In order to better understand the optical properties of carotenoids in photoreceptor membrane, the absorption spectroscopic studies of a carotenoid canthaxanthin (Cantha) in solution and in artificial membrane model system, i.e., the monolayers, have been carried out. The monolayers deposited on hydrophilic and hydrophobic quartz lamelles at surface pressure of 20 mN/m display a large hypsochromic shift relative to solution spectra in various organic solvents, for example, acetone, benzene, chloroform, etc. This blue shift in monolayers has been attributed to the formation of card-pack or H- aggregates brought about by water.
The presence of water is necessary to induce the formation of aggregates. This is suggested by the blue-shifted spectra of acetonic solution of Cantha in which small amount (10%, v/v) of water has been added. The spectra of Cantha monolayers placed in vacuum (10-6 Torr) for 24 hours or heated at 50 degree celsius for one hour also confirm the requirement of water. These spectra show absorption maxima similar to those observed in organic solvents. The vacuum or the heating seems to remove the water and disrupt the aggregates. The importance of intermolecular separation in the formation of card-pack aggregates in Cantha is demonstrated by the spectra of mixed monolayers of Cantha with a phospholipid phosphatidylcholine in varying molar ratios.