Bacteriochlorophylls

Holzapfel W, Finkele U, Kaiser W, Oesterhelt D, Scheer H, Stilz H U and Zinth W 1989 Observation of a bacteriochlorophyll anion radical during the primary charge separation in a reaction center Chem. Rhys. Lett. 160 1-7... 

Fig. 13. Arrhenius plot of k(T) for electron transfer from cytochrome c to the special pair of bacteriochlorophylls in the reaction center of c-vinosum.

Figure 12.13 Photosynthetic pigments are used hy plants and photosynthetic bacteria to capture photons of light and for electron flow from one side of a membrane to the other side. The diagram shows two such pigments that are present in bacterial reaction centers, bacteriochlorophyll (a) and ubiquinone (b). The light-absorbing parts of the molecules are shown in yellow, attached to hydrocarbon "tails" shown in green.

This pair of chlorophyll molecules, which as we shall see accepts photons and thereby excites electrons, is close to the membrane surface on the periplasmic side. At the other side of the membrane the symmetry axis passes through the Fe atom. The remaining pigments are symmetrically arranged on each side of the symmetry axis (Figure 12.15). Two bacteriochlorophyll molecules, the accessory chlorophylls, make hydrophobic contacts with the special pair of chlorophylls on one side and with the pheophytin molecules on the other side. Both the accessory chlorophyll molecules and the pheophytin molecules are bound between transmembrane helices from both subunits in pockets lined by hydrophobic residues from the transmembrane helices (Figure 12.16). 

Figure 12.17 Computer-generated diagram of the stmcture of light-harvesting complex LH2 from Rhodopseudomonas acidophila. Nine a chains (gray) and nine p chains Bight blue) form two rings of transmembrane helices between which are bound nine carotenoids (yellow) and 27 bacteriochlorophyll molecules (red, green and dark blue). (Courtesy of M.Z. Papiz.)...

Figure 12.18 Ribbon diagram showing the a (red) and the P (blue) chains of the light-harvesting complex LH2. Each chain forms one transmembrane a helix, which contains a histidine residue that binds to the Mg atom of one bacteriochlorophyll molecule. (Adapted from G. McDermott et al.. Nature 374 517-521, 1995.)...

Figure 12.21 Schematic diagram of the relative positions of bacteriochlorophylls (green) in the photosynthetic membrane complexes LHl, LH2, and the reaction center. The special pair of bacteriochlorophyll molecules in the reaction center is located at the same level within the membrane as the periplasmic bacteriochlorophyll molecules Chi 875 in LHl and the Chi 850 in LH2. (Adapted from W. Kiihlbrandt, Structure 3 521-525, 1995.)...

Bacteriochlorophyll- Light-absorbing pigment found in green sulfur and purple sulfur bacteria. 

Transformations which alter the bacteriochlorin chromophore are quite rare. An important reaction in the structural elucidation of the bacteriochlorophylls is the dehydrogenation to chlorophyll derivatives. Thus, bacteriopyromethylpheophorbide a (1) can be smoothly dehydrogenated with 3,4,5,6-tetrachloro-l,2-benzoquinone to the corresponding chlorin 3-acetyl-pyromethylpheophorbide a (2) in high yield.1 la,b... 

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 ... 

Structures and Chemical and Spectroscopic Properties of Major Chlorophylls and Bacteriochlorophylls... 

Bacteriochlorophyll a 365, 772 Purple bacteria Grey-pink -C0CH3 -CH3 -CHjCH3 -CH CHjCOO-phytyl Single Single... 

Bacteriochlorophyll c 428, 660 Green sulfur bacteria Green -C2CH3-0H -CH3 -CH2CH3 -CH CH COO-farnesyl Single Single... 

In accordance with the structure of chlorophyll c, it is hypothesized that its biosynthesis comes from protochlorophyllide a by dehydrogenation of the side chain at C-17. Chlorophyll d should arise from chlorophyll a by oxidation of the C-3-vinyl residue, but at which stage of chlorophyll biosynthesis this occurs is unknown. The biosynthesis of bacteriochlorophylls seems to follow the same general pathway of higher plants, according to studies performed with chlorophylhde and bacterio-chlorophyU enzymes. ... 

Rudiger, W., Biosynthesis of chlorophylls a and b the last steps, in Chlorophylls and Bacteriochlorophylls Biochemistry, Biophysics, Functions and Applications, 25, Grimm, B. et al., Eds., Springer, Dordrecht, 2006, chap. 14. 

Marine roseobacters that contain bacteriochlorophyll a have been described (Oz et al. 2005), and the bacteriochlorophyll a-containing marine bacterium Porphyrobacter sanguineus was able to degrade biphenyl and dibenzofuran, though unable to use them as sole substrates for growth (Hiraishi et al. 2002). 

Hiraishi A, Y Yonemitsu, M Matsushita, YK Shin, H Kuraishi, K Kawahara (2002) Characterization of Porphybacter sanguineus sp. nov., an aerobic bacteriochlorophyll-containing bacterium capable of degrading biphenyl and dibenzofuran. Arch Microbiol 178 45-52. 

It should be noted that in many algae, chlorophyll b is replaced by chlorophyll c where the phytyl ester is replaced by a methyl ester and the side chain carrying the function is unsaturated. In a similar manner, photosynthetic bacteria contain the closely related bacteriochlorophylls rather than the normal chlorophylls [29]. 

Finally, we should note that a particularly important area of application where density functional techniques, in spite of the deficiencies noted above, are virtually without competition is provided by biochemically relevant molecules, such as enzymes or nucleic acids. The techniques discussed in this section are virtually the only quantum chemical methods which can be applied in this context due to their outstanding price/performance ratio. For example, the 13C and 15N chemical shifts in bacteriochlorophyll A have been studied by Facelli, 1998, and in another investigation the 57Fe, 13C and 170 shifts in iron porphyrin derivatives gave important clues as to the structural details of these species, as shown by McMahon et al 1998. 

Facelli, J. C., 1998, Density Functional Theory Calculations of the Structure and the l5N and l3C Chemical Shifts of Methyl Bacteriopheophorbide a and Bacteriochlorophyll a , J. Phys. Chem. B, 102, 2111. 

By this method, for example, the absolute configurations of the following compounds were established (-)-2-phenylbutyric acid,[48] (-)-hydratropic acid,[48] (+)-0-acetyl-mandelic acid,t48] (-)-2-(7/-carbazolyl)propionic acid,t48] (+)-1 -phenylethanol,[48] (-)-menthol,t48] (+)-1 -phenylethylamine,[48] and 1-alanine ethylester.[48] The determination of the absolute configuration of bacteriochlorophylles c, d and e was made possible by the esterification of the phaeophorbides by CDI to imidazolides.[49]... 

The various chlorophyll types are listed in Table 12.1. An alternate resonance form is available for all the chlorophylls listed in Table 12.1 except for bacteriochlorophyll. Chlorophyll a is found in all green plants. In most land plants and algae one also finds chlorophyll b or d. Thus a more correct statement than the one made previously would be that chlorophyll a is the photochemically active pigment in all green plants, as shown by experimenta-tion.<8)... 

Fiedor, J., L. Fiedor, J. Winkler et al. 2001. Photodynamics of the bacteriochlorophyll-carotenoid system. 1. Bacteriochlorophyll-photosensitized oxygenation of (3-carotene in acetone. Photochem. Photobiol. 74 64-71. 

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