Open-Chain Tetrapyrroles

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... 

In terms of synthetic strategy, approaches to porphyrins from open-chain tetrapyrroles are the only truly general routes. The principle is to construct, in a stepwise manner, an open-chain tetrapyrrole bearing a pre-determined arrangement of peripheral substituents. Cyclization to produce a porphyrin, or an immediate precursor, under mild conditions which do not cause redistribution of the pyrrole rings, should be accomplished after full characterization of the open-chain intermediate. 

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. 

Once the porphyrin ring is open, the metal is deposited on the catalyst. Open chain tetrapyrrolic structures (M-biliverdenates) are unstable under catalytic hydrogenation conditions (Subramanian and Fuhrhop, 1978). The ease of this sequence with Ni-etioporphyrin (Ni-etio) is apparent by the lack of stable metal intermediates other than the chlorin (M-PH2). 

Recently a major improvement of this latter procedure had been reported [17]. An efficient route for the preparation of both symmetrical and asymmetrical 1,19-dibromo-dihydrobilins has been developed. The cyclization of these open chain tetrapyrroles has been investigated under a variety of conditions and the best results for the preparation of corroles have been obtained in methanol. 

Two major strategies have been considered in order to obtain metallocorrolates the first one consists of the reaction of the preformed macrocycle with metal ions, while the second involves the oxidative, base induced cycliz-ation of the open-chain tetrapyrrolic precursor dihydrobilin (1,19-dideoxybiladiene-a,c) in buffered alcoholic solution in the presence of metal salts. By proper tuning of the experimental conditions (reaction time, solvent, metal carrier) several metal complexes have been obtained and characterized. 

Figure 5 Open-chain tetrapyrroles used in porphyrin synthesis ...

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. 

This is a general term for porphyrins and bilane derivatives (cyclic and open-chain tetrapyrroles, respectively), centered aronnd a nnmerically limited nnmber of natnral products. Other lUPAC parent skeletons are phorbine, corrin, chlorin, and phthalocyanine (synthetic). See Pure Appl. Chem., 59, 779-782,1987, for the nnmbering of these skeletons. 

Anodic oxidation in DMF of the open-chain tetrapyrrole, 1, 8 -dimethyl-a,c-bila-diene, yielded porphyrin one methyl group is lost from a cyclic intermediate with four conjugated pyrrole subunits by attack of a nucleophile [32]. 

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