Tetrapyrrole pigments, open

Figure 24-24 The degradation of heme and the formation of open tetrapyrrole pigments.

The analytical situation becomes much more complicated with open-chain tetrapyrrole pigments, which occur as bile pigments in nature (Falk, 1989). Their conformation is by no means limited to planar or ruffled-planar, but all kinds of cw-tran -configured diastereomers are known and produce extremely differentiated and often complicated UV/vis spectroscopic shifts and extreme changes in solubility, caused by enormous changes in neighbor group interactions. The latter are best analyzed by NOE (nuclear Overhauser effects) (Sanders, 1996). 

Bile Pigments. The oxidative degradation of heme yields open-chain tetrapyrrole as a waste product in humans and other higher animals. The yellow color of the skin in jaundice victims is caused by the presence of biluubin [635-65-4] (32, R = (CH2)2COOH). 

The phycobiliproteins are accessory photosynthetic pigments aggregated in cells as phycobilisomes that are attached to the thylakoid membrane of the chloroplast. The red phycobiliproteins (phycoerythrin) and the blue phycobiliprotein (phycocy-anin) are soluble in water and can serve as natural colorants in foods, cosmetics, and pharmaceuticals. Chemically, the phycobiliproteins are built from chro-mophores — bilins — that are open-chain tetrapyrroles covalently linked via thio-ether bonds to an apoprotein. ... 

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... 

A third class of accessory pigment of more limited distribution are the open tetrapyrroles, sometimes called "plant bile pigments" because of their relationship to the pigments of animal bile (Fig. 24-24). 

The open tetrapyrroles of algae and the chro-mophore of phytochrome (Chapter 23) are all derived from phycoerythrobilin, which is formed from biliverdin, as indicated in Fig. 24-24. The animal bile pigments have not been found in prokaryotes. How-... 

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. 

Carotenoids are found in all native photosynfhetic organisms. They serve a dual function, as both accessory antenna pigment and also are essential in photoprotection of photosynfhetic systems from the effects of excess light, especially in the presence of oxygen. Bilins are open-chain tetrapyrroles that are present in antenna complexes called phycobilisomes. These complexes are found in cyanobacteria and red algae. Structures of representative carotenoid pigments are shown in Figure 3. 

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