THE Mössbauer parameters of various iron porphyrin complexes have been investigated as model systems for haem proteins1,2. Topics which require further consideration are the effect of porphyrin structure, basicity and esterification on the Mössbauer spectra. It has been shown that the basicity towards protons of meso and diacetyl deuterohaemins differs by a factor of one hundred whereas their isomer shifts (δ0) and quadrupole splittings (ΔE) were insensitive to this change2. We have studied the water soluble N-methyl substituted meso tetrapyridyl porphine (unpublished results of Fleischer and Hambright), which seems to be the most acidic porphyrin known. The (pK3+pK4) of this compound is the same as the unmethylated derivative3, whereas pK2=12.9. No other porphyrin has a measurable pK2 in aqueous solution. Table 1 shows that the isomer shift of this structurally different haemin compound and protohaemin chloride are almost identical This indicates4 in both a 5 per cent 4s electron contribution to the d5 configuration of Fe(III), and that regardless of structure or basicity, the metal–ligand bonding in these high spin square pyramid chelates is primarily ionic. It is also noted that low spin octahedral hemichromes have approximately the same sigma electron density1 at the iron nucleus (δ0≅ 0.041 cm/sec) as the high spin haemin chlorides, whereas ΔE of the low spin derivatives is, as expected, larger (ΔE≅0.21 cm/sec) than that of the high spin compounds. This unexplained similarity is novel in that typical inorganic low spin complexes have more sigma density than high spin varieties4, presumably because of their increased pi bonding capacities. Thus the similarity in δ0 values might indicate a limited role for pi bonding in low spin ferric porphyrin systems.
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WYNTER, C., HAMBRIGHT, P., CHEEK, C. et al. Mössbauer Spectroscopy of Some Iron Porphyrins. Nature 216, 1105 (1967). https://doi.org/10.1038/2161105a0
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