EVIDENCE FOR THE FORMATION OF A CHLOROPHYLL a/ZEAXANTHIN COMPLEX IN LECITHIN LIPOSOMES FROM FLUORESCENCE DECAY KINETICS

The interaction of Chi a with zeaxanthin (Zea), which is an analogue of lutein, has been studied in soya bean lecithin liposomes using the fluorescence of Chi as monitor. The fluorescence emission spectrum at 4.2 K of Chi a showed characteristic changes in the presence of Zea: the emission maximum shifted from 688 nm to 680 nm, and a peak at 731 nm appeared. The fluorescence decay kinetics of Chi a alone could be described by the sum of two exponential components (T₁,≅0.8 ns, T₂≅2.5 ns). In the presence of Zea a component with a long lifetime, T≅5 ns, appeared with a large relative amplitude (40%). This indicated the formation of a Chl a /Zea complex, in which Chl a /Chl a interaction is negligible, presumably because of strong interaction between Chl a and Zea. The fluorescence anisotropy decay kinetics supported the hypothesis of the formation of a large Chl a containing complex in the presence of Zea. A rotational correlation time, φ≅14 ns at 4°C and φ≅21 ns at 30°C, was found, which is distinctly larger than for samples containing Chl a only. We interpret these results as further evidence for a strong interaction between Chl a and Zea in the hydrophobic environment of the lecithin liposomes. This interaction may also occur in the Chl-proteins of the Chi alb light-harvesting complex of plant photosynthesis.