Absorption and photoacoustic spectroscopies of lutein and zeaxanthin Langmuir–Blodgett films in connection with the Haidinger's brushes

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Abstract
The molecular state of a single monolayer of lutein, zeaxanthin, and of a mixture of lutein and L-α-phosphatidylcholine-β-oleoyl-γ-stearoyl has been investigated using absorption and photoacoustic spectroscopies. Both methods, confirmatively, reveal that the two carotenoids are mostly in a monomer state coexisting with carotenoid crystals. We observe that the absorption as well as the photoacoustic bands are red-shifted relative to the bands in solution. The absorption bands are found to peak at mean values of 440.0, 464.8, and 503.8 ± 0.5 nm for lutein, and at 452.7, 475.9, and 513.7 ± 0.5 nm for zeaxanthin. The photoacoustic peaks are situated at 444.0, 467.2, and 502.4 ± 0.5 nm for lutein, and 455.4, 473.8, and 509.0 ± 0.5 nm for zeaxanthin. Diluting lutein monolayer with the phospholipid results into a blue-shift of the absorption bands compared with those of the pure carotenoid monolayer and the macular pigment spectra. The results are discussed in connection with the Haidinger's brushes. It is concluded that lutein molecules might not be mixed with membrane lipids in the macular tissue, but form a monolayer protruding from the Henle fiber membranes to form the Haidinger's brushes in the macula.