Hemes and hemoproteins. Part 8. Co-ordination of amines and amino acids 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 by 20 amines and amino acids have been determined in 20% aqueous MeOH at 25 °C. Values of log K increase approximately linearly with the basicity (pK ranging from 5.3 to 10.7) of the primary amine, including the unsubstituted amino acid glycine, according to the relationship log K= 0.43 pK– 0.5; this is the first demonstration of such a linear relationship for any iron-(II) or -(III) porphyrin. The values are decreased by substitution (i.e. branching) on either the co-ordinated N_α or neighbouring C_β atoms; comparison with published data shows a sensitivity to β substitution in the co-ordination of amines by the d⁵ iron(III), d⁶ cobalt(III) and d⁸ nickel(II) ions, but not by the larger d¹⁰ silver(I) and HgMe⁺ ions. The values may be increased through π–π interaction of the donor–acceptor type between the conjugated porphyrin ring and phenyl [phenylalanine or NH₂(CH₂Ph)] or indolyl (tryptophan) substituents. The compounds NH₂NH₂ and NH₂OH both show an enhanced value of log K which can be ascribed to the so-called α effect; this appears to be the first reported example of this effect in metal–ligand bonding.