Tetrapyrrole classes 2.1.2.1 -Tetrapyrrolic macrocycles

As a class, metal-ion derivatives of tetrapyrrole macrocyclic rings, such as the corrins or porphyrins (see Chapter 1 for the parent ring structures), are of major biological importance. 

Chls and all tetrapyrroles are heteroaromatic compounds and the aromatic character of the underlying tetrapyrrole moiety and the reactivity of the functional groups in the side chains govern their chemistry. Three different classes of tetrapyrroles, differentiated by their oxidation level, occur in nature porphyrins (11, e.g. hemes), chlorins (12, e.g. chls) and bacteriochlorins (13, e.g. bchls). As a cyclic tetrapyrrole with a fused five-membered ring, the overall reactivity of chi is that of a standard phytochlorin 7. Such compounds are capable of coordinating almost any known metal with the core nitrogen atoms. Together with the conformational flexibility of the macrocycle and the variability of its side chains, this accounts for their unique role in photosynthesis and applications ... 

Another interesting class of anion receptors based upon protonated nitrogen atoms are the expanded porphyrin macrocycles such as 4.17 (diprotonated sapphyrin) and compound 4.18. The tetrapyrrole porphyrin macrocycles are excellent hosts for metal cations such as Fe2+ and Mg2+ (e.g. haemoglobin and chlorophylls, Sections 2.3-2.5) however, their cavity dimensions are too small to accommodate anions. Conversely, expanded porphyrins such as 4.17 comprising five or more pyrrole residues present a rigid macrocyclic cavity about 5.5A in diameter, in which (particularly when protonated) the Nff... 

A class of novel sulfur-bridge pyrrole macrocycles was discovered <05CC2122>. For example, treatment of dipyrromethane 85 with disulfur dichloride led to the formation of disulfide linked tetrapyrrole 86. 

Macrocyclic tetrapyrrole compounds such as heme (iron), chlorophyll (magnesium), siroheme (iron), and E12 (cobalt) contain specific metal ions at the center of their tetrapyrrole rings [17]. Metal ion chelatases can be divided into two classes based on their structural architecture. Class 1 chelatases are heteromultimeric enzymes that require three gene products for efficient catalysis [18] of the ATP-dependent chelation reaction [19]. Enzymes in this class include chlorophyll and bacteriochlorophyll magnesium chelatases [18] and aerobic cobalt chelatase (CobNST) [20]. 

D.K.R Ng, J. Jiang, K. Kasuga and K. Machida conclude this volume with an overview of rare-earth and actinide half-sandwich tetrapyrrole complexes. When tetrapyrrole molecules, such as porphyrins or phthalocyanines, are reacted with the rare earths and actinides, they are split in half, forming the half-sandwich complexes because the metal atoms are larger than the core size of the macrocyclic ligands. They also can form sandwich-type complexes in which the metal centers are sandwiched between the macrocycles. However, this chapter is devoted to the former class of compounds. Ng and co-workers discuss the synthesis, structure, and spectroscopic and electrochemical properties of half-sandwich complexes of porphyrins and phthalocyanines. The authors... 

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