Porphyrin octaacetic acid

Table 44. Transition temperatures for the esters of porphyrin octaacetic acid (see Scheme 43 for structures).

The enzymatic conversion of a-(aminomethyl)pyrroles is also used by nature to produce porphyrinogens like uroporphyrinogen III (see introduction, compound 8), which is the key building block in the biosynthesis of all known porphinoid natural products. This biomimetic method is a powerful tool for the synthesis of different porphyrins, e.g. for the preparation of JV,Af, V ,Ar"-tetramethylporphyrin-2,3,7,8,12,13,17,18-octaacetic acid dibromide 12.36... 

Figure 4.14 Octaacetic acid porphyrin 17 is reversibly adsorbed on vesicle surfaces below pH 4.5. At pH 5, the porphyrin dissolves in water. It is now protonated at the pyrrolenine nitrogen atom, whereas the acetic acid side chains are half-deprotonated. At higher pH, the two-banded spectrum of the N-diprotonated porphyrin dication is again replaced by a four-banded spectrum of the neutral porphyrin base.

A similar synthesis to give octaacetic acid porphyrin is much less productive (Chiusoli et al., 1989). Highly substituted porphyrins with eight P-pyrrolic ethyl groups and four phenyl groups in the meso positions have been obtained similarly. The latter, highly overcrowded porphyrins (see chlorophyll synthesis) are totally stable (Evans et al., 1977). 

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