Light harvesting antenna/pigments

Carotenoid is the main light harvesting antenna pigment of RC II in some algal phyla [1]. Energy transfer process from carotenoid to chi is less characterized, compared with the situation of chi or phycobiliproteins. The electron exchange mechanism [2] is proposed for this process, contrary to the case of Forster mechanism [3] for chi or phycobiliproteins. 

Typical photoreceptor proteins are chlorophyllic protein complexes, and proteins belonging to the carotenoid (rhodopsin), phytochrome, and cryptochrome families [10], These function as light-harvesting antenna pigments and auxiliary cofactors in the photosynthetic process, or they may play a regulatory role in biological processes. The chemical structures of typical chromophoric groups contained in these proteins are presented in Table 3.4. 

Two experimental systems will be briefly described to illustrate some of the ideas presented in the previous section. The examples span the range of system complexity from a diatomic molecule (I2) [28] to a supramolec-ular pigment-protein complex (the core light-harvesting antenna of photosynthetic bacteria, LH1 [18, 19]). 

For each photon absorbed by any of the accessory pigments or Chi s whose excitations are funneled into a reaction center, one electron can be removed from its trap chi. Because four electrons are involved per 02 derived from water, the evolution of this molecule of 02 requires the absorption of four photons by Photosystem II or the light-harvesting antennae feeding into it (see Eq. 5.8 and Fig. 5-15). An additional four photons whose excitations arrive at the trap chi of Photosystem I are required for the reduction of the two molecules of NADP+ to NADPH necessary for the subsequent reduction of one C02 molecule (Eq. 5.9 Figs. 5-1 and 5-15). Hence eight photons are needed for the evolution of one molecule of 02 and the fixation of one molecule of C02. (In Chapter 6, Section 6.3D, we will consider how many photons are used to provide the ATP s required per C02 fixed.) The series representation (Fig. 5-15) proposed by Hill and Fay... 

Figure 5-15. Schematic model for a series representation of the two photosystems of photosynthesis, indicating the stoichiometry of various factors involved in the reduction of C02 to a carbohydrate ( CH20)). Some of the photons (hv) are captured by the accessory pigments and Chi a in the light-harvesting antennae these excitations are then fed into the two photosystems, but primarily to Photosystem II.

The two photochemical reactions are performed by two photosystems. Each photosystem consists of a so-called reaction centre, where the primary energy conversion takes place, associated with a few hundred pigment molecules (chlorophylls and carotenoids see Fig. 2) serving as light-harvesting antennas, which transfer the absorbed energy as electronic excitation energy to the reaction centres. 

DJ Lundell, RC Williams and AN Glazer (1981) Molecular architecture of a light-harvesting antenna. In vitro assembly of the rod substructures of Synechococcus 6301 phycobilisomes. J Biol Chem 256 3580-3592 S Brody and E Rabinowitch (1957) Excitation lifetime of photosynthetic pigments. Science 125 555 G Tomita and E Rabinowitch (1962) Excitation energy transfer between pigments in photosynthetic cells. Biophys J 2 483-499... 

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