Photosynthetic organisms, chlorophylls

Structural studies related to photosynthesis a model for chlorophyll aggregates in photosynthetic organisms. C. E. Strouse, Prog. Inorg. Chem., 1976, 21,159-177 (31). 

This chapter brings together information concerning structural features, spectral characteristics, distributions, and functions of major chlorophylls in photosynthetic organisms. Other topics discussed include biosynthesis and degradation in senescent plants and ripening fruits and potential biological properties of chlorophylls. 

Esterification increases the lipophilic character of the pigments that has been recogiuzed as an important factor for interactions with the peptide chains of proteins. The hydrolysis of this side chain results in chlorophyllides and the concomitant removal of the Mg + ion in pheophorbides. Only a Umited number of natural chlorophylls in plants and photosynthetic organisms has been described and is well... 

Thus far we have used the word chlorophyll as if this term related to a unique chemical species. In actuality there are a number of structurally related molecules present in photosynthetic organisms which are collectively referred to as chlorophyll. The general chlorophyll structure is ... 

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]. 

With the exception of certain archaebacteria, all known photosynthetic organisms take advantage of the photochemical reactivity of one or another type of chlorophyll 7fig. 15.4). Chlorophylls resemble hemes but differ from them in four major respects ... 

Chlorophylls a and b contain the alcohol phytol bacteriochlorophylls a and b have either phytol or geranylger-aniol, depending on the species of bacteria. Photosynthetic organisms also contain small amounts of pheophytins or bacteriopheophytins, which are the same as the corresponding chlorophylls or bacteriochlorophylls except that two hydrogens replace the Mg2+ (see fig. 15.4). We will see that... 

Photosynthetic organisms take advantage of the fact that chlorophyll becomes a strong reductant when it is excited with light. Photooxidation of chlorophyll or bacteriochlorophyll occurs in pigment-protein reaction centers. Chloroplasts have two types of reaction cen-... 

Fig. 3 Schematic model of light-harvesting compartments in photosynthetic organisms and their position with respect to the membrane and the reaction centers. RC1(2) Photosystem I(II) reaction centre. Peripheral membrane antennas Chlorosome/FMO in green sulfur and nonsulfur bacteria, phycobilisome (PBS) in cyanobacteria and rhodophytes and peridinin-chlorophyll proteins (PCP) in dyno-phytes. Integral membrane accessory antennas LH2 in purple bacteria, LHC family in all eukaryotes. Integral membrane core antennas B808-867 complex in green nonsulfur bacteria, LH1 in purple bacteria, CP43/CP47 (not shown) in cyanobacteria and all eukaryotes.

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