Phospholipids line tension

Figure 12. Epifluorescence (fluorescent probe, 23) photomicrograph of a mono-molecular film of the phospholipid dipalmitoyl phosphatidyl choline (10, R = R = n-CisHsi) at the air-water interface. The black regions are composed of solid-phase lipid, and the white (fluorescent) regions are fluid-phase lipid containing about 1 mol% of a fluorescent lipid probe. (Top) Micrograph showing the onset of solid phase formation bar, 50 pm. Middle) Micrograph showing formation of chiral solid domains when the phospholipid is one of the enantiomeric forms (R) bar, 50 pm. Bottom) Micrograph showing spiral forms of enantiomeric lipid when 2 mol% of cholesterol is included in the monolayer so as to reduce the line tension bar, 30 pm. Reproduced from ref. 146 (McConnell and Keller, Proc. Natl. Acad. Sci. USA 1987, 84,4706) with permission of the Academy of Sciences of the USA.

A key factor in formation of structures that are not compact is the line tension between the phases, which suppresses instabilities at an interface. The addition of cholesterol to a monolayer lowers the line tension and favors the formation of ramified structures. Complex lamellar structures of many types, including spirals (Fig. 22), are formed in the LE LC transition region of phospholipids doped with cholesterol. If the amphiphiles are chiral, the spirals have a unique sense of rotation. 

Wallace JA, Schiirch S. Line tension of a sessile drop on a fluid-fluid interface modified by a phospholipid monolayer. J Colloid Interface Sci 1988 124 452-461. 

Phospholipids play an important role in lung functions. The surface active material to be found in the alveolar lining of the lung is a mixture of phospholipids, neutral lipids and proteins. Lowering of surface tension by the lung surfactant system and the surface elasticity of the surface layers assists alveolar expansion and contraction. Deficiency of lung surfactants in newborns leads to a respiratory distress syndrome and this led to the suggestion that instillation of phospholipid surfactants could cure the problem. 

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