Interaction of isomeric forms of xanthophyll pigment zeaxanthin with dipalmitoylphosphatidylcholine studied in monomolecular layers

Abstract Two-component monomolecular layers were formed with DPPC and two stereoisomers of zeaxanthin 9- cis and 13- cis at the argon–water interface. Very distinct over-additivity which represents affection of a lipid arrangement in the membrane has been observed in the case of zeaxanthin 9- cis (maximum at 20 mol%) but not in the case of zeaxanthin 13- cis . The differences in the organization of the isomers of zeaxanthin–DPPC monolayers are interpreted in terms of the different orientation of both xanthophylls at the interface observed at relatively high surface pressures (>25 mN/m) comparable to the surface pressures of biomembranes. The results are consistent with the model according to which zeaxanthin 9- cis adopts a vertical orientation at the polar–nonpolar interface in contrast to zeaxanthin 13- cis , which is oriented horizontally owing to the fact that it interacts by two hydroxyl groups with the same hydrophobic–hydrophilic interface in the monolayer. The findings are discussed in comparison with the behavior of zeaxanthin in the conformation all- trans in the same system. Zeaxanthin all- trans forms efficiently molecular aggregates in the mixed monolayers in contrast to cis isomers. Circular dichroism measurements show the formation of molecular structures by zeaxanthin 13- cis that are interpreted as dimers. FTIR measurements show that these dimers are stabilized by van der Waals interactions unlike aggregated structures formed by all- trans zeaxanthin that are stabilized by hydrogen bonding. Physiological importance of the differences in aggregation and orientation of stereoisomers of zeaxanthin in lipid environment is discussed.