THE CHELATION OF MONOCHELATED PROTOHEME

摘要

Photolysis of CO and 1-methylimidazole (1-MeIm) mixtures of protoheme monomethyl ester, mono[3-(1-imidazoyl)propyl]amide (monochelated protoheme, or mcph) in toluene is followed over a time period of minutes. Reactions are followed under pseudo-first-order conditions, and the difference spectra recorded for each kinetic run show multiple isosbestic points. Intercepts from plots of the reciprocals of the observed rate constants versus 1-methylimidazole concentration suggest the initial mcph-CO complex has the internal base displaced by external 1-methylimidazole, Photolysis of this mcph(1-MeIm)(CO) complex yields mcph(1-MeIm) (5) which is five-coordinated, with the internal imidazole not attached to the iron atom. Prior to CO recombination, 5 forms a rapid equilibrium with the two hexacoordinated forms, mcph(1-MeIm)(2) (6) and mcph(1-MeIm) (3), which decays to form the CO complex mcph(1-MeIm)(CO) (species 6, like species 5, has the internal base unbound to the iron atom whereas species 3 has the internal imidazole bound to the iron atom). Based on values relative to the rare of CO association to 5, estimates are made for the binding constant of a second mole of 1-MeIm to form 6, (6 +/- 2) x 10(5) M(-1), and the chelation constant of the internal imidazole to form 3, (8 +/- 3) x 10(3). Comparison with similar compounds taken from the literature reveal that, in addition to the dependence on the length of the side-arm (F.A. Walker and M. Benson, J. Am. Chem. Soc., 102 (1980) 5530-5538), chelation is dependent on solvent, structure of the base, structure of the porphyrin, and whether the iron is four- or five-coordinated-the binding of side-arm bases is poorer to a five-coordinated heme than to a four-coordinated heme, contrary to traditional behavior. [References: 34]