Differences in the interactions of a monoglyceride with cholesterol and with a bile salt

The differences in the interaction of monoglycerides with bile acids or cholesterol have been investigated from a physico-chemical point of view. A Langmuir trough and fluorescence microscope was used to study mixed monolayers of dl-α-palmitin (a monoglyceride) and cholesterol or deoxycholic acid (a bile acid) at the air/aqueous interface. The surface pressure–area per molecule isotherms of the monolayers were analyzed to give the thermodynamic properties. The deoxycholic acid–dl-α-palmitin monolayer showed stronger repulsions between the film components than was observed with the cholesterol acid–dl-α-palmitin monolayer. Mixed monolayers containing dl-α-palmitin and cholesterol or deoxycholic acid phase separated at high surface pressures and high fractions of dl-α-palmitin, the conditions that resulted in the most repulsions between the two components of the monolayer. The mixed cholesterol and dl-α-palmitin monolayer phase separated in a random pattern. The deoxycholic acid and dl-α-palmitin mixed monolayer gave smaller domains that were distributed in a homogeneous fashion within the monolayer at high molecular packing densities. The difference in the cholesterol and deoxycholic acid interactions with dl-α-palmitin were explained by the fact that while both cholesterol and deoxycholic acid molecules do not pack efficiently with the dl-α-palmitin molecules, the attractive interactions between the alcohol groups on dl-α-palmitin and the carboxylic groups on deoxycholic acid cause attractive interactions between the deoxycholic acid and dl-α-palmitin domains, which causes the interaction abilities of deoxycholic acid with dl-α-palmitin to be higher than cholesterol.