Properties of Polymeric 2-Methylimidazole-Heme Complexes

Visible absorption spectra of the heme complexes of poly (1-vinyl-2-methylimidazole) (PMI), poly (1-vinylimidazole) (PVI), their copolymers of 1-vinylpyrrolidone (PMIP, PIP) and the corresponding monomeric imidazoles were measured in aqueous solutions. PVI and PIP formed the six-coordinate, low-spin type heme complexes, but 2-substituted imidazoles, PMI and PMIP, formed the five-coordinate, high-spin type ones. The apparent formation constants (K) of the heme complexes with these imidazole ligands were estimated spectroscopically. The K values of the polymeric complexes were about 102 to 104 times those of monomeric ones. This result is thought to be caused by the apparent higher concentration of ligands in the polymer domain and by the hydrophobic interaction of heme with the polymeric ligand. PMIP could form the five-coordinate heme complex, even if the large amount of imidazole coexisted.