PH-INDUCED CLEAVAGE OF THE PROXIMAL HISTIDINE TO IRON BOND IN THE NITRIC-OXIDE DERIVATIVE OF FERROUS MONOMERIC HEMOPROTEINS AND OF THE CHELATED PROTOHEME MODEL-COMPOUND

The EPR properties of the nitric oxide derivative of several monomeric hemoproteins (sperm whale myoglobin (Mb), horse Mb, Coryphaena hippurus Mb, Dermochelys coriacea Mb and Chironomus thummi thummi erythrocruorin) and of the ''chelated'' protoheme model compound were investigated in the pH range from 7.0-3.9. At neutral pH, the EPR spectra of all the NO-Fe complexes investigated have a rhombic shape with a different degree of resolution of the superhyperfine structure in the g/z region of the spectrum. By lowering pH, EPR spectra undergo a reversible change, characterized by the appearance of a 3 line pattern in the high magnetic field region, which is essentially complete at pH 3.9. This transition, which has pKa values of 4.7 and 5.0 for sperm whale Mb and for the ''chelated'' protoheme model compound respectively, is interpreted as indicative of the protonation of the N.epsilon. of the proximal histidine and of the cleavage of the Fe-N.epsilon. bond.