STEREOCHEMISTRY OF THE FE(II)-CYANIDE AND FE(III)-CYANIDE COMPLEXES OF THE HOMODIMERIC SCAPHARCA-INAEQUIVALVIS HEMOGLOBIN - A RESONANCE RAMAN AND FTIR STUDY

Resonance Raman measurements carried out in parallel on ferrous and fe rric Scapharca inaequivalvis dimeric hemoglobin cyanide derivatives al lowed the identification of the electron density marker bands and heme core size marker bands in both derivatives in comparison with those o btained for the carbonmonoxy and deoxy adducts. The iron cyanide stret ching 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 correspon ding 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 frequencie s of the ligand in the ferric and ferrous derivatives have been identi fied 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 c omplexes and horseradish peroxidase. The main feature emerging from th ese data is that cyanide, at variance with oxygen and carbon monoxide, binds to ferrous iron with only a minor perturbation of the electroni c structure of the heme. The functional counterpart of this effect is the absence of cooperative cyanide binding in HbI.