Tetrapyrrole pigments, open structures

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... 

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. 

Structure.— Mild oxidation converts bilirubin into the green pigment, biliverdin more powerful oxidants resolve it into four P3TTole units. Bilirubin does not combine with metals to form porphyrans, thus indicating that the formula is an open chain of pyrrole units, not a closed tetrapyrrole or porph3u in. 

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