Tetrapyrroles chlorophylls pigments, tetrapyrrole

Chlorophylls are macrocyclic tetrapyrrole derivatives containing in their natural form a chelated magnesium ion. The basic structure of chlorophyll pigments is shown in Fig. 2.122. 

Tetrapyrroles The red heme and green chlorophyll pigments, so important in the energy-... 

Cyanobacteria and red algae employ phycobilins such as phycoerythrobilin and phycocyanobilin (Fig. 19-40b) as their light-harvesting pigments. These open-chain tetrapyrroles have the extended polyene system found in chlorophylls, but not their cyclic structure or... 

In chemical terms, chlorophylls are porphyrins, such as the pigments of blood (hemoglobin) and muscles (myoglobin). The tetrapyrrole ring of chlorophylls is formed by four pyrrole residues joined together by methine groups the four nitrogen atoms are coordinated with a cen-... 

E. McDonald, Biosynthesis of the pigments of life Formation of the macrocycle. Nature 285 17, 1980. This paper discusses the steps in tetrapyrrole biosynthesis and the pathways diverting this nucleus to chlorophylls, hemes, cytochromes, and other macrocyclic pigments. 

While the following discussion centres on the Mg2+-containing pigments, it should be noted that there are two additional major categories of non-chlorophyll-type pigments — the carotenoids and the phycobilins. The latter compounds are open-chain tetrapyrroles. 

Chlorophylls compounds consisting of cyclic tetrapyrroles coordinated around a Mg-chelated complex, and in the case of chlorophyll-a, represents a dominant pigment used to estimate algal biomass in aquatic ecosystems. 

Similar reactions occur in red algae and cyanobacteria but because the full spectrum of sunlight does not always penetrate very far below the surface of the sea they make use of different pigments. These may be structurally similar to those used by plants, and are therefore classed as bacteriochlorophylls, or belong to the structurally distinct phycobilin class of compounds. Unlike chlorophyll, which has a cyclic tetrapyrrole structure containing magnesium, the phycobilins are acyclic tetrapyrroles with similarities to the breakdown product of the haem ring, bilirubin. 

The chlorophylls are tetrapyrrole pigments in which the porphyrin ring is in the dihydro form and the central metal atom is magnesium. There are two chlorophylls, a and b, which occur together in a ratio of about 1 25. Chlorophyll b differs from chlorophyll a in that the methyl group on carbon 3 is replaced with an aldehyde group. The structural for-... 

The substances from the tetrapyrrole class are remarkable both through the very deep colour and through the extreme stability to chemical and physical agents, which naturally go together according to the resonance theory. To this class belong the coloured constituent of the blood haemin and of plants chlorophyll, all derivatives of porphyrin and also the artificial pigments discovered by Linstead, the phthalocya-... 

Interestingly, only two types of pigments appear to be involved in all known photochemical reactions in plants and algae. These are the carotenoids and the tetrapyrroles, the latter class including the chlorophylls, the phycobilins, and phytochrome. The maximum absorption coefficients for the most intense absorption bands are slightly over 104 m2 mol-1 in each case, with 7 to 12 double bonds in the main conjugated system. Cytochromes, which are involved in the electron transport reactions in chloroplasts and mitochondria, are also tetrapyrroles (considered later in this chapter). Table 5-1 summarizes the relative frequency of the main types of photosynthetic pigments. 

Hundreds of distinct pigments are found in various photosynthetic organisms. They can be grouped into three main classes chlorophylls (including bacteriochloro-phylls), carotenoids, and bilins. The chlorophylls are cyclic tetrapyrroles with a central Mg. They are found in both antenna and reaction center complexes of all types of photosynthetic... 

The chemical nature of P-700 is difficult to establish. The absorption bleachings correspond approximately to the peaks of Chi a. which appears to be virtually the only tetrapyrrolic pigment in purified PS I particles. It has thus been assumed that P-700 is Chi a bound to a protein. A few recent results, however, may require this hypothesis to be refined. An examination of the spectroscopic and redox properties of P-700 led Wasielewski et al. [13] to propose that P-700 could be the enol form of Chi a where enolization was of the keto ester on ring V. This has not been confirmed by chemical extraction. Extraction experiments, however, have evidenced two other chlorophyll derivatives. A species named Chl-RC I has been isolated from PS I, at a nearly 1/1 molar ratio with P-700 and its structure shown to be a chlorinated derivative. It is not yet clear whether Chl-RC I is a native constituent of PS I or an extraction artefact. Chl-RC I has not been obtained in a recent chemical analysis by HPLC, which instead revealed two Chi a per P-700 [14]. 

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