Porphyrins Uroporphyrins

A mild procedure which does not involve strong adds, has to be used in the synthesis of pure isomers of unsymmetrically substituted porphyrins from dipyrromethanes. The best procedure having been applied, e.g. in unequivocal syntheses of uroporphyrins II, III, and IV (see p. 251f.), is the condensation of 5,5 -diformyldipyrromethanes with 5,5 -unsubstituted dipyrromethanes in a very dilute solution of hydriodic add in acetic acid (A.H. Jackson, 1973). The electron-withdrawing formyl groups disfavor protonation of the pyrrole and therefore isomerization. The porphodimethene that is formed during short reaction times isomerizes only very slowly, since the pyrrole units are part of a dipyrromethene chromophore (see below). Furthermore, it can be oxidized immediately after its synthesis to give stable porphyrins. 

The arrangement of the acetate (A) and propionate (P) substituents in the uroporphyrin shown in Figure 32-2 is asymmetric (in ring IV, the expected order of the A and P substituents is reversed). A porphyrin with this type of asymmetric substitution is classified as a type III porphyrin. A porphyrin with a completely symmetric arrangement of the substituents is classified as a type I porphyrin. Only types I and III are found in nature, and the type III series is far more abundant (Figure 32-3)—and more important because it includes heme. 

The above account of selectivity of inorganic plus organic chemistry in synthesis is given rather extensively to stress three points. All the four (Mg, Fe, Co and Ni) porphyrin products came from one source, the synthesis of uroporphyrin. The basis of selection is very different from that in primitive centres which use thermodynamic stability constant selectivity based on different donor atoms for different metal ions. Here, all ion complexes have the same donor atoms, nitrogen, the most constrained being the coordination of Mg2+ by five nitrogens exactly as is seen for Fe in haemoglobin. Hence, there also has to be a new control feedback to ensure that the appropriate quantities of each metal cofactor is produced in a balanced way, that is synthesis from uroporphyrin has to be divided based upon... 

Hb and Ni-reconstituted Hb were prepared according to reported methods (13). Mb was made by the method of Alston and Storm (14) Solutions of the proteins in 0.05 M phosphate buffer at pH 7.5 were used for obtaining spectra. The proteins are stable in air and do not photodecompose. Ni protoporphyrin IX (Ni(ProtoP)), Ni uroporphyrin I (Ni(UroP)), and Ni octaethylporphyrin (Ni(OEP)) were obtained from Porphyrin Products and used without further purification. All solvents were of highest purity obtainable from commercial sources. All materials showed the literature uv-visible absorption spectra. Absorption spectra were obtained on a Perkin-Elmer Model 330 spectrophotometer. 

Let me put it to Professor Lehn that biology can do better than he can with small molecules. From uroporphyrin biology makes (1) copper porphyrins (turacin feathers), (2) heme (iron) porphyrins, (3) magnesium chlorins, (4) cobalt corrins (B12). These compounds are formed with very little confusion between metals and partners. I cannot give selectivity factors, but they are very big. How is this managed Is it not better than man can do Maximum size selectivity for anions is not yet understood as few anion-binding sites in biology are known. 

The first porphyrin intermediate of the biosynthetic pathway is uroporphyrinogen, which is stepwise decarboxylated by uroporphyrinogen decarboxylase to heptacarboxy-, hexacarboxy-, pentacarboxy-, and coproporphyrinogen. This latter compound proceeds, as indicated in Fig. 7.3.1, to protoporphyrinogen and protoporphyrin. The oxidized uroporphyrin and its decarboxylation products up to coproporphyrin are assayed in urine. Coproporphyrin and the further downstream intermediaries can be recovered from feces as described below. 

Increased porphyrins in clear fluid such as urine may be detected directly by their pink fluorescence if exposed to long ultraviolet (Fig. 7.3.2). The specificity of this screening assay may be improved if porphyrins are extracted by talcum [8]. These isolated porphyrins may be quantified using a spectrofluorimeter. As different porphyrias show specific excretion patterns, separation of the main porphyrins is desirable. The formerly used fractionated extraction enabled to separate the uroporphyrin fraction from the coproporphyrin fraction. In addition to uroporphyrin, the first fraction includes heptacarboxy- and part of hexacarboxyporphyrins, and in addition to coproporphyrin, the second fraction includes part of hexacarboxy- and pentacar-boxyporphyrins. Later on, thin-layer chromatography of methylester derivatives is used. 

All acute porphyrias show similar urinary porphyrin patterns with predominant elevation of uroporphyrin and coproporphyrin III isomer in addition, hepta-, hexa-, and especially pentacarboxyporphyrins are increased (see Fig. 7.3.3). 

Table 7.3.3 Example mean reference values for uroporphyrin, coproporphyrin I, and coproporphyrin III (these can only be interpreted as a guidance) collected by means of high-performance liquid chromatography (HPLC). The intermediary porphyrins were below the detection limit...

Porphyrins in plasma are mainly used to distinguish between PCT and pseudoporphyria in patients with chronic hemodialysis. CEP patients also show characteristic elevations with dominance of I-isomers, especially of uroporphyrin and its decarboxylation products, in both plasma and erythrocytes. Plasma porphyrins may be used for follow-up of the patients. 

Each laboratory should establish its own reference values. Those given here can thus only be interpreted as a guide. Porphyrins in plasma are rarely detectable in healthy individuals. Patients on chronic dialysis but without signs of PCT may show some measurable values, which are given here as reference values (nmol/1, mean 2SD) uroporphyrin <24.3 heptacarboxyporphpyrin <2.7 hexacarboxyporphyrin <1 pentacarboxyporphyrin < 1 coproporphyrin I < 1 coproporphyrin III < 1. 

Side chains Different porphyrins vary in the nature of the side chains that are attached to each of the four pyrrole rings. For example, uroporphyrin contains acetate (-CH2-COO-) and propionate (-CH2-CH2-COO-) side chains, whereas coproporphyrin, is substituted with methyl (-CH3) and propionate groups. 

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. 

Acetate (A) and propionate (P) are reversed in ring D of uroporphyrin III compared with uroporphyrin I. Only type III porphyrins are physiologically important in humans. 

Methods using condensations of two dipyrrolic units suffer from inherent symmetry restrictions, though these are of course less serious than those involved in the use of monopyrrole tetramerization. Thus the routes through dipyrroles are limited to synthesis of porphyrins which are centrosymmetrically substituted (by way of self-condensation of a suitably activated dipyrrole), or to porphyrins which possess subunit symmetry in one or both halves of the molecule. It is very fortunate that this latter restriction is not serious for the synthesis of porphyrins from natural materials because rings c and d of uroporphyrin-III, coproporphyrin-III and protoporphyrin-IX (Table 1), for example, are symmetrically substituted about the C-15 atom. 

Figure 16-5 Structures of some biologically important porphyrins. (A) Uroporphyrin I.

Porphyria may result from several different enzyme deficiences in the porphyrin biosynthetic pathway. The condition is often hereditary but may be induced by drugs or other xenobiotic substances and may be continuous or intermittent.425 426 429 In one type of congenital porphyria uroporphyrin I is excreted in large quantities. The biochemical defect appears to be a deficiency of the cosynthase that is required for formation of protoporphyrin IX. Another type of porphyria results from overproduction in the liver of... 

If an appropriate electron acceptor is available, the photoexcited porphyrin may transfer an electron to it. The acceptors may be organic molecules such as JV-alkylnicotinamide, metal complexes such as [Fe(CN)6]3- or another porphyrin molecule.131 The second order reaction rate for the Zn(uroporphyrin)-acceptor system at infinite ionic strength is estimated to be 108M 1s l. The electron travels from Zn(Por) to V-benzylnicotiamide or to [Fe(CN)6]3 over a distance of 15 A or 30 A respectively. Since the reaction creates charged species, polar media accelerate the process. The back reaction or other photosensitization processes such as 02 generation become favourable in nonpolar solvents. 

Studies on porphyrins in aqueous solution are complicated by their lack of solubility (in acidic solutions) and their tendency to associate (in basic solutions). These difliculties may be overcome by using soluble porphyrins with carboxylate or sulfonate substituents, although the uroporphyrins are convenient to study as they are freely soluble, and do not aggregate at concentrations necessary for many solution studies. 

Tetraanion of tetrakis(4-carboxyphenyl)porphyrina Tetraanion of tetrakis(4-sulphonatophenyl)porphyrin Tetracation of tetrakis(4-pyridinio)porphyrin Tetraanion of tetrasulphophthalocyanine Tetrakis(4-tolyl)porphyrin Octaanion of uroporphyrin ... 

The important role of chlorophylls in photosynthesis inspired people looking for ways to apply these natural porphyrins in DSSC as light harvesting sensitizers. The use of zinc and antimony metallo-uroporphyrins M-UP (105) as photosensitizers... 

It may be noted that although the human organism is capable of degrading metal-containing porphyrins such as heme, it has no means of degrading porphyrins containing no metal, for example coproporphyrins and uroporphyrins. These must be excreted as such. The latter two are water-soluble and are thus found in urine and to some extent in the feces. Protoporphyrins, however, are largely water-insoluble and are excreted in the feces. Finally, it may be mentioned that porphyrias make their appearance after puberty. This has been associated with the appearance of 5/3-steroid reductases, as discussed above. 

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