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Pigment configuration in the light-harvesting protein of the xanthophyte alga Xanthonema debile

Streckaite, Simona, Gardian, Zdenko, Li, Fei, Pascal, Andrew A., Litvin, Radek, Robert, Bruno, Llansola-Portoles, Manuel J.
Photosynthesis research 2018 v.138 no.2 pp. 139-148
Raman spectroscopy, algae, carotenoids, chlorophyll, high performance liquid chromatography, light harvesting complex, magnesium, soil, stoichiometry
The soil chromophyte alga Xanthonema (X.) debile contains only non-carbonyl carotenoids and Chl-a. X. debile has an antenna system denoted Xanthophyte light-harvesting complex (XLH) that contains the carotenoids diadinoxanthin, heteroxanthin, and vaucheriaxanthin. The XLH pigment stoichiometry was calculated by chromatographic techniques and the pigment-binding structure studied by resonance Raman spectroscopy. The pigment ratio obtained by HPLC was found to be close to 8:1:2:1 Chl-a:heteroxanthin:diadinoxanthin:vaucheriaxanthin. The resonance Raman spectra suggest the presence of 8–10 Chl-a, all of which are 5-coordinated to the central Mg, with 1–3 Chl-a possessing a macrocycle distorted from the relaxed conformation. The three populations of carotenoids are in the all-trans configuration. Vaucheriaxanthin absorbs around 500–530 nm, diadinoxanthin at 494 nm and heteroxanthin at 487 nm at 4.5 K. The effective conjugation length of heteroxanthin and diadinoxanthin has been determined as 9.4 in both cases; the environment polarizability of the heteroxanthin and diadinoxanthin binding pockets is 0.270 and 0.305, respectively.