterol. A quantitative analysis of these resonance spectra supports the view that at temperatures below .theta.m, the chain-melting temperature of the phospholipid, and at cholesterol mole fractions Xc < 0.2, these mixtures consist of 2 phases, a solid phase of essentially pure DMPC and a fluid phase having a mole fraction of cholesterol equal to 0.2. The spin-label data also provide evidence for fluid-fluid immiscibility in the bilayer membrane at temperatures above the chain-melting transition temperature of DMPC.