Interaction between chlorophyll a and violaxanthin in different steric conformations : Model studies in monomolecular layers

Abstract

Monomolecular layers of pure chlorophyll a (Chl a), violaxanthin in the conformation all- trans, 9- cis, 13- cis and two-component monomolecular layers formed with Chl a and all isomers of violaxanthin, at a stoichiometric ratio 1:1, were formed and compressed at the argon–water interface. The specific molecular areas of Chl a and the isomers of violaxanthin in monomolecular layers were determined on the basis of the isotherms of compression. The isotherms of compression of the Chl a–violaxanthin-all trans monolayers correspond well to the theoretical ones, calculated on the basis of the additivity rule. The theoretical isotherms of compression of monolayers formed with Chl a and violaxanthin in conformations 9- cis and 13- cis are shifted towards higher molecular areas with respect to the experimental ones, indicating molecular interaction between components of the system. Monolayers were also deposited to a solid support by means of the Langmuir–Blodgett technique and subjected to spectroscopic studies: to quartz lamella in the case of electronic absorption and fluorescence spectroscopy and to ZnSe crystal in the case of Fourier Transform Infrared Spectroscopy (FTIR). The comparison of the absorption and Chl a fluorescence excitation spectra enabled to make a calculation of the rate of excitation energy transfer from violaxanthin to Chl a. Efficiency of energy transfer from a carotenoid to Chl a was 32% in the case of violaxanthin all- trans, 21% in the case of violaxanthin 9- cis and 15% for violaxanthin 13- cis. Such findings suggest a structural role of violaxanthin in conformation 9- cis and 13- cis and an accessory role of violaxanthin all- trans in the photosynthetic apparatus. The analysis of FTIR spectra reveals molecular interaction between Chl a and violaxanthin in conformations cis via the hydroxyl groups of xanthophylls located in the C3 and C3′ positions.