Hemes and hemoproteins. Part 9. Co-ordination of pyridines and diazines 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 substituted pyridines and the three diazines have been determined in 20% aqueous MeOH at 25 °C. Values of log K for the co-ordination of the 1,3-diazine pyrimidine and the 1,4-diazine pyrazine as well as the 4-substituted pyridines increase approximately linearly with the pK of the base according to the relationship log K= 0.36 pK+ 0.8. The value of log K observed for the 1,2-diazine pyridazine is significantly greater (by ca. 0.7) than that expected from basicity alone, which is ascribed to operation of the α effect. It is suggested that this effect, which reflects the breakdown in correlation between basicity towards the proton and towards other Lewis acids, originates from the existence of two opposing factors involving the vicinal atoms (viz. lone pair–lone pair repulsion and the inductive effect) which have a different dependence on the degree of negative charge removed by the Lewis acid.