Abstract
The mixed monomolecular film system dimyristoylphosphatidic acid/cholesterol has been studied at 10 °C and pH 11 over the concentration range 0–25 mol % cholesterol by using a combination of classical surface pressure/area per molecule isotherm determinations and fluorescence microscopy. The latter technique permitted the direct observation of the quasi-crystalline condensed phases of this system and allowed both their number and shape to be documented. From the isotherms, the fluid/condensed-phase transition is observed to shift to higher surface pressures over the 0–20 mol % cholesterol concentration range. Broadening of the transition is detectable between 5 and 10 mol % and increases with increasing cholesterol concentration until at 25 mol % the transition is no longer detectable. The fluorescence micrographs parallel the isotherm behavior. Between 1 and 5 mol % cholesterol spiral condensed domains are observed. Below 1 mol %, and between 7.5 and 20 mol %, other structures are found, but at 25 mol % condensed domains are no longer detectable. Interpretation of these results suggests that, at or below 5 mol % cholesterol, the sterol acts to reduce the line tension between the fluid mixed phase and a nearly pure phosphatidic acid condensed phase. Changing line width as a function of compression or degree of crystallization and quasi-crystallite nucleation is consistent with this picture. At higher cholesterol concentrations the situation is complicated by probable cholesterol incorporation into the condensed phase, ultimately resulting in an elimination of the fluid/gel transition. The situation is further complicated by possible artifactural effects at high compression due to high fluorescent probe concentrations in the residual “fluid” phase.
Original language | English |
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Pages (from-to) | 1352-1358 |
Number of pages | 7 |
Journal | Langmuir |
Volume | 4 |
Issue number | 6 |
DOIs | |
State | Published - 1 Nov 1988 |