The miscibility of milk sphingomyelin and cholesterol is affected by temperature and surface pressure in mixed Langmuir monolayers

- The interactions between milk sphingomyelin and cholesterol are involved in the formation of membrane domains.
- The melting temperature of milk-SM is Tm = 34 °C.
- Below Tm, cholesterol has a weak condensing effect on milk-SM.
- Above Tm, cholesterol and milk-SM are miscible with strongest area condensation for 30 mol% chol.
- The physical state of milk-sphingomyelin controls its assembly with cholesterol.

The miscibility of milk sphingomyelin (milk-SM) and cholesterol was investigated in this study. The effect of different physical states of milk-SM on its interactions with cholesterol was determined by the recording of isotherms of compression of Langmuir films for temperatures above and below the gel to Lα phase transition of milk-SM (Tm ∼ 34 °C). For T = 15 °C < Tm, the liquid expanded (LE) to liquid condensed (LC) phase transition of milk-SM monolayers was observed at surface pressures of 10-15 mN/m. For T = 43 °C > Tm, the milk-SM molecules were in a LE phase regardless of the surface pressure applied. A phase diagram pressure - milk-SM/cholesterol composition was established. This study demonstrated that both temperature and surface pressure affected the miscibility between the milk-SM and cholesterol. The strongest attractive forces (i.e. condensing effect) were identified for 30 mol% cholesterol when the milk-SM was in the LE phase state.

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