Xanthophylls absorption

The maximum spectrometric absorption of lutein (C40H56O2, mol wt 568.9, xanthophyll, (3R,3.S,6.R)-p,8-carotene-3,3.-diol) is found between 453 and 481 nm. Its solubihty in ethanol is greater than that of the carotenoids. It is somewhat less sensitive to oxidation and heat degradation than P-carotene. It contributes yellow color." ... 

In contrast with the hydrocarbon carotenes primarily located in the cores of the CM particles, xanthophylls are present at the surfaces of the CM particles, making their exchanges with other plasma lipoproteins easier." Therefore, if some exchanges occur between lipoproteins, AUC (or absorption) values of the newly absorbed compound in the TRL fraction will be underestimated. Based on all these considerations, the present approach is more appropriate to determine the relative bioavailability of a compound derived from various treatments within one snbject and/or within one study. 

Ruban AV, Horton P, and Young AJ. 1993. Aggregation of higher-plant xanthophylls—Differences in absorption-spectra and in the dependency on solvent polarity. Journal of Photochemistry and Photobiology B-Biology 21(2-3) 229-234. 

Solvents with different polarities and refractive indexes significantly affect carotenoid optical properties. Because the refractive index is proportional to the ability of a solvent molecule to interact with the electric held of the solute, it can dramatically affect the excited state energy and hence the absorption maxima positions (Bayliss, 1950). Figure 7.2a shows three absorption spectra of the same xanthophyll, lutein, dissolved in isopropanol, pyridine, and carbon disulfide. The solvent refractive indexes in this case were 1.38, 1.42, and 1.63 for the three mentioned solvents, respectively. 

The identification of xanthophylls in vivo is a complex task and should be approached gradually with the increasing complexity of the sample. In the case of the antenna xanthophylls, the simplest sample is the isolated LHCII complex. Even here four xanthophylls are present, each having at least three major absorption transitions, 0-0, 0-1, and 0-2 (Figure 7.4). Heterogeneity in the xanthophyll environment and overlap with the chlorophyll absorption add additional complexity to the identification task. No single spectroscopic method seems suitable to resolve the overlapping spectra. However, the combination of two spectroscopic techniques, low-temperature absorption and resonance Raman spectroscopy, has proved to be fruitful (Ruban et al., 2001 Robert et al., 2004). 

FIGURE 7.4 Absorption (a) and resonance Raman (b) spectra of the four major xanthophylls of I.HCII antenna zeaxanthin (Zea), lutein (Lut), violaxanthin (Vio), and neoxanthin (Neo). 

Taking into consideration that antenna xanthophylls not only possess original absorption but also resonance Raman spectra, and the fact that the Raman signal is virtually free from vibrational spectroscopy artifacts (water, sample condition, etc.), it seemed of obvious advantage to apply the described combination of spectroscopies for the identification of these pigments. 

Absorption and Raman analysis of LHCII complexes from xanthophyll biosynthesis mutants and plants containing unusual carotenoids (e.g., lactucoxanthin and lutein-epoxide) should also be interesting, since the role of these pigments and their binding properties are unknown. Understanding the specificity of binding can help to understand the reasons for xanthophyll variety in photosynthetic antennae and aid in the discovery of yet unknown functions for these molecules. 

The contribution of lutein and zeaxanthin to the risk reduction of AMD is mainly based on two properties of the xanthophylls one is their blue-light absorption and the other is their antioxidant... 

Carotene (a carotenoid) and (d) lutein (a xanthophyll) are accessory pigments in plants. The areas shaded pink are the conjugated systems (alternating single and double bonds) that largely account for the absorption of visible light. 

The problem relating to chlorophylls can be overcome to some extent by saponification of the sample, which will remove the chlorophylls however, care must be taken in the choice of conditions, as some carotenoids, particularly the xanthophylls, may be degraded (see UNITF2.1). On the other hand, it is possible to use an alternate wavelength for the carotenoids. For example most of the major carotenoids of interest in foods have an absorption peak around 480 nm, where any absorption of chlorophylls causes less interference however, it is then necessary to use alternate extinction coefficients (e.g., 2180 for (3-carotene). 

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