Stereochemistry of the Fe(II)− and Fe(III)−Cyanide Complexes of the Homodimeric Scapharca inaequivalvis Hemoglobin. A Resonance Raman and FTIR Study

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Abstract
Resonance Raman measurements carried out in parallel on ferrous and ferric Scapharca inaequivalvis dimeric hemoglobin cyanide derivatives allowed the identification of the electron density marker bands and heme core size marker bands in both derivatives in comparison with those obtained for the carbonmonoxy and deoxy adducts. The iron cyanide stretching mode, measured for the first time in a ferrous hemoprotein, has been detected at 455 cm-1, only 6 cm-1 lower than in the corresponding ferric derivative. This finding demonstrates that the large free energy difference for complex formation between the two derivatives is not concentrated on the Fe−C bond. The internal stretching frequencies of the ligand in the ferric and ferrous derivatives have been identified by FTIR and Raman measurements using different cyanide isotopes. The frequency decreases in the order Fe(III)−CN adduct, free cyanide, Fe(II)−CN adduct, consistent with the behavior observed in inorganic complexes and horseradish peroxidase. The main feature emerging from these data is that cyanide, at variance with oxygen and carbon monoxide, binds to ferrous iron with only a minor perturbation of the electronic structure of the heme. The functional counterpart of this effect is the absence of cooperative cyanide binding in HbI.