Abstract
The ligand binding properties of the Fe(III) Porphyrin microperoxidase-8 (MP-8) and several Co(III) Corrinoids towards nitrogenous bases have been studied. More than 70 equilibrium constants for axial ligand substitution of three series of nitrogenous bases (Amines, Pyridines, Imidazole and their derivatives) with Fe(III)MP-8 and five different corrinoid complexes varying in the trans ligand X (X = Methyl, Vinyl, CN, Bzm and H2O) have been determined spectrophotometrically in aqueous solution. A base line verifying the general equation log K =a.pK +b has been established for each metal complex with these nitrogenous bases, when the pK is the only significant variable, the value of (a) is comparable but values of (b) fall in the order 5-heterocycles > 6-heterocycles > amines. These base lines are characterised by the wide range of pK's (pK =5.3 -10.6 for primary amines, 0.4- 9.76 for 6- membered heterocyclic compounds and 2.3 - 14.3 for 5-membered heterocyclic compounds). The most striking base line was found in the case of the 5-membered heterocyclic compounds which covered 12 pK units, including imidazole, 1,2,4-triazole, their conjugated anions, together with certain substituted imidazoles. These base lines include the first linear basicity correlations for any Fe Porphyrin or for 5- heterocycles with any transition metal ion. The value of the slope (a) increased linearly with published data on the trans effect and furthermore the established base lines pass through a focal point which is different for each family in the range pK =-2 to 0, log K =-2 to 0. These base lines showed the enhancement of log K in the case of hydroxyl amine, hydrazine and pyridazine above the established base lines, this was ascribed to the so called a-effect for the first time in metal-ligand bonding. Also, the base lines help to quantify the suppression in log K value due to the steric hindrance as in the case of a and p-branching of amines and also the two substituted pyridines. A new mechanistic pathway was established for the methylation of Co(II) with methyl iodide in presence of the thiol (RSH) complex of Co(II). This reaction showed that increasing the electron density on the Co(II) ion by coordination of the polarisable RSH ligand induced some of the reactivity towards Mel characteristic of the fully reduced Co(I) corrinoids.