Mössbauer Spectroscopy of Some Iron Porphyrins

Abstract

THE Mössbauer parameters of various iron porphyrin complexes have been investigated as model systems for haem proteins^1,2. 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 We have studied the water soluble N-methyl substituted meso tetrapyridyl porphine (unpublished results of Fleischer and Hambright), which seems to be the most acidic porphyrin known.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.