And where porphyrins go to

Chlorophyll in plants is also continually turned over, and its creation and destruction seems to depend on the amount of light available. Try camping in a field for a week and then leave. The grass where your tent has been standing will be yellow from lack of chlorophyll, compared with the grass that has been continually exposed to the ambient light. 

Living systems know how to synthesise porphyrins. They also know how to break them down. What are the mechanisms of this breakdown, and what are the compounds they are broken down into We shall see how such a question becomes, at least as far as the animal kingdom is concerned, increasingly scatological. 

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The Group 13 porphyrins usually show similar electrochemical behavior upon going down the Periodic Table, although differences do exist in the stability of the oxidized and reduced forms of the compounds upon going from Al(III) to Tl(III). All complexes of the type (P)MX, where M = Al(III) [360], Ga(III) [355], In(III) [356], or Tl(III) [353], and P = TPP or OEP are oxidized at the porphyrin macrocycle to give ranion radicals and dianions upon reduction. The Tl(III) complexes undergo what appears in many cases to be a metal-centered two-electron reduction prior to demetallation [339],... 

Starting from the iron(III) complex, three reversible waves are observed in an aprotic solvent such as DMF (Figure 4.5a). The last one corresponds to generation of the iron(0) porphyrin. The latter does react with CO2, as attested to by the fact that the Fe(I)/Fe(0) wave becomes irreversible under 1 atm of C02 (Figure 4.5b). However, the reaction is sluggish and the current does not go beyond a two-electron-per-molecule stoichiometry. This is confirmed by preparative-sc ale electrolysis where catalyst turnover numbers are found not to be larger than a few units. 

Ultimately, all porphyrin syntheses start from pyrroles (which themselves need to be made), so that a special section with this title might appear pointless. However, this section really deals with those porphyrins where the number of chemical manipulations that the pyrrole has to go through prior to porphyrin formation, is minimal the porphyrin is synthesised directly from the final pyrrole. This means that such synthetic routes are usually confined to those porphyrins with a symmetrical arrangement of peripheral substituents, e.g., 5,10,15,20-meso-tetrakis(aryl)porph)rrin (e.g. TPP) and 2,3,7,8,12,13,17,18-octa-substituted porphyrins (e.g. OEP). 

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