Two phase coexistence in ternary mixtures of saturated and polyunsaturated phosphatidylcholines with cholesterol.

Combining ²H and ¹⁴N nuclear magnetic resonance measurements on ternary lipid/cholesterol mixtures it is possible to quantitate the phase separation into coexisting ℓ_o and ℓ_d domains using a single sample whose composition is within the two phase region. Mixtures of each of the unsaturated phospholipids dioleoyl phosphatidylcholine, dilinoleoyl phosphatidylcholine and dilinolenoyl phosphatidylcholine with the saturated dipalmitoyl phosphatidylcholine and cholesterol all exhibit ℓ_o and ℓ_d phase coexistence over a substantial range of compositions and temperatures. A higher degree of unsaturation broadens the temperature range of two phase coexistence and results in the ℓ_d phase domains actually being significantly more 'fluid' at lower temperatures than they are at higher temperatures.