Carotenoids in Lipid Membranes - Carotenoids: Physical, Chemical, and Biological Functions and Properties - page 24

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FIGURE 2.3 Temperature dependency of the absorption spectra of zeaxanthin incorporated into the
liposomes formed with DPPC. The initial concentration of zeaxanthin in the medium used to prepare lipo-
somes was 5 mol% with respect to lipid. (Based on the results presented in Sujak, A. et al., Biochim. Biophys.
Acta , 1509, 255, 2000.)

differences expressed in much higher over-additivity of the molecular area in the lutein-containing
membranes as compared to the zeaxanthin-containing membranes (Sujak and Gruszecki, 2000).
Such a difference has been interpreted in terms of the different orientation of the xanthophylls in
the lipid environment. The fact that the differences were not observed at the higher concentrations
of carotenoids, promoting their aggregation, allowed the evaluation of the aggregation threshold
concentration above which pigments remained in the form of molecular assemblies within the lipid
phase. The aggregation threshold values for lutein and zeaxanthin in monomolecular layers, 30
and 20 mol% respectively, correspond to the values of 15 and 10 mol% with respect to a lipid bilayer
(Sujak and Gruszecki, 2000). Below those concentrations, the pigments are distributed between the
pools of monomeric and aggregated molecules. Interestingly, the aggregation threshold determined
for canthaxanthin, using the same approach, was considerably lower than in the case of lutein and
zeaxanthin in the monomolecular layers, equal to 2 mol%, which corresponds to 1 mol% in the case
of a lipid bilayer. Such a low aggregation threshold of canthaxanthin in the membranes formed with
DPPC has been coni rmed in the spectroscopic studies of lipid bilayers (Sujak et al., 2005). The very
strong ability of canthaxanthin to form molecular aggregates is most probably directly responsible
for the formation of the crystal inclusions in the natural biomembranes of retina (Goralczyk et al.,
1997, 2000).

2.3 EFFECTS OF CAROTENOIDS ON LIPID MEMBRANES

2.3.1 M ODEL M EMBRANES
Carotenoid molecules incorporated into the lipid membranes considerably interfere with both the
structural and the dynamic membrane properties. Both effects are directly related to the chemical
structure of carotenoid molecules. Importantly, it is the rigid, rod-like backbone of the carotenoids,