There is broad interest in the question of fluid-fluid phase coexistence in membranes, in particular, whether evidence for liquid-disordered (l(d))-liquid-ordered (l(o)) two-phase regions or membrane "rafts" can be found in natural membranes. In model membrane systems, such phase behavior is observed, and we have used
deuterium nuclear magnetic resonance spectroscopy to map the phase boundaries of ternary mixtures containing 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), chain-perdeuterated 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC-d(62)), and
cholesterol. For both this ternary
model system and the binary DPPC-d(62)/
cholesterol sytem, we present clear evidence for l(d)-l(o) two-phase coexistence. We have
selected sample compositions to focus on this region of fluid-fluid phase coexistence and to determine its temperature and composition ranges. The
deuterium nuclear magnetic resonance spectra for compositions near the l(d)-l(o)
phase boundary at
high cholesterol concentrations show evidence of exchange broadening or critical fluctuations in composition, similar to that reported by Vist and Davis. There appears to be a line of critical compositions ranging from 48 degrees C for a DOPC/DPPC-d(62)/
cholesterol composition of 0:75:25, to approximately -8 degrees C for the composition 57:14:29. At temperatures below this two-phase region, there is a region of three-phase coexistence (l(d)-l(o)-
gel). These results are collected and presented in terms of a partial ternary
phase diagram that is consistent with previously reported results of Vist and Davis.
