Antennae, pigments

The interiors of rhodopseudomonad bacteria are filled with photosynthetic vesicles, which are hollow, membrane-enveloped spheres. The photosynthetic reaction centers are embedded in the membrane of these vesicles. One end of the protein complex faces the Inside of the vesicle, which is known as the periplasmic side the other end faces the cytoplasm of the cell. Around each reaction center there are about 100 small membrane proteins, the antenna pigment protein molecules, which will be described later in this chapter. Each of these contains several bound chlorophyll molecules that catch photons over a wide area and funnel them to the reaction center. By this arrangement the reaction center can utilize about 300 times more photons than those that directly strike the special pair of chlorophyll molecules at the heart of the reaction center. 

Antenna pigment proteins assemble into multimeric light-harvesting particles... 

Figure 10.16 Solar energy transfer from accessory pigments to the reaction centre, (a) The photon absorption by a component of the antenna complex transfers to a reaction centre chlorophyll, or, less frequently, is reemitted as fluorescence, (b) The electron ends up on the reaction centre chlorophyll because its lowest excited state has a lower energy than that of the other antenna pigment molecules. (From Voet and Voet, 2004. Reproduced with permission from John Wiley Sons., Inc.)...

Nowadays, most reaction center models carry suitable antenna pigments and acceptor groups and in effect are photosystem models. A typical example for a state-of-the-art system that incorporates many aspects of a photosystem consisted of a boron dipyrrin covalently linked to a zinc(II) porphyrin, which carried a suitably modified C60 derivative as axial ligand. Selective excitation of the boron dipyrrin as antenna pigment resulted in energy transfer to a zinc(II) porphyrin followed by electron transfer to the acceptor109. 

Figure 23-50 Proposed mechanism of DNA photoylase action. The antenna pigment (top) absorbs a photon, then transfers energy to the anion of bound FADH2 (step a) which donates an electron to the thymine dimer in DNA (step b). After free-radical-driven steps an electron is returned to the blue FAD radical (step c).

Heathcote, P. and Clayton, R.K. 1977. Reconstituted energy transfer from antenna pigment-protein to reaction centers isolated from Rhodopseudomonas sphaeroides. Biochim. Biophys. Acta, 459. 506-515. 

Other pigments, called antenna pigments, or accessory pigments, absorb light at other wavelengths. The accessory pigments are responsible for the brilliant colors of plants in the autumn (in the Northern Hemisphere). The breakdown of chlorophyll allows us to see the colors of the accessory pigments. 

Carotenoids are found in all native photosynfhetic organisms. They serve a dual function, as both accessory antenna pigment and also are essential in photoprotection of photosynfhetic systems from the effects of excess light, especially in the presence of oxygen. Bilins are open-chain tetrapyrroles that are present in antenna complexes called phycobilisomes. These complexes are found in cyanobacteria and red algae. Structures of representative carotenoid pigments are shown in Figure 3. 

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