Photosynthesis bacteriochlorophylls

Flowever, many photosynthetic bacteria, such as purple sulfur and green sulfur bacteria contain special bacteriochlorophyll compounds (not chlorophyll a) and carry out anoxygenic photosynthesis without producing oxygen ... 

Analogies of Rare Earth Porphyrin Doubledeckers with the Special Pair of Bacteriochlorophylls in Bacterial Photosynthesis... 

Chlorins (2) are undoubtedly the most important dihydroporphyrins, since the chlorin chromophore is found in chlorophylls and some bacteriochlorophylls and, as the magnesium complex, is the catalyst in photosynthesis. The method of choice for formation of trans-chlorins involves reduction of iron porphyrins with sodium in boiling isopentyl alcohol (57JCS3461), but methods involving photochemical reduction of tin(IV) porphyrins, isomerization of phlorins, reduction of metalloporphyrins with sodium anthracenide followed by protonolysis, heating with sodium ethoxide, and photoreductions of zinc(II) porphyrins in the presence of ascorbic acid have also been employed. The best method for formation of c/s-chlorins (note that all natural chlorophylls possess the trans arrangement) appears to... 

The initial or primary processes of photosynthesis occur in the reaction centers in which chlorophyll or bacteriochlorophyll absorbs a photon.3183 Then, the chlorophyll, in its singlet excited state (Chi ), donates an electron to some acceptor A to form a radical A-and to leave an oxidized chlorophyll Chl+ radical (Eq. 23-31). 

Reaction centers of purple bacteria. The exact composition varies, but the properties of reaction centers from several genera of purple bacteria are similar. In Rhodopseudomonas viridis there are three peptide chains designated H, M, and L (for heavy, medium and light) with molecular masses of 33,28, and 24 kDa, respectively. Together with a 38-kDa tetraheme cytochrome (which is absent from isolated reaction centers of other species) they form a 1 1 1 1 complex. This constitutes reaction center P870. The three-dimensional structure of this entire complex has been determined to 0.23-nm resolution288 319 323 (Fig. 23-31). In addition to the 1182 amino acid residues there are four molecules of bacteriochlorophyll (BChl), two of bacteriopheophytin (BPh), a molecule of menaquinone-9, an atom of nonheme iron, and four molecules of heme in the c type cytochrome. In 1984, when the structure was determined by Deisenhofer and Michel, this was the largest and most complex object whose atomic structure had been described. It was also one of the first known structures for a membrane protein. The accomplishment spurred an enormous rush of new photosynthesis research, only a tiny fraction of which can be mentioned here. 

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. 

Later, temperature independence was also observed for the efficiency of reducing the cation of bacteriochlorophyll P890 with the reduced primary acceptor A in the range T = 1.5-80 K [33,34]. This reaction is the inverse of the process of charge separation during photosynthesis. 

A depending on the size of the lanthanide metals. Delocalization of electron density on four equivalent nitrogen atoms causes elongation of the Ln-N bonds at about 0.10-0.15 A compared to silylamides. The close proximity of the macrocyclic 7i-systems in sandwich complexes proved to be useful as structural and spectroscopic models for the bacteriochlorophyll [Mg(Bchl)]2, the special pair in the reaction center of bacterial photosynthesis [211,212]. The distance between the pyrrole rings in [Mg(Bchl)]2 is about 3 A. 

The primary photochemical processes of photosynthesis take place within membrane bound complexes of pigments and protein, reaction centers (Shuvalov and Krasnovsky, 1981 Deisenhofer et al., 1986, Rees et al., 1989 Norris and Shiffer, 1990 Kirmaier and Holten, 1991 Feher et al, 1992 Stowell et al, 1997). One mole of a reaction center from different bacteria contains 4 moles of bacteriochlorophyl (Bchl), 2 moles of bacteriopheophytin (Bph), two moles of ubiquinone (Q) and a non-heme Fe atom. In RC from Rhodobacter speroides, a total of 11 hydrophobic a-helixes create a framework that organizes the cofactor and a hydrophobic band approximately 35 A wide. RC from Rhodopseudomonus viridus has three polypeptides having pronounced hydrophobic properties. The molecular mass of the polypeptides are 37 571 (L), 35902 (M) and 28902 (H). The H subunit does not carry pigments but it is sufficient for the photochemical activity. The protein components of reaction centers from different-bacteria are similar. 

Arlt, T., Schmidt, S., Kaiser, W., Lauterwasser, C., Meyer, M., Scheer, H., and Zinth, W., 1993, The accessory bacteriochlorophyll A real electron carrier in primary photosynthesis. Proc. Natl. Acad. Sci. USA, 90 11757911761. 

Bixon, M., Michel Beyerle, M. E., and Jortner, J., 1988, Formation dynamics, decay kinetics and singlet-triplet splitting of the (bacteriochlorophyll dimer)-positive (bacteriopheophytin)-negative radical pair in bacterial photosynthesis. Isr. J. Chem., 28 1559168. 

In a book written by specialists in the various areas of photosynthesis research, there are bound to be some overlaps and some gaps. One area that may not have been adequately covered, althovigh its impact can be discerned in various chapters, is the wealth of information regarding energy and electron transfer and structure derived from studies of prompt and delayed fluorescence of chlorophyll and bacteriochlorophyll. However, the reader interested in this area should find enough information in this book for further literature on the subject. 

---