Intermolecular interactions in lipid/carotenoid monolayers.

The collapse pressure at the air-water interface of monomolecular films of 1,2-distearoyl derivatives of phosphatidylcholine and digalactosyldiacylglycerol containing various proportions of the carotenoid astaxanthin was related to the composition of the monolayer. The results were analysed by using a regular-association approximation by which it is assumed that there is a stepwise formation of ABi-type associations where A and B represent the diacyl lipid and astaxanthin respectively and 1 less than or equal to i less than or equal to 6. This treatment provides an adequate description of the experimental data and permits calculation of equilibrium constants for the steps in complex-formation; each step is said to have the same equilibrium constant. The value for the collapse-surface-pressure increment associated with formation of ABi complexes was also derived. Calculated values of equilibrium constants and collapse-surface-pressure increments are greater for phosphatidylcholine/astaxanthin mixed monolayers than for digalactosyldiacylglycerol/astaxanthin mixed monolayers. These differences are discussed in terms of intermolecular interactions between components in the two systems.