THE CHELATION OF MONOCHELATED PROTOHEME

Photolysis of CO and 1-methylimidazole (1-MeIm) mixtures of protoheme monomethyl ester, mono[3-(1-imidazoyl)propyl]amide (monochelated proto heme, or mcph) in toluene is followed over a time period of minutes. R eactions are followed under pseudo-first-order conditions, and the dif ference 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-methylim idazole, Photolysis of this mcph(1-MeIm)(CO) complex yields mcph(1-MeI m) (5) which is five-coordinated, with the internal imidazole not atta ched to the iron atom. Prior to CO recombination, 5 forms a rapid equi librium with the two hexacoordinated forms, mcph(1-MeIm)(2) (6) and mc ph(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 a tom whereas species 3 has the internal imidazole bound to the iron ato m). Based on values relative to the rare of CO association to 5, estim ates are made for the binding constant of a second mole of 1-MeIm to f orm 6, (6 +/- 2) x 10(5) M(-1), and the chelation constant of the inte rnal imidazole to form 3, (8 +/- 3) x 10(3). Comparison with similar c ompounds taken from the literature reveal that, in addition to the dep endence on the length of the side-arm (F.A. Walker and M. Benson, J. A m. Chem. Soc., 102 (1980) 5530-5538), chelation is dependent on solven t, structure of the base, structure of the porphyrin, and whether the iron is four- or five-coordinated-the binding of side-arm bases is poo rer to a five-coordinated heme than to a four-coordinated heme, contra ry to traditional behavior.