Carotenoids peridinin, structure

Hoffman, E., et al., 1996. Structural basis of light harvesting by carotenoids Peridinin-chlorophyll-protein from Amphidinium carterae. Science 272 1788-1791. 

The unique water-soluble peridinin- Chi a-protein (PCP) complexes are found in many dynoflagellates in addition to intrinsic membrane complexes. [64] It contains Chi a and the unusual carotenoid peridinin in stoichiometric ratio of 1 4. Unlike other families of antennas, the main light-harvesting pigments are carotenoids, not chlorophylls. The structure of the PCP consists of a protein that folds into four domains, each of which embeds four peridinin molecules and a single Chi a. The protein then forms trimers, suggested to be located in the lumen [64] in contact with both LHCI and LHCII [66], allowing efficient EET to occur. 

A survey10 of several dinoflagellates has revealed the presence of some interesting new carotenoids in addition to the main carotenoid peridinin [3 -acetoxy-5,6-epoxy-3,5 -dihydroxy-6, 7 -didehydro-5,6,5, 6 -tetrahydro-12, 13, 20 -trinor-/3,/ -caroten-19,11-olide (19)]. Pyrrhoxanthin was assigned the trinor structure 3 -acetoxy-5,6-epoxy-3-hydroxy-7, 8 -didehydro-5,6-dihydro-12, 13, 20 -trinor-/3,/3-caroten-19,11-olide (20) from a consideration of its spectroscopic properties and by chemical correlation with peridinin, and dinoxanthin was shown to be an acetate of neoxanthin, i.e. 3 -acetoxy-5,6-epoxy-6, 7 -didehydro-5,6,5, 6 -tetrahydro-/3,j8-carotene-3,5 -diol (11). Small amounts were also obtained of pyrrhoxanthinol and peridininol which were shown to be the deacetylated analogues (21) and (22) of pyrrhoxanthin and peridinin respectively. 

Fig. 5. Crystal structure ofthe A. carterae PCP monomer. (A) stereogram ofthe ribbon model ofthe PCP monomer. The scaffold formed by the helices is likened to a boat the various parts of a boat (bow, stern, deck and keel) are marked. The chromophores in the hydro-phobic cavity ofthe monomer complex are likened to "cargos. (B) stereogram ofthe arrangement of peridinins and chlorophyll in the N-terminal domain [oniy two peridinins ofthe C-terminai domain are shown by thinner iines]. Figure source Hofmann, Wrench, Sharpies, Hiller, Werte and Diederichs (1996) Structural basis of light harvesting by carotenoids Peridinin-chlorophyll-protein from Amphidinlum carterae. Science 272 1789,1790. A color stereogram of (A) kindiy provided by Dr. Eckhard Hofmann and Dr. Wolfram Weite is shown in Color Plate 7.

Amphidinium carterae also shows precisely where the carotenoid peridinin is located in this antenna complex (Hofmann et al., 1996 Fig. 11). Kiihlbrandt et al. (1994) have provided the atomic level structure of LHCllb, the major light harvesting Chi a/Chl b complex of plants and green algae this has allowed the rationalization of the proposed mechanisms of excitation energy transfer among the Chls. 

The unusual structure of the carotenoid pigment peridinin required for its solution the collaboration of four laboratories and a combination of all available physical techniques (Chapter 5). 

P Wegfahrt and H Rapoport (1971) The structure of peridinin, the characteristic dinoflegellate carotenoid. Am Chem Soc 93 1823-1825... 

In the singlet manifold, carotenoids have, like all polyenes, an unusual electronic structure The hrst excited state (Si) has the same symmetry, A, as the ground state, and thus one-photon transitions from So to Si are forbidden. In other words, the Si state does not appear in the absorption (or emission) spectrum of carotenoids (with more than 9 double bonds), which is dominated by the very strong So S2 (B ) transition. Carotenoids also possess a state of symmetry, which may lie near S2, though evidence for the spectroscopic observation of this state remains controversial [132-135]. Finally, some unusual carotenoids with polar substituents, such as peridinin, may also have low-lying charge transfer states [42, 136, 137]. 

Fig. I. Structures of some carotenoids known to play a major role in light-harvesting in eukaryotic algae a) peridinin b) fuco-xanthin c) siphonaxanthin d) prasinoxanthin.

In some important examples of naturally occurring carotenoids, the polyene chain is modified by the presence of one or two acetylenic or allenic groups. These and other interesting features are illustrated by peridinin (558) and pyrrhoxanthin (556) which contain allenic and acetylenic groups, respectively, as well as unusual modifications such as a C37-skeleton with an abnormal arrangement of side-chain methyl groups, and the presence of a butenolide structure. The synthesis of such carotenoids, particularly in the natural optically active form, provides a major challenge and the syntheses that have been developed are described in this Chapter. 

The water solubility of carotenoid sulphates is a special property among carotenoids and depends not only on the number of sulphate groups present, but on the total carotenoid structure. The following water solubilities in mg/ml have been reported r,2 -dihydro-( ), /-caroten-T-ol (88) sulphate > 0.01, astaxanthin (406) disulphate > 0.02, zeaxanthin (119) disulphate > 0.05, capsorubin (413) disulphate >0.14, fucoxanthin (369) sulphate > 0.20 and peridinin (558) sulphate > 0.36 [12]. It should be noted that the water solubility is drastically reduced in the presence of inorganic salts, as expected from solubility product considerations. 

NMR was first used in structural elucidation of carotenoids for peridinin (30) (108, 160, 161). Support for the presence of the allo-xanthin (31) end group in isomytiloxanthin (20) was obtained by direct comparison of NMR spectra (114, 175). From the characteristic shifts of the 14-methyl and C-8 signals violeoxanthin was shown to be the 9-ds isomer of violaxanthin (32) (136). 

Rapoport The structure of peridinin — the characteristic dinoflagellate carotenoid. J. Amer. Chem. Soc. 43, 1823 (1971). 

Allenic carotenoids are very limited in algae (Table 106.2). From their chemical structures, all-trani-neoxanthin 18 might be converted to fucoxanthin 75, dinoxanthin 74, peridinin 72, vaucheriaxanthin 69, and diadinoxanthin 67, but the pathways and enzymes remain unknown (Fig. 106.3) [6]. 

Fig. 1 (C) Structure of the complete peridinin-chlorophyll-protein trimer complex from the dinoflagellate Amphidinium carterae. The protein is depicted in ice blue, the carotenoids in red and the chlorophylls in green. Figure kindly produced by Robielyn Hagan from PDB file IPPR using VMD.

---