Add time:08/31/2019         Source:sciencedirect.com

Ferrihematoporphyrin-IX, prepared by insertion of iron into hematoporphyrin-IX, is shown to be high-spin and monomeric in methanol. Equations which describe the variation of absorbance at a fixed wavelength with increasing ligand concentration from which binding constants for the coordination of ligands by the porphyrin can be determined spectroscopically, are derived. It is shown that N-donor ligands, L (L = ethanolamine, imidazole, N- acetyl imidazole, and pyridine) bind at 25 °C in a stepwise manner, to form initially the high-spin mono(L) intermediate before a second ligand is bound to form the low-spin bis(L) complex. The binding constants K1 and K2 for the two steps can be determined, although the results are dependent on the monitoring wavelength because of the very small spectroscopic changes accompanying the coordination of the first ligand. There is a linear relationship between the magnitude of the binding constant for the first ligand and the pKa of the ligand, but no such simple relationship is found for binding of the second ligand. Some factors which are likely to control the tendency of the mono(L) intermediate to bind a second ligand are discussed. Only one benzimidazole ligand is coordinated because of steric interaction with the porphyrin.

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