220 MHz proton NMR studies of hemoproteins High-spin-low-spin equilibrium in ferric myoglobin and hemoglobin derivatives

Abstract

The proton NMR spectra at 220 MHz were observed at various temperatures for azide, imidazole, and deuteroxide complexes of sperm whale and horse myoglobin, and for azide and imidazole complexes of human hemoglobin. For myoglobin complexes, the size of the observed hyperfine shift and signal width increased as follows: cyanide < azide < imidazole < deuteroxide, which corresponded to the order of the effective Bohr magneton numbers. Hyperfine shifts of these samples below −10 ppm showed abnormal temperature dependence, except purely low-spin complexes such as the imidazole complex of human hemoglobin and cyanide complexes of myoglobin and hemoglobin. These abnormal shifts were discussed and analyzed on the basis of the spin equilibrium of the heme iron confirmed previously by magnetic susceptibility measurements.