Resonance Raman spectra of protohemin and protohemin‐imidazole complex

Resonance Raman spectra of aqueous solutions of protohemin and its complex with imidazole have been obtained using several exciting lines from Ar+ and HeCd lasers. The effects of different axial ligands to iron in hemin are reflected in the resonance Raman spectra of their respective complexes. Within experimental limits, hemin does not show any anomalously polarized bands (Q1 > ¾) while its complex with imidazole shows several such bands when excitation is in the α and β absorption regions. When excitation is in the Soret region, a polarized band ∼ 1500 cm−1 appears in the spectra of hemin‐imidazole complex and heme proteins. It is proposed as a spin state marker as well as to characterize the position of the iron ion relative to the porphyrin plane. Excitation profiles for some of the bands of hemin confirm the assigment of the α band near 5300 Å in the unresolved visible absorption spectrum.