Aromatic porphyrin

Reflecting their aromaticity, porphyrins usually give strong M(Por) + and M(Por) 2+ mass peaks, though some fragmentations of the side chains and loss of the metal from labile metalloporphyrins are often observed.2 The ionization energy of M(TPP) (M = H2, Cu) was reported to be 8.0eV by mass spectrometry. MCD, CD, resonance Raman, Zeeman, ENDOR, ESR and Mossbauer spectroscopies have been reviewed in detail.2... 

Fio. 3. Nonporphyrinic metals (a) vanadyl hydroporphyrin, (b) vanadyl arylporphyrin (highly aromatic porphyrin), (c) porphyrin-degraded product (bilirubin). (Yen, 1975). 

Finally, the iron compound haem, part of the haemoglobin molecule we use to carry oxygen around in our bloodstream. It contains the aromatic porphyrin ring system with its eighteen elec-trons arranged in annulene style. Chlorophyll, mentioned earlier in this chapter, has a similar aromatic ring system. 

Thianthrene radical cation is also an excellent one-electron oxidant of iron porphyrin complexes. Such oxidation of Fem(0Cl03)(TPP), where TPP is meso-tetraphenylporphyrin, provides the corresponding porphyrin 7r-cation radical analytically pure [32]. Similar oxidation of the AT-methyl porphyrin complex (N-MeTPP)FenCl, where AT-MeTPP is AT-methyl-meso-tetraphenylporphyrin, afforded [N-MeTPPFemCl]+ which was not further oxidized [33]. Thus thianthrene radical cation selectively oxidized the aromatic porphyrin ligand in one case and the metal center in the other. Ligand oxidation at a phenolic moiety has also been reported [34] on treatment of a 1,4,7-triazacyclononane appended with one or two phenol moieties ligated to Cu(II) complex with thianthrene radical cation. 

Hydrogenation of a 3,4-pyrrolic double bond produces chlorins, the mother chromophores of chlorophyll, and the whole aromatic porphyrin spectrum changes to a polyene-type chlorin spectrum. It consists mainly of two bands of comparable intensity at 400 and 650 nm and several smaller bands in between (see Fig. 6.2.9) (Cox et al., 1974 Smith, 1975 Gouterman, 1978). 

Figure 1.11. Formation of highly-fused aromatic porphyrin system under high temperature.

Several reports have called into question the correlation between sterically enforced, out-of-plane deformation and the electronic properties, redox potential and spectroscopic signatures. Based on UV-visible spectra, structural analysis, and TD-DFT calculations, it has been suggested that the shifts in UV-visible transitions result from electronic effects of the peripheral substituents on the aromatic porphyrin core. These effects drive a bond-alternating rearrangement of the porphyrin core atoms called in-plane nuclear reorganization. ... 

In compound 2", the porphyrin incorporates a trivalent gold which was selected for two reasons (i) it forms very stable porphyrin complexes and will thus not be lost during the synthesis of the second porphyrin nucleus [13] and (ii) because of its strong electropositive character, it confers to the aromatic porphyrin ring to which it is complexed a remarkable electron-accepting abihty with a resulting very accessible reduction potential [41-44[. The reactions leading to the transition metal-complexed [2]- and [3[rotax-anes 8 and respectively, are indicated in Fig. 5 [45[. 

One obvious place to control porphyrin biosynthesis is to control the synthesis of 5-AL. This is the only step which requires a high enei bond in the form of succinyl-CoA. All the other steps involve reactions which are largely irrevermble and thermodynamically favored, such as the formation of a pyrrole ring, decarboxylation, and oxidation to the aromatic porphyrin ring. The rate of synthesis of 5-AL may be controlled by the amount of the enzyme 5-AL-f thetase, by the concentration of its coenzyme-pyridoxal phosphate, by the steady state level of succinyl-CoA and of glycine, and possibly by inhibitors of the enzyme such as cysteine. Once formed the 5-AL may be converted to porphyrin or may be oxidized via the Shemin cycle. It is difficult to obtain a quantitative estimate of the importance of this oxidative pathway (65, S91). 

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