Bacteriochlorophyll structure

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.)...

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... 

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

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. ... 

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. 

Reaction centers of bacteria contain polypeptides, bacteriochlorophylls, bacteriopheo-phytins, two quinines, and nonheme iron atom. In some bacterial species, both the quinones are ubiquinones, whereas in some others one of the quinones is menaquinone [37,39]. Depending on the bacterial species chloroplasts contain plastoquinone and phyl-loquinone. Structures of ubiquinone, menaquinone, and phylloquinone are provided in Figures 7.12 through 7.14, respectively. 

Fig. 2.136. Molecular structures of (a) naturally occurring magnesium chlorin, bacteriochlorophylls-d (Bchls-d), (b) their synthetic analogues, methyl bacteriopheophorbides-d (MBPhes-d, and (c) 3 -epimeric zinc complexes (Zn-MBPhes-d). Reprinted with permission from S. Sasaki et al. [307],...

Figure 23-26 (A), (B) Arrangement of bacteriochlorophyll chromophores in the cyclic LH2 array of Rhodopseudomonas acidophila. The B850 subunits are gray while the B800 subunits are black. (C) Fluorescence-excitation spectra. Top trace, for an ensemble of LH2 complexes, other traces, for several individual LH2 complexes at 1.2K. Fine structure is evident for the B800 but not for the B850 chromophores. From van Oijen et al.299 with permission.

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. 

Spectral measurements suggesting exciton splitting were among early observations that led to the conclusion that the bacteriochlorophyll involved in the initial photochemical process exists as a dimer or special pair (Fig. 23-31),319/324 a conclusion verified by the structure determination. The special pair of BChl b lies in the center of the helical bundle that is embedded in the membrane. Nearly perpendicular to the rings of the special pair are two more molecules of BChl b. The central magnesium atoms of all four bacteriochlorophylls are held by imidazole groups of histidine side chains.319 325 Below the chlorophylls are... 

The crystal structure of reaction centers from R. viridis was determined by Hartmut Michel, Johann Deisenhofer, Robert Huber, and their colleagues in 1984. This was the first high-resolution crystal structure to be obtained for an integral membrane protein. Reaction centers from another species, Rhodobacter sphaeroides, subsequently proved to have a similar structure. In both species, the bacteriochlorophyll and bacteriopheophytin, the iron atom and the quinones are all on two of the polypeptides, which are folded into a series of a helices that pass back and forth across the cell membrane (fig. 15.1 la). The third polypeptide resides largely on the cytoplasmic side of the membrane, but it also has one transmembrane a helix. The cytochrome subunit of the reaction center in R. viridis sits on the external (periplasmic) surface of the membrane. 

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. 

Chlorophyll is a magnesium tetrapyrrole of the chlorin class that has a unique alicyclic ring. Figure 19 shows the structure of chlorophyll, together with parts of the structure of chlorophyll b and bacteriochlorophyll a. The fifth and sixth coordination positions may be filled by solvent water, and also offer the possibility of dimer or higher aggregate formation. 

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