Interactions in mixed monolayers between dioleoyl-l-phosphatidylcholine and 11-cis retinal. A comparison with all-trans retinal

Abstract

The surface pressure isotherms of 11-cis retinal, diolcoyl-l-phosphatidylcholine and seven mixtures of these components covering the whole range of molar fractions have been studied at the nitrogen-water interface at 20.5°C. For all the mixtures, an almost constant collapse pressure at 44 ± 1 mN m−1 is observed. The surface pressure isotherms also show an apparent collapse pressure at 14, 16, 20, 24, and 28 mN m−1 at a molar fraction of dioleoyl-l-phosphatidylcholine of 0.100, 0.200, 0.400, 0.500, and 0.600, respectively. Below this apparent collapse pressure, it is shown that the components are completely miscible at the interface. However, this pressure represents the onset of the rejection of 11-cis retinal from the monolayer, this rejection appearing to be total at the true collapse pressure. The miscibility region of the isotherms is studied in terms of the additivity rule. Negative deviations to ideality are observed and negative excess free energies of mixing are calculated for all the systems at four different surface pressures. A comparison between 11-cis retinal and all-trans retinal mixtures with dioleoyl-l-phosphatidylcholine shows that the two isomers have about the same thermodynamics of interaction with this phospholipid.