Porphyrins Haemoglobin

Delocalization of c -electron spin from a transition metal has been used to study structure, equilibria, and reactions of complexes, particularly ligand exchange and solvation. Other applications are to organometallics, such as metallocenes, metallobiomolecules such as porphyrins (haemoglobins, cytochromes), iron-sulfur clusters, and so on. Organic radicals such as nitroxides or aromatic radical ions are useful spin probes also. The field of paramagnetic NMR spectroscopy has been reviewed biennially. ... 

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... 

Figure 5 Schematic representation of absorbance of porphyrin compounds in relation to tissue transmittance at various wavelengths (see text). The lowest energy band (Band I) is shown in each case, apart from the porphyrin spectrum (etio type shown) on the left. The transmittance curve refers to a fold of human scrotal sac 0.7 cm thick (Wan, S. Parrish, J. A. Anderson, R. R. Madden, M. Photochem. Photobiol. 1981, 34, 679-681). The broad feature at ca. 500-600 nm is ascribed to haemoglobin (reproduced by permission of the Royal Society of Chemistry from Chem. Soc. Rev. 1995, 24, 19-33).

After this first step, where some samples are eliminated because they are not suited to the protocol, the positive ion mode is used to investigate haem, an iron porphyrin which is a blood marker. Spectra taken of the haem reference show that it can be detected due to [M]+ and [M+H]+ ions (respectively, at m/z 616.2 and 617.2), and also due to a large distribution of fragment peaks between m/z 350 and m/z 550 (Figure 15.12a). The same spectrum has also been obtained for haemoglobin leading to the conclusion that the presence of protein does not disturb the detection of haem. 

Related behaviour to that just described occurs in haemoglobin, although the latter is a tetramer consisting of four subunits each containing an iron porphyrin. The latter are located within hydrophobic pockets in the globin portion of the molecule (which, in this case, is composed of four linked chains - consisting of two a and two / chains, which differ in their respective amino acid compositions). 

Glycine Creatine Porphyrins (for the synthesis of haemoglobin and cytochromes) Purine nucleotides... 

These are rarely found free but occur as a common prosthetic group of proteins. The first such protein to be extensively studied was the iron porphyrin (Greek porphyra, purple) haemoglobin. This and other iron-porphyrin proteins play a vital role in the physiological activity of nearly all forms of life.146 These forms have the same basic structure (39) but differ in the nature of the pyrrole substituents these are shown for the major porphyrins in Table 13. It has become common practice to refer to all the iron-porphyrin proteins as haem proteins. The function of haemoglobin is, of... 

Respiratory pigments similar to the vertebrate haemoglobins have also been identified in many invertebrates. These vary from small proteins with two Fe-porphyrin units to large molecules containing up to 190 Fe-porphyrin units. Myoglobin, the 02 storage protein in muscle tissue, is also a small iron-protoporphyrin protein. The crystal structures of this and a number of other porphyrin proteins are now known (Chapter 20.2, Table 11). 

Figure S.A4. shows the structure of haemoglobin, a conjugate protein where there are extensive regions of a helix around the prosthetic group which is an iron porphyrin.

Fig. 5.A4. The structure of haemoglobin and its iron porphyrin prosthetic group. This diagram is highly simplified and the dotted line indicates the overall shape of the folded molecule and the zones of the a helical structure within the molecule are also shown...

The haemoglobin molecule (Hb) has a molecular weight of about 64000. Four subunits can be identified, each consisting of a polypeptide chain to which is attached a porphyrin group (Fig. 8.6), with an iron atom near its centre. The iron atom is in the II oxidation state (high-spin d6) and is further bonded to a nitrogen atom from an amino-acid residue below the porphyrin ring. The conformation of the polypeptide chain prohibits... 

Another interesting class of anion receptors based upon protonated nitrogen atoms are the expanded porphyrin macrocycles such as 4.17 (diprotonated sapphyrin) and compound 4.18. The tetrapyrrole porphyrin macrocycles are excellent hosts for metal cations such as Fe2+ and Mg2+ (e.g. haemoglobin and chlorophylls, Sections 2.3-2.5) however, their cavity dimensions are too small to accommodate anions. Conversely, expanded porphyrins such as 4.17 comprising five or more pyrrole residues present a rigid macrocyclic cavity about 5.5A in diameter, in which (particularly when protonated) the Nff... 

The porphyrin motif, four pyrrole rings linked by carbon atoms to give a planar cyclic compound, is found in many biomolecules. It is present in haemoglobin and myoglobin and its derivatives are also at the core of vitamin B12, hydrogenases, cytochrome c, the chlorophyll photosystem and similar essential proteins. 

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