Porphyrin ring system

Green coloration, present in many vegetable oils, poses a particular problem in oil extracted from immature or damaged soybeans. Chlorophyll is the compound responsible for this defect. StmcturaHy, chlorophyll is composed of a porphyrin ring system, in which magnesium is the central metal atom, and a phytol side chain which imparts a hydrophobic character to the stmcture. Conventional bleaching clays are not as effective for removal of chlorophylls as for red pigments, and specialized acid-activated adsorbents or carbon are required. 

There are, however, some crown type compounds which contain no structural feature except the thiophene subunit, and these deserve some comment here. This is especially true since one of these compounds was prepared very early in the history of crown compounds. Ahmed and Meth-Cohn were interested in sulfur analogs of the porphyrin ring system and prepared compound 7 in 1969 by the method shown in Eq. 

FIGURE 15.27 The displaceiTient of the Fe ion of the heme of deoxymyoglobin from the plane of the porphyrin ring system by the pull of His F8. In oxymyoglobin, the bound O9 counteracts this effect. 

In deoxyhemoglobin, histidine F8 is liganded to the heme iron ion, but steric constraints force the Fe His-N bond to be tilted about 8° from the perpendicular to the plane of the heme. Steric repulsion between histidine F8 and the nitrogen atoms of the porphyrin ring system, combined with electrostatic repulsions between the electrons of Fe and the porphyrin 77-electrons, forces the iron atom to lie out of the porphyrin plane by about 0.06 nm. Changes in... 

The porphyrin ring system (the parent compound 1 is also known as porphin) consists of four pyrrole-type subunits joined by four methine ( = CH-) bridges to give a macrotetracycle. The macrocycle contains 227i-electrons from which 1871-electrons form a delocalized aromatic system according to Huckel s 4n + 2 rule for aromaticity. The aromaticity of the porphyrin determines the characteristic physical and chemical properties of this class of compounds. The aromatic character of porphyrins has been confirmed by determination of their heats of combustion.1"3 X-ray investigations4 of numerous porphyrins have shown the planarity of the nucleus which is a prerequisite for the aromatic character. 

The high stability of porphyrins and metalloporphyrins is based on their aromaticity, so that porphyrins are not only most widespread in biological systems but also are found as geoporphyrins in sediments and have even been detected in interstellar space. The stability of the porphyrin ring system can be demonstrated by treatment with strong acids, which leave the macrocycle untouched. The instability of porphyrins occurs in reduction and oxidation reactions especially in the presence of light. The most common chemical reactivity of the porphyrin nucleus is electrophilic substitution which is typical for aromatic compounds. 

The large number of cytochromes identified contain a variety of porphyrin ring systems. The classification of the cytochromes is complicated because they differ from one organism to the next the redox potential of a given cytochrome is tailored to the specific needs of the electron transfer sequences of the particular system. The cytochromes are one-electron carriers and the electron flow passes from one cytochrome type to another. The terminal member of the chain, cytochrome c oxidase, has the property of reacting directly with oxygen such that, on electron capture, water is formed ... 

The preceding sections of Chapter 7 have discussed iron-containing proteins and enzymes having a porphyrin ring system. Section 7.9 presents a short introduction to the many non-heme iron-containing proteins and enzymes. Two of these are iron-sulfur proteins (Section 7.9.2) and iron-oxo proteins (Section 7.9.3). 

Histidine Plane of residue porphyrin ring system... 

FIGURE 5-2 The heme group viewed from the side. This view shows the two coordination bonds to Fe2+ perpendicular to the porphyrin ring system. One of these two bonds is occupied by a His residue, sometimes called the proximal His. The other bond is the binding site for oxygen. The remaining four coordination bonds are in the plane of, and bonded to, the flat porphyrin ring system. 

Coenzyme B12 is the cofactor form of vitamin B 2, which is unique among all the vitamins in that it contains not only a complex organic molecule but an essential trace element, cobalt. The complex corrin ring system of vitamin B12 (colored blue in Fig. 2), to which cobalt (as Co3+) is coordinated, is chemically related to the porphyrin ring system of heme and heme proteins (see Fig. 5-1). A fifth coordination position of cobalt is filled by dimethylbenzimidazole ribonucleotide (shaded yellow), bound covalently by its 3 -phosphate group to a side chain of the corrin ring, through aminoisopropanol. 

Plants are green because of the chlorophyll they contain. Chlorophyll, however, requires the porphyrin ring system, which contains nitrogen. If a plant is deficient in nitrogen, it will be less able to pro-... 

Figure 10.13 Perspective view of a picket fence model of the porphyrin ring system of hemoglobin. Note that the oxygen molecule is bound at an angle (there is a four-fold statistical distribution of the terminal O because of rotation about the Fe—O bond). [From J. P. Collman, R. R. Gagne, C. A. Reed, W. T. Robinson, and G. A. Rodley, Proc. Natl. Acad. Sci. USA 71,1326 (1974).]...

In order to explain their high activity and stability it was postulated that polyhalogenation of the porphyrin ring system not only stabilizes the latter towards oxidative destruction but also stabilizes the oxoiron intermediate with respect to p-oxo dimer formation. In principle, it should also be possible to design stable solid catalysts capable of mediating analogous selective oxidations in the liquid phase. 

It should be pointed out that no X-ray structure of a metalloenzyme capable of catalyzing a redox reaction has been reported. Thus, the detailed environment of the metal ion in most redox enzymes is largely unknown. The porphyrin ring system is known to be present in many metalloenzymes, including certain oxygenases. These ligands are probably intimately involved in catalysis carried out by these enzymes. 

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. 

The reactions of the heme pigments in meat and meat products have been summarized in the scheme presented in Figure 6-13 (Fox 1966). Bilin-type structures are formed when the porphyrin ring system is broken. 

These data and the conclusions derived from them apply to heme groups of the protoporphyrin IX ring and the heme a ring. The results also apply to Fe(III) OEP models. The porphyrin ring systems, which do not conform to these rules, are the partially reduced rings such as haem d and Fe(III) TPP. It seems probable that other partially reduced porphyrin rings as found in chlorin, bacteriochlorin, and siroheme will not show NIR CT bands in this region. However, much further experimental work is required to establish where the CT bands lie in these systems. 

The two major classes of heme oxidized above the Fe(III) state, namely, the one-electron oxidized ferryl state and the two-electron oxidized state containing Fe(IV)=0 and the porphyrin ring cation radical, FedVKport), have MCD spectra that provide two useful pieces of information (76). First, the form of the spectrum can indicate whether or not the porphyrin ring system is oxidized, and whether the... 

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