Peridinin structure

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

Scheme 18.2 Structure of the allenic carotinoids fucoxanthin (5) and peridinin (6).

Absorption spectra of peridinin in different solvents are shown in Fig. 2a. In the nonpolar solvent M-hexane, the absorption spectrum exhibits the well-resolved structure of vibrational bands of the strongly allowed S0-S2 transition with the 0-0 peak located at 485 nm. In polar solvents, however, the vibrational structure is lost and the absorption band is significantly wider. In addition, there are also differences between the various polar solvents. Although the loss of vibrational structure is obvious, a hint of shoulder is still preserved in methanol and acetonitrile, but in ethylene glycol and glycerol the absorption spectrum is completely structureless with a broad red tail extending beyond 600 nm. 

Early work on peridinin demonstrated that its structure leads to breaking of the idealized C2h symmetry resulting in relatively strong fluorescence from the Si state [16], Recent studies demonstrated that the intensity of the peridinin Si emission depends on solvent polarity [8,9], and time-resolved studies revealed that the polarity-dependent change in the Si emission yield... 

Fig. 6. Potential energy surface diagram showing the polarity effect on structure of the Si/ICT potential surface of the peridinin molecule. The transitions corresponding to the observed signals are denoted by arrows. Due to the lack of knowledge about Sn potential surface, it is visualized only as a line representing a final state for Si-S and ICT-Sn transitions. See text for details.

It is necessary to emphasize that Fig. 6 represents a model for only one particular peridinin conformation. As discussed above, a distribution of peridinin conformers exists in polar solvents. A slight structural change in the excited state is also responsible for the rise component observed in polar solvents, but this dynamic cannot be included in the model depicted in Fig. 6 as it includes a change of the S /ICT potential surface. In protic solvents the dynamics of peridinin are further complicated as hydrogen bonding leads to the formation of a red peridinin form with different properties of both ground and excited states. 

Figure 23-29 (A) Stereoscopic drawing of light-harvesting complex from the dinoflagellate protozoan Amphidinium carterae. The central cavity contains eight molecules of peridinin, two of which can be seen protruding from the top. Deeply buried toward the bottom are two molecules of Chi a. Also present are two molecules of digalactosyl diacylglycerol. From Hofmann et al.268 Courtesy of Wolfram Welte. (B) Structure of peridinin.

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. 

Figure 7 Rogue s gallery of structures of peripheral anteima complexes. As labelled these include Chlorosomes from green sulfur bacteria, fused antenna domains of the Photosystem I core, the CP43 and CP47 proteins of Photosystem II, the Fenna-Matthew-Olson (FMO) protein associated with chlorosomes, LHI proteins surrounding a purple bacterial photo synthetic core, the peridinin-chlorophyll a protein of dinoflagellate algae, the LHCI and LHCII proteins found in plants and many algae, and the LHII protein complex that is associated with LHI in purple bacteria...

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). 

Peridinin (1) was first isolated by Schutt in 1890. Nearly a century later, between 1971 and 1980, its structure was elucidated by Liaaen-Jensen and coworkers. Ito et al. synthesized peridinin (1) as a racemic mixture of diastereomers in 1990 and accomplished an enantio-selective total synthesis in 1993. In 2002 and 2004, Katsumura et al. published two syntheses of peridinin (1) with stereochemical control of the six stereogenic centers and of the /Z-geometry of the seven double bonds. 

A. Structure and Function of the Peridinin-Chlorophyll-Protein (PCP) Complex.233... 

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.

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. 11. A2 k resolution structure of a peridinin Chi a complex of a dinoflagellate. In the color version, the eight peridinin molecules are shown in red, whereas the two Chls are in green. A lipid molecule is shown in blue, and two proteins are shown in gray The proximity of peridinin molecules to each other and to Chi a molecules explains the efficient excitation energy transfer from peridinin to Chi a, observed by Haxo et al. (1976). The diagram is reproduced from Hoffman et al. (1996). See also Color Plate 2.

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. 

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.

Structure and organization ofthe peridinin-chlorophyU a-binding protein from the dinoflageUate Gonyaulax polyedra. Mol Gen Genet 255 595-604... 

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