Reaction centre antenna

With tlie development of femtosecond laser teclmology it has become possible to observe in resonance energy transfer some apparent manifestations of tire coupling between nuclear and electronic motions. For example in photosyntlietic preparations such as light-harvesting antennae and reaction centres [32, 46, 47 and 49] such observations are believed to result eitlier from oscillations between tire coupled excitonic levels of dimers (generally multimers), or tire nuclear motions of tire cliromophores. This is a subject tliat is still very much open to debate, and for extensive discussion we refer tire reader for example to [46, 47, 50, 51 and 55]. A simplified view of tire subject can nonetlieless be obtained from tire following semiclassical picture. 

Kranss, N., et al., 1996. Photosystem I at 4 A resolution represents the first structural model of a joint photosynthedc reaction centre and core antenna system. Nature Structural Biology 3 965-973. 

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.)...

Nonradiative energy transfer has a major role in the process of photosynthesis. Light is absorbed by large numbers of chlorophyll molecules in light-harvesting antennae and energy is transferred in a stepwise manner to photosynthetic reaction centres, at which photochemical reactions occur. This fundamental energy-transfer process will be considered in more detail in Chapter 12. 

Figure 12.7 The photosynthetic unit, in which an antenna chlorophyll molecule is excited by photon absorption and the energy is transferred to the chlorophyll dimer at the reaction centre...

Rapid multistep Coulombic energy transfer takes place as the excitation energy is transferred between the antenna chromophores and the special pair of bacteriochlorophyll molecules (P) in the reaction centre. 

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. 

Antennae for light harvesting Reaction centre for charge separation Membrane for physical separation... 

Of all the systems where Forster dipole—dipole energy transfer has been identified, the most important is light harvesting by antennae chloro-phyll-b molecules and donation of singlet energy to the chlorophyll-a reaction centres in photosynthetic organisms. Typical values of R0 have been estimated to be 4—5nm. Further details of photosynthesis may be found in articles by Birks [6, 141,142], Berlman [127], Gregory [144], and Jortner [145]. 

The study of artificial photosynthesis has been the subject of ongoing attention for many years now due to the need for sustainable energy resources. In natural photosynthesis a lightharvesting antenna system with a large optical cross-section (for example the LH2 complex) absorbs a photon that is funneled by energy transfer (ET) to the reaction centre [1-3]. Excellent candidates to mimic the natural antenna system are molecules that efficiently absorb light and are able to transfer the captured energy to other parts of the molecule. Molecules based on Zn and free-base porphyrins are examples of compounds that can be used as models for the LID complex [4]. 

The photosynthetic apparatus is found in and on membrane structures, which, in plant cells and algae, are located in chloroplasts and are called thylakoids. In bacteria the photosynthetic membrane is derived by complex invagination of the cytoplasmic membrane. The photosynthetic apparatus is made up of antennae, which contain light-harvesting pigment molecules (usually chlorophylls or bacteriochlorophylls) and photochemical reaction centres, which also contain pigments, together with the necessary enzymes and coenzymes. 

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. 

Fig. 3 Schematic model of light-harvesting compartments in photosynthetic organisms and their position with respect to the membrane and the reaction centers. RC1(2) Photosystem I(II) reaction centre. Peripheral membrane antennas Chlorosome/FMO in green sulfur and nonsulfur bacteria, phycobilisome (PBS) in cyanobacteria and rhodophytes and peridinin-chlorophyll proteins (PCP) in dyno-phytes. Integral membrane accessory antennas LH2 in purple bacteria, LHC family in all eukaryotes. Integral membrane core antennas B808-867 complex in green nonsulfur bacteria, LH1 in purple bacteria, CP43/CP47 (not shown) in cyanobacteria and all eukaryotes.

The present paper summarises our recent results relating to the location and structure of the reaction centres of photosystems I and II, with particular reference to the organisation of the chlorophyll-proteins and the transfer of excitation energy from antennae to the reaction centres. 

In contrast to LHCI, the light-harvesting chlorophyll a/b-antennae complex of photosystem II (LHCII) is the major component of the particles on the complementary protoplasmic fracture face of appressed membranes (PFs) (Simpson, 1979, Olive et al., 1979), and does not appear to be a significant component of the reaction centre EFs particles, although this is disputed. The LHCII in PFs particles is, nevertheless, in contact with the reaction centre particles and may provide a pathway for excitation energy transfer between several photosystem II reaction centres. 

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