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. 2006 May 1;90(9):3176-83.
doi: 10.1529/biophysj.105.072959. Epub 2006 Feb 3.

Phase behavior of lipid monolayers containing DPPC and cholesterol analogs

Benjamin L Stottrup  1 , Sarah L Keller
Affiliations

Phase behavior of lipid monolayers containing DPPC and cholesterol analogs

Benjamin L Stottrup et al. Biophys J. .

Abstract

We investigate the miscibility phase behavior of lipid monolayers containing a wide variety of sterols. Six of the sterols satisfy a definition from an earlier study of "membrane-active sterols" in bilayers (cholesterol, epicholesterol, lathosterol, dihydrocholesterol, ergosterol, and desmosterol), and six do not (25-hydroxycholesterol, lanosterol, androstenolone, coprostanol, cholestane, and cholestenone). We find that monolayers containing dipalmitoyl phosphatidylcholine mixed with membrane-active sterols generally produce phase diagrams containing two distinct regions of immiscible liquid phases, whereas those with membrane-inactive sterols generally do not VSports手机版. This observation establishes a correlation between lipid monolayers and bilayers. It also demonstrates that the ability to form two regions of immiscibility in monolayers is not one of the biophysical attributes that explains cholesterol's predominance in animal cell membranes. Furthermore, we find unusual phase behavior for dipalmitoyl phosphatidylcholine monolayers containing 25-hydroxycholesterol, which produce both an upper and a lower miscibility transition. The lower transition correlates with a sharp change of slope in the pressure-area isotherm. .

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FIGURE 1
FIGURE 1
Progression of phase diagrams determined by epifluorescence microscopy of monolayers containing phospholipids and sterols. (a) Sketch of early reports of immiscibility for a binary system (e.g., DOPC and dihydrocholesterol (20)), with a shaded region of coexisting liquid phases (l-l) at low surface pressure, and one uniform liquid phase (liq) at higher pressures. (b) Sketch showing two distinct shaded regions of coexisting liquid phases: the α-region at low cholesterol and the β-region at high cholesterol (e.g., DPPC and dihydrocholesterol (22)). (c) Sketch showing five distinct shaded regions of coexisting phases, either solid-liquid (s-l) or liquid-liquid (l-l) (e.g., DMPS/GM1/dihydrocholesterol (45)).
FIGURE 2
FIGURE 2
All sterols and steroids here share a common structural element of three 6-carbon rings fused to a 5-carbon ring.
FIGURE 3
FIGURE 3
Phase diagrams for monolayers of DPPC containing membrane-active sterols. Regions of immiscible liquid phases are shaded: the α-region at low cholesterol and the β-region at high cholesterol. No attempt was made to identify solid phases at high surface pressure (compare with Fig. 1 c) or to explore sterol concentrations <10 mol % or >70 mol %. Concentrations at which no coexisting phases were observed are denoted by “x” symbols along the x axis.
FIGURE 4
FIGURE 4
Ternary phase diagram for monolayers containing DOPC, DPPC, and cholesterol (31). Two different regions of immiscible liquid phases are observed, the α-region at low cholesterol (•) and the β-region at high cholesterol (○). Transition pressures in the α-region are recorded in color, and regions between points are calculated by a polynomial fit. Transition pressures in the α-region are highest near equimolar ratios of the two phospholipids, DOPC and DPPC. Transition pressures in the β-region are greater than in the α-region, often well above 15 mN/m. Regions with no symbols represent regions of the phase diagram that were not investigated (rather than regions with no coexisting liquid phases).
FIGURE 5
FIGURE 5
Phase diagrams for monolayers of DPPC-containing membrane-inactive sterols. Monolayers containing androstenolone, lanosterol, cholestenone, and coprostanol exhibit no liquid-liquid immiscibility for all concentrations studied (denoted by “x” symbols) along the x axis. In contrast, monolayers containing 25-hydroxycholesterol exhibit two distinct regions of immiscibility: an α-region at low sterol and a β-region at high sterol. The α-region is bounded by both an upper and a lower miscibility transition. At low cholestane concentrations, monolayers form two liquid phases, but cholestane is squeezed out of the monolayer at higher concentrations (see Fig. 9).
FIGURE 6
FIGURE 6
Solid domains in a background of a liquid phase in a monolayer of (a) 4:1 DPPC/coprostanol at ∼9 mN/m and (b) 100% DPPC at ∼9 mN/m (plus 0.5% TR-DPPE). The addition of coprostanol increases the perimeter of solid domains. Scale bar 20 μm.
FIGURE 7
FIGURE 7
Pressure-area isotherm of a monolayer containing 85 mol % DPPC and 15 mol % 25-hydroxycholesterol (with 0.5 mol % TR-DPPE). A kink in the isotherm (at the asterisk) corresponds to the lower miscibility transition sketched in the inset. The full phase diagram of DPPC/25-hydroxycholesterol is found in Fig. 5.
FIGURE 8
FIGURE 8
(a) Molecular areas for isobaric cuts at 5 mN/m through pressure-area isotherms of DPPC/sterol monolayers. Errors are ±2Å2. (b) Inverse compressibilities calculated from pressure-area isotherms at 25 mN/m. As membrane-active sterol composition increases in lipid monolayers, so does the inverse compressibility. Inverse compressibilities are lowest for membrane-inactive sterols (open symbols), with the exception of 25-hydroxycholesterol.
FIGURE 9
FIGURE 9
Pressure-area isotherms of monolayers containing DPPC and either cholestane (top) or desmosterol (bottom) at the mole fractions labeled on each curve. At concentrations ≥40 mol %, cholestane is squeezed out of the monolayer. In contrast, desmosterol isotherms show no evidence of squeeze-out. The plateau at ∼7 mN/m for cholestane and ∼10 mN/m for desmosterol marks the onset of gel (solid) domains as in Fig. 6.
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References

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