Chlorophyll bacteriochlorophylls

When the components of the PS II reaction centre are drawn on a redox scale and compared in this way to those of the purple bacterial reaction centre, a remarkable similarity can be seen between the electron acceptors in each system (Fig. 4). The chemical natures of these components are extremely similar, being made up of a complex of two quinones, an iron atom and a pheophytin (a bacteriopheo-phytin in bacteria). The donor side of PS II in the redox scheme is, however, not comparable to that in bacteria. P-680 may appear to be structurally similar to P-870 in bacteria in that it is made up of chlorophyll (bacteriochlorophyll in bacteria) and that is acts as the primary electron donor however, the P-680/P-680+ redox couple is approximately 600-800 mV more oxidizing than the equivalent bacterial redox couple P-870/P-870, = +450 mV). In addition, PS II has an array of high-potential components which make up the 02-evolving enzyme and which are clearly unique to that system. 

The main focus of our work is photosynthetic pigments (chlorophylls, bacteriochlorophylls, and carotenoids). The important absorptions cover the spectral range from 300 to around 850 nm. Single HPLC runs last typically 30 min and use gradient elution, and 10-15 compounds are not uncommon. No dedicated machine was available at the time when the system was installed we therefore use a general purpose spectrometer with a semimicro flow cell (30 fil) and without beam focusing. Since the... 

The green sulfur bacteria are the "classical" green bacteria. They contain relatively large amounts of chlorobium chlorophyll (bacteriochlorophyll c, d or e), in addition to BChl a. The latter pigment is mainly contained in the cytoplasmic membrane, whereas most of the BChl c is situated in the chlorosomes, oblong bodies of several hundred A diameter that are bound to the cytoplasmic membrane. Green sulfur bacteria are strict anaerobes which are found in fresh water as well as in marine habitats. 

Keywords Self-assembly, chlorophyll, bacteriochlorophyll, antenna complex, chlorosome, biomimetic models, porphyrinoids. 

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). 

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... 

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. 

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]. 

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)... 

For convenience of discussion, a schematic diagram of bacterial photosynthetic RC is shown in Fig. 1 [29]. Conventionally, P is used to represent the special pair, which consists of two bacterial chlorophylls separated by 3 A, and B and H are used to denote the bacteriochlorophyll and bacteriopheophytin, respectively. The RC is embedded in a protein environment that comprise L and M branches. The initial electron transfer (ET) usually occurs from P to Hl along the L branch in 1—4 picoseconds (ps) and exhibits the inverse temperature dependence that is, the lower the temperature, the faster the ET. It should be noted that the distance between P and Hl is about 15 A [53-55]. 

Once the special pair has absorbed a photon of solar energy, the excited electron is rapidly removed from the vicinity of the reaction centre to prevent any back reactions. The path it takes is as follows within 3 ps (3 X 10 12 s) it has passed to the bacteriopheophytin (a chlorophyll molecule that has two protons instead of Mg2+ at its centre), without apparently becoming closely associated with the nearby accessory bacteriochlorophyll molecule. Some 200 ps later it is transferred to the quinone. Within the next 100 ps the special pair has been reduced (by electrons coming from an electron transport chain that terminates with the cytochrome situated just above it), eliminating the positive charge, while the excited electron migrates to a second quinone molecule. 

Absorption spectra of chlorophyll a and bacteriochlorophyll a. (Modified from Archer and Bolton, in preparation)... 

PERCENTAGE COMPOSITION OF CHLOROPHYLL-A AND BACTERIOCHLOROPHYLL-A DERIVATIVES IN TWO SURFICIAL FLOCS AND A 1.2 M CORE FROM WHIPRAY BASIN, SITE NO. 2... 

R.L. Airs and B.J. Keely, A high resolution study of the chlorophyll and bacteriochlorophyll pigment distributions in a calcite/gypsum microbial mat. Org. Geochem. 34 (2003) 539-551. 

Tetrapyrroles Divinyl chlorophylls a and b Monovinyl chlorophyll b Chlorophylls Cp c2 and c3 Bacteriochlorophyll a Prochlorococcus spp. Chlorophytes, prasinophytes Chromophyte microalgae Anoxygenic photosynthetic bacteria... 

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