Phase Equilibria in DOPC/DPPC-d62/Cholesterol Mixtures

Abstract

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 system, 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.

Effect of cholesterol on the molecular structure and transitions in a clinical-grade lung surfactant extract

“…The observed temperature dependence of the order parameters in the LSE-cholesterol mixtures is in clear contrast to the order parameters of the simple model system composed of DPPC:chol (1:1), which shows an L α(o) phase over this whole temperature range (38,49). However, there are close similarities between the |SCH| profile of the LSE + 10 wt % cholesterol system and the ternary model system composed of DPPC:DOPC:chol (1:1:1) in the same temperature range (34,49), with a critical point near physiological temperatures. Based on previous phase diagrams, one can predict that also the DPPC:chol binary system goes through a gradual change from L α(d) to L α(o) at higher temperatures above its critical point (50,51).…”

“…Based on previous phase diagrams, one can predict that also the DPPC:chol binary system goes through a gradual change from L α(d) to L α(o) at higher temperatures above its critical point (50,51). The marked change of the |SCH| profile with temperature as observed for both the LSE-chol and DPPC:DOPC:chol systems (34,49) is likely related to the lowering of the critical temperature where the L α(d) -L α(o) coexistence merge into a single Lα phase, due to the presence of unsaturated lipids with low melting temperature. Finally, the |SCH| profile obtained from the LSE + 5 wt % cholesterol approaches the |SCH| profile of the L α(d) phase in the clinicaltype LSE at increasing temperature (Fig.…”

Cholesterol enhances surface water diffusion of phospholipid bilayers

“…[17][18][19] In lipid membrane systems without embedded proteins; however, water content within the bilayer decreases in the presence of Chol, as evidenced by electron paramagnetic resonance (EPR) 20,21 and fluorescence anisotropy measurements. 16,17 Although extensive research has been performed in model cell membranes to characterize the Chol-containing lipid mixtures at various lipid phases and compositions, [21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36] the molecular basis underlying the biological effect of Chol in lipid-water assemblies remains unclear.…”

Adventures in Physical Chemistry

“…However, the NMR spectra of the putative two-liquid phase region is particularly interesting and is discussed elsewhere (5,36,45,72,73).…”