Axial histidyl imidazole non-exchangeable proton resonances as indicators of imidazole hydrogen bonding in ferric cyanide complexes of heme peroxidases

Proton NMR spectra of a model of low-spin cyanide complexes of ferric hemoproteins indicate that two broad single-protein resonances from the axial imidazole can be resolved outside the diamagnetic spectral region. Upon deprotonation of the imidazole in the model, the upfield resonance shifts dramatically to higher field, suggesting that its position may reflect the degree of hydrogen bonding or proton donation of the imidazole. Met-cyano myoglobin reveals a pair of such broad peaks in the regions expected for an essentially neutral axial imidazole. In the cyano complexes of horseradish peroxidase and cytochrome c peroxidase, a pair of single-proton resonances are located which are assigned to the same imidazole protons on the basis of their linewidth and shift changes upon altering the heme substituents. The upfield proton, however, is found at much higher field than in metMbCN. The upfield bias of this resonance is taken as evidence for appreciable imidazolate character for the axial ligand in these heme peroxidases.