Experimental Observation of Anharmonic Coupling of the Heme-Doming and Iron-Ligand Out-of-Plane Vibrational Modes Confirmed by Density Functional Theory

In the deoxy ferrous state of histidine-ligated heme proteins, the iron-histidine band (νFe-His) has been assigned as a stretching mode that involves a two-body motion involving the iron and histidine combined with a minor amount of heme doming. An analogous Raman band, νFe-L has been observed in the proximal cavity mutant of H93G myoglobin where the Raman band of a series of nonnative axial ligands, L, can be compared. The H93G mutant of myoglobin consists of substitution of the proximal histidine, H93, by glycine. This replacement abolishes the sole covalent connection between the globin and the heme iron and creates a cavity that can be occupied by exogenous ligands, L, by dialysis. In the present study, the iron-axial-ligand out-of-plane vibration, νFe-L, for a series deoxy ferrous heme-iron adducts H93G(L) has been measured as a function of temperature, where L ) imidazole (Im), 4-methyl imidazole (4-Me Im), 2-methyl imidazole (2-Me Im), 1-methyl imidazole (1-Me Im), 4-bromo imidazole (4-Br Im), and 2,4-dimethyl imidazaole (2,4-diMe Im). Density functional theory calculations show that anharmonic coupling between low wavenumber heme-doming and iron-ligand out-of-plane modes can account for the calculated νFe-L frequency shifts. These calculations parallel the experimental observed trends for temperature-dependent Raman spectra for all of the adducts except H93G(1-Me Im).