Xanthophylls epoxidation

The HPLC analysis of milkweed, the food-plant source for Monarch butterflies, demonstrates that it contains a complex mixture of carotenoids including lutein, several other xanthophylls, xanthophyll epoxides, and (3-carotene, Figure 25.3b. There is a component in the leaf extract that is observed to elute near 8min, which has a typical carotenoid spectrum but is not identical to that of the lutein metabolite observed at near the same retention time in the extracts from larval tissue. 

Epoxy-5,6-dihydro-p,e-carotene-3,3 -diol, 9CI. Xanthophyll epoxide. Lutein epoxide [28368-08-3]... 

In order to obtain nearly absolute purity of the spectra of these xanthophylls, it was necessary to calculate the difference Raman spectra. Therefore, for zeaxanthin, two spectra of samples, one containing violaxanthin and the other enriched in zeaxanthin, were measured at 514.5 nm excitation. After their normalization using chlorophyll a bands at 1354 or 1389 cm-1, a deepoxidized-minus-epoxidized difference spectrum has for the first time been calculated to produce a pure resonance Raman spectrum of zeaxanthin in vivo (Figure 7.10b). A similar procedure was used for the calculation of the pure spectrum for violaxanthin. The only difference is that the 488.0nm excitation wavelength and epoxidized-minus-deepoxidized order of spectra have been applied in the calculation. The spectra produced using this approach have remarkable similarity to the spectra of xanthophyll cycle carotenoids in pure solvents (Ruban et al., 2001). The v, peaks of violaxanthin and zeaxanthin spectra are 7 cm 1 apart and in correspondence to the maxima of this band for isolated zeaxanthin and violaxanthin, respectively. The v3 band for zeaxanthin is positioned at 1003 cm-1, while the one for violaxanthin is upshifted toward 1006 cm-1. 

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. 

Structurally, vitamin A (retinol) is essentially one half of the molecule of (3-carotene. Thus, (3-carotene is a potent provitamin A to which 100% activity is assigned. An unsubstituted (3 ring with a Cn polyene chain is the minimum requirement for vitamin A activity, y -Carotene, a-carotene, (3-cryptoxanthin, a-cryptoxanthin, and (3-carotene 5,6-epoxide, all having one unsubstituted ring, have about half the bioactivity of (3-carotene (Table7.4) On the other hand, the acyclic carotenoids, devoid of (3-rings, and the xanthophylls, in which the (3-rings have hydroxy, epoxy, and carbonyl substituents, are not provitamin A-active for humans. 

Leaves were dark-adapted therefore, there is no detectable level of zeaxanthin. Concentrations are nmol pigment (mol chi a — b), the P value from one factor ANOVA is displayed below each column. V-A-Z = xanthophyll pool (violaxanthin, antheraxanthin, zeaxanthin) EPS = epoxidation state. Reprinted with permission from P. J. Ralph et al. [76]. 

Light-harvesting Chi a/b complexes contain carotenoids of the /3-/3 type (/8-carotene, violaxanthin, antheraxanthin, zeaxanthin, neoxanthin) and of the /3-e type (lutein). Some higher plants like lettuce contain an , -carotenoid, lactucaxanthin, both in the major and in minor Chi a/b complexes which may replace either lutein or other xanthophylls (Phillip and Young, 1995). Among /3,/3-carotenoids, violaxanthin, antheraxanthin, and neoxanthin contain epoxides whereas the others do not. In order to... 

F/g. 14. Characterization of (left panel) the relative levels of the D1 and LHCIl proteins during the transition from summer to winter (1993 to 1994), and (right panel) the relative level of the D1 protein, the epoxidation state, EPS, of the xanthophyll cycle (0.5A+V)/(V+A+Z), and the efficiency of open PS II units (F /F determined after a 30 min period of dark adaptation) during the transition from winter to summer (1994) from needles of Scots pine growing in Sweden. Data redrawn from Ottander et al. (1995). 

Gilmore AM and Bjorkman O (1994) Adenine nucleotides and the xanthophyll cycle in leaves. 1. Effects of CO2- and temperature-limited photosynthesis on adenylate energy charge and violaxanthin de-epoxidation. Planta 192 526-536... 

The structural differences between violaxanthin and zeaxanthin may provide an explanation of how the xanthophyll cycle could indirectly control qE. De-epoxidation would change the interaction between LHCII and the xanthophyll. Violaxanthin would stabilize the unquenched conformation and zeaxanthin the quenched conformation (see Fig. 2). But what is the evidence for this kind of indirect control of qE ... 

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