Tetrapyrrole ring

After the protein part (globin) has been removed, the tetrapyrrole ring of heme is oxidatively cleaved between rings A and B by heme oxygenase. This reaction requires molecular oxygen and NADPH+H", and produces green biliverdin, as well as CO (carbon monoxide) and Fe which remains available for further use (see p. 286). 

Members of the vitamin B12 or corrinoid group, containing a highly substituted tetrapyrrole ring which donates four nitrogen atoms to form coordinate covalent linkages with a centrally positioned hexacoordinate cobalt ion. 

Porphyrins consist of a tetrapyrrole ring with variable side chains. The physiological compounds are the reduced forms, the porphyrinogens that do not fluoresce. They react spontaneously with oxygen in the air to the strongly fluorescent porphyrins. 

In type I porphyrins, the side chains are arranged I symmetrically, that is, for uroporphyrin I, A (acetate) I alternates with propionate (P) around the tetrapyrrole ring. 

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-... 

Metalloporphyrins are formed by the chelation of a metal ion into the porphyrin structure. This involves the incorporation of the metal ion into the center of the tetrapyrrole ring with the simultaneous displacement of two protons from the pyrrolic nitrogen atoms. Nickel is present in petroleum as Ni (II), and it sits in the plane of the four pyrrole rings comprising the porphyrin. The vanadium is present as V (IV), but exists in... 

This section will not be concerned with the detailed description of the synthetic methods leading to the appropriate precursors we will limit our attention to the crucial step of the synthesis of corrinoids, i.e. the formation of the tetrapyrrolic ring. The corrinoid macrocycle has been synthesized following two different procedures the first one involves the cyclization of a proper linear precursor, while the second involves ring contraction of a porphyrinoid structure. 

The synthetic pathways leading to tetradehydrocorrins and isobacteriochlorins are very similar and it is just by fine variations of the reaction conditions that the preparation is driven towards specific tetrapyrrolic rings. The linear precursors have been synthesized using the sulfide contraction method (also indicated by other authors as sulfur extrusion ). The corrin skeleton is formed by alkaline hydrolysis of the cyano protecting group present at the 19 position and subsequent acid catalyzed coupling of pyrroles A and D, as described in Fig. 26. 

Comparing the reactants and the products, the reactions are apparently nonredox processes. Using a spin-trapping EPR technique it was shown [114] that irradiation of the complexes leads to an alkyl radical formation (CH3 or C2Hj). The efficiency of the homolytic metal-carbon bond splitting depends on the electronic properties of the other axial ligand. The ostensibly non-redox photoinsertions are thus a product of two redox reactions. As far as the photoreactive excited state is concerned, the metal-carbon bond is either indirectly activated by a ir-nt excitation localized on the tetrapyrrole ring [112] or there is an... 

A characteristic feature of nearly all complexes discussed in this section is the presence of a transition metal as the central atom. In contrast, tetrapyrrole ring localized redox reactions are typical for non-transition metal complexes having redox stable central atoms (e.g. Mg(II), Zn(II), Al(III)). 

The third class (c) of photoredox processes, namely electron transfer from the coordinated tetrapyrrole ring (its jr-system) to the central atom or vice versa, comprises a few cases, represented in Table 4 by the neutral complex Sn(Pc)2... 

The course of the photoformation of the tetrapyrrole ring-localized radicals (group 1 of the above classification) can be expressed by four equations... 

Back electron-transfer processes of n-anion and n-cation radicals with reversible electron donors or acceptors (e.g. aquated Fe3+, [Fe(CN)6]3, quinones) are fast reactions realized in nano- or picosecond time scale. In cases when irreversible redox partners are used (e.g. S20, CBr4, CC14, EDTA) tetrapyrrole ring ring localized radicals dimerize [193], decompose [212], undergo disproportionation [215] or other stabilization reactions. Photoformation of stable products will be discussed later. 

Table 5. Tetrapyrrole ring localized electron-transfer and atom-transfer photoredox reactions ...

The third group (class 3 of the above classification) of the tetrapyrrole ring localized photoreactions can be exemplified [231] by the formation of oxonia-chlorins from chlorophyll derivatives (X = Cl, CF3COO)... 

---