Hemes and hemoproteins. Part 10. Co-ordination of imidazole and other azoles by the iron(III) porphyrin microperoxidase-8

Abstract

Equilibrium constants K for the substitution of co-ordinated H₂O in the iron (III) porphyrin microperoxidase-8 (MP-8) by various azoles have been determined by UV/VIS spectrophotometry in 20% aqueous MeOH at 25 °C and I ca. 0.1 mol dm^–3. The results establish that the azoles obey (subject to a larger error at higher pK values) the relationship log K=a·pK+b, where a= 0.34 and b= 2.1, over 12 pK units from 1,2,4-triazole (pK_b2.3) to imidazolate (pK_a 14.3). Comparison with previous results on the co-ordination of nitrogen-containing bases to MP-8 shows that the five-membered heterocycles form a group distinct from the six-membered heterocycles as well as the alkylamines, and establishes the order of importance of factors which determine the preference of iron (III) porphyrins for imidazole (as His) over pyridine and amines (as Lys) as ‘group-specific factors’ > basicity > π bonding. A further comparison of these linear relationships for the three groups as ligands with those published for the three groups as acceptors in hydrogen bonding shows significant parallels which demonstrate that the factors which distinguish five- from six-membered heterocycles are intrinsic to the bases (ligands) and independent of the nature of the Lewis acid, when referred to the pK for protonation in aqueous solution as the standard.