Photosynthesis reaction centres

From SCRP spectra one can always identify the sign of the exchange or dipolar interaction by direct exammation of the phase of the polarization. Often it is possible to quantify the absolute magnitude of D or J by computer simulation. The shape of SCRP spectra are very sensitive to dynamics, so temperature and viscosity dependencies are infonnative when knowledge of relaxation rates of competition between RPM and SCRP mechanisms is desired. Much use of SCRP theory has been made in the field of photosynthesis, where stnicture/fiinction relationships in reaction centres have been connected to their spin physics in considerable detail [, Mj. 

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. 

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]. 

These bacteria cannot in general oxidize water and must live on more readily oxidizable substrates such as hydrogen sulfide. The reaction centre for photosynthesis is a vesicle of some 600 A diameter, called the chromato-phore . This vesicle contains a protein of molecular weight around 70 kDa, four molecules of bacteriochlorophyll and two molecules of bacteriopheophy-tin (replacing the central Mg2+ atom by two H+ atoms), an atom Fe2+ in the form of ferrocytochrome, plus two quinones as electron acceptors, one of which may also be associated with an Fe2+. Two of the bacteriochlorophylls form a dimer which acts as the energy trap (this is similar to excimer formation). A molecule of bacteriopheophytin acts as the primary electron acceptor, then the electron is handed over in turn to the two quinones while the positive hole migrates to the ferrocytochrome, as shown in Figure 5.7. The detailed description of this simple photosynthetic system by means of X-ray diffraction has been a landmark in this field in recent years. 

Pairwise distribution can occur, for example, in the case of recombination of the trapped electrons with the parent counterions or with the products of their transformation provided that the two reagents are produced in sufficiently low concentration (see Chap. 6). The pairwise distribution is also characteristic of the recombination processes in the reaction centre of photosynthesis (see Chap. 8). 

The present chapter discusses briefly modern ideas on the mechanisms of electron transfer during photosynthesis and the experimental data pointing to the important part played by electron tunneling reactions in the operation of the reaction centres of photosynthesizing systems. 

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. 

The key feature of photosynthesis is the ability to carry charge spatially away from an excited state reaction centre before the usually highly efficient and biochemically useless recombination can take place. In photosynthetic systems, charge separation occurs about 108 times faster than recombination, a ratio that is impossible to reach in normal chemical reactions. This phenomenon is achieved by the spatial anchoring of the components at particular orientations to one another within a non polar region of the membrane anchored protein, thus preventing free diffusion and allowing a vectorial uphill chemical reaction. 

Synthesis of PP-L-A molecules consisting of bisporphyrin PP linked to a pyromellitimide rather than quinone acceptor A was also reported in [162], For the cofadal bisporphyrin strong quenching of fluorescence was found, while for the side-by-side bisporphyrin relatively weak quenching was observed. Fluorescence quenching data are supported by the direct ps laser studies of PET in PP-L-A molecules with cofacial and side-by-side bisporphyrin. These results show that the proximity of PP and Q is not sufficient for high efficiency of PET. Other factors, such as appropriate geometry of PP play an important role for efficient PET. Note that cofacial bisporphyrin models the special pair electron donor in the reaction centre of photosynthesis. 

But one may go further and say that these are the primary steps, the only ones which are true photochemistry and which are, therefore, unique to photosynthesis. The pigment-protein complexes responsible for light harvesting and for the electron transfer sequences within the reaction centres of Photosy stem 1 (PS1) and Photosy stem 2 (PS2) are the heart of the photosynthetic process. 

---