HEMES AND HEMOPROTEINS .8. COORDINATION OF AMINES AND AMINO-ACIDS BY THE IRON(III) PORPHYRIN MICROPEROXIDASE-8

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-degrees-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(alpha) or neighbouring C(beta) atoms; comparison with published data shows a sensitivity to beta 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 pi-pi 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 alpha effect; this appears to be the first reported example of this effect in metal-ligand bonding.