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Abstract:
For canonical lipid raft mixtures of cholesterol (chol),
N
-palmitoylsphingomyelin (PSM), and 1-palmitoyl-2-oleoyl-
phosphatidylcholine (POPC), electron paramagnetic resonance (EPR) of spin-labeled phospholipids—which is insensitive to
domain size—is used to determine the ternary phase diagram at 23
C. No phase boundaries are found for binary POPC/chol
mixtures, nor for ternary mixtures with PSM content
<
24 mol %. EPR lineshapes indicate that conversion from the liquid-disor-
dered (L
a
) to liquid-ordered (L
o
) phase occurs continuously in this region. Two-component EPR spectra and several tie lines
attributable to coexistence of gel (L
b
) and fluid phases are found for ternary mixtures with low cholesterol or low POPC content.
For PSM/POPC alone, coexistence of L
a
and L
b
phases occurs over the range 50
95.5 mol % PSM. A further tie line is found at
3 mol % chol with endpoints at 50 and
R
77 mol % PSM. For PSM/chol, L
b
L
o
coexistence occurs over the range 10
38 mol %
chol and further tie lines are found at 4.5 and 7 mol % POPC. Two-component EPR spectra indicative of fluid-fluid (L
a
L
o
) phase
separation are found for lipid compositions: 25%
<
PSM
<
65%, 5%
<
chol
<
30–35%, 65%
>
POPC
>
10%, and confirmed by
nonlinear EPR. Tie lines are identified in the L
a
L
o
coexistence region, indicating that the fluid domains are of sufficient size
to obey the phase rule. The three-phase triangle is bounded approximately by the compositions 40 and 75 mol % PSM with
10 mol % chol, and 60 mol % PSM with 25 mol % chol. These studies define the compositions of raft-like L
o
phases for a minimal
realistic biological lipid mixture.
