Antenna complex

Knaff, D. B., 1991. Regnlatory phosphoryladon of chloroplast antenna proteins. Trends in Biochemical Sciences 16 82-83. Additional discussion of the structure of light-harvesdng antenna complexes associated widi photo-synthedc reaction centers can be found in Trends in Biochemical Sciences 11 414 (1986), 14 72 (1989), and 16 181 (1991). 

In photosynthetic bacteria, (i.e., Chlorobiaceae) the bacteriochlorophyUs support photosynthesis at low light intensities, and they accomplish this activity by using a unique antenna complex known as a chlorosome in which the pigments are located. Since these bacteria are strict anaerobes, photosynthesis is nonoxygenic. ... 

It is therefore important to bear in mind the dependency of the carotenoid spectrum upon properties of the environment for in vivo analysis, which is based on the application of optical spectroscopies. This approach is often the only way to study the composition, structure, and biological functions of carotenoids. Spectral sensitivity of xanthophylls to the medium could be a property to use for gaining vital information on their binding sites and dynamics. The next sections will provide a brief introduction to the structure of the environment with which photosynthetic xanthophylls interact—light harvesting antenna complexes (LHC). 

IDENTIFICATION OF XANTHOPHYLLS ASSOCIATED WITH THE TRANSMEMBRANE HELIXES OF LHCII ANTENNA COMPLEX ... 

The analysis of carotenoid identity, conformation, and binding in vivo should allow further progress to be made in understanding of the functions of these pigments in the photosynthetic machinery. One of the obvious steps toward improvement could be the use of continuously tuneable laser systems in order to obtain more detailed resonance Raman excitation profiles (Sashima et al 2000). This technique will be suitable for the investigation of in vivo systems with more complex carotenoid composition. In addition, this method may be applied for the determination of the energy of forbidden Sj or 2 Ag transition. This is an important parameter, since it allows an assessment of the energy transfer relationship between the carotenoids and chlorophylls within the antenna complex. 

Ruban, A.V., Pascal, A.A., Lee, P.J., Robert, B., and Horton, P. 2002a. Molecular configuration of xanthophyll cycle carotenoids in photosystem II antenna complexes. J. Biol. Chem 111 42937-42942. 

Leegwater, J. A. (1996). Coherent versus incoherent energy transfer and trapping in photosynthetic antenna complexes. J. Phys. Chem. 100, 14403-9. 

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.)...

We have seen that, in photosynthetic bacteria, visible light is harvested by the antenna complexes, from which the collected energy is funnelled into the special pair in the reaction centre. A series of electron-transfer steps occurs, producing a charge-separated state across the photosynthetic membrane with a quantum efficiency approaching 100%. The nano-sized structure of this solar energy-conversion system has led researchers over the past two decades to try to imitate the effects that occur in nature. 

A typical example involves the perylene monoimide donors described above. They have been used to construct antenna models in which up to eight perylene units were covalently bound to a zinc(II) porphyrin94. Osuka and coworkers windmill porphyrins also present excellent antenna complex building blocks (vide infra)95. In addition, several porphyrin wheel antenna complexes were prepared by template-assisted synthesis96,97 or self-aggregation98. Intramolecular cyclization of porphyrin dodecamers to a porphyrin wheel has also been achieved99. 

In the case of the carotenoid-containing LH2 complex, the triplet states of BChl a and carotenoid (spheroidene) were generated immediately after excitation, but the triplet-state BChl a was quenched efficiently by the carotenoid so that no BChl a cation-radical was generated. Thus, the photoprotective function of the carotenoid in this antenna complex has been proven. 

Zuber, H. (1986) Structure of light-harvesting antenna complexes of photosynthetic bacteria, cyanobacteria and red algae. Trends Biochem. Sci. 11, 414 419. 

The structure of the reaction centre complexes appears to be well conserved in organisms carrying out oxygen-evolving photosynthesis. In contrast, the organization of antenna complexes seems to be more varied. 

Overview of photosynthetic organisms and their Light-Harvesting Antenna complexes... 

In section 2.2 an overview of the organisms capable of photosynthesis will be outlined, with particular attention to the evolutionary relationships between their antenna complexes, tn section 2.3 some of the most representative antenna families will be described and discussed in more detail. Finally, a deeper look in the EET mechanism will be given in section 2.4, in which the dynamics of EET in a particular antenna complex will serve as an example. 

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