Xanthophylls structures

FIGURE 2.2.1 Structures of common carotenoids (I. Main carotenes. II. Xanthophylls. III. Animal carotenoids). 

The carotenoids are subdivided into two main groups, the carotenes (hydrocarbon carotenoids) and the xanthophylls (oxygenated carotenoids). The structure of /8-carotene is as follows ... 

Structure of the Photosystem II Antenna Xanthophylls in LHCII Structure.117... 

The variations in the end group structure and conformation are determined by the carotenoid biosynthesis enzymes. These structural features are likely to determine localization as well as functions of these xanthophylls in vivo (Hashimoto et al., 2001 Young et al., 2002). 

FIGURE 7.1 (a) Structures of the five most common xanthophylls. (b) HPLC separation profile of the... 

It is therefore important to bear in mind the dependency of the carotenoid spectrum upon properties of the environment for in vivo analysis, which is based on the application of optical spectroscopies. This approach is often the only way to study the composition, structure, and biological functions of carotenoids. Spectral sensitivity of xanthophylls to the medium could be a property to use for gaining vital information on their binding sites and dynamics. The next sections will provide a brief introduction to the structure of the environment with which photosynthetic xanthophylls interact—light harvesting antenna complexes (LHC). 

The structure of the major trimeric LHCII complex has been recently obtained at 2.72 A (Figure 7.3) (Liu et al., 2004). It was revealed that each 25kDa protein monomer contains three transmembrane and three amphiphilic a-helixes. In addition, each monomer binds 14 chlorophyll (8 Chi a and 6 Chi b) and 4 xanthophyll molecules 1 neoxanthin, 2 luteins, and 1 violaxanthin. The first three xanthophylls are situated close to the integral helixes and are tightly bound to some amino acids by hydrogen bonds to hydroxyl oxygen atoms and van der Waals interactions to chlorophylls, and hydrophobic amino acids such as tryptophan and phenylalanine. 

Identifying electronic and vibrational properties of xanthophylls should provide not only structural information. Gaining information about excited state energy levels would help to design and interpret kinetic experiments, which probe molecular interactions and the energetic relationship between the xanthophylls and chlorophylls. 

Neoxanthin and the two lutein molecules have close associations with three transmembrane helixes, A, B, and C, forming three chlorophyll-xanthophyll-protein domains (Figure 7.5). Considering the structure of LHCII complex in terms of domains is useful for understanding how the antenna system works, and the functions of the different xanthophylls. Biochemical evidence suggests that these xanthophylls have a much stronger affinity of binding to LHCII in comparison to violaxanthin... 

FIGURE 7.5 Structural domains of LHCII xanthophylls. Aromatic amino acids tyrosine in the neoxanthin domain and tryptophan and phenylalanine in the violaxanthin domain are labeled as Y, W, and F, respectively. 

The v4 region enhancement and structure in the resonance Raman spectra of xanthophylls reviewed in this chapter shows that it can be used for the analysis of carotenoid-protein interactions. Figure 7.8 summarizes the spectra for all four major types of LHCII xanthophylls. Lutein 2 possesses the most intense and well-resolved v4 bands. The spectrum for zeaxanthin is very similar to that of lutein with a slightly more complex structure. This similarity correlates with the structural similarity between these pigments. It is likely that they are both similarly distorted. The richer structure of zeaxanthin spectrum may be explained by the presence of the two flexible P-end rings... 

The use of selective isotope replacement of carbon and hydrogen atoms in the structure of xanthophylls in combination with LHCII reconstitution should greatly aid the assignment of multiple v4 twisting bands. This assignment would help localize the areas of distortion within the carotenoid molecule and understand the possible causes of this distortion. 

FIGURE 13.2 Chemical formulas of macular xanthophylls. It can be noted that the chemical structures of (meso)-zeaxanthin and lutein differ only by the position of a single double bond. 

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