Electron Paramagnetic Resonance Study of the Stereochemistry of Nitrosylhemoglobin

Paramagnetic resonance spectra of single crystals of nitrosylhemoglobin (HbNO) have been obtained at − 195°C. There are four nonequivalent paramagnetic atoms which form two sets; the members of each set are related by the 010 dyad axis. The principal values of the g tensor are: gxx = 2.0820, gyy = 2.0254, and gzz = 1.9909; the isotropic g value is 2.039. The principal values of the hyperfine A tensors are: Tξξ = 77 MHz, Tηη = 54 MHz, and Tξξ = 18 MHz; the isotropic coupling constant has a value of 50 MHz. The calculated principal values for the dipolar T′ tensors are: tξξ′ = 27 MHz, tηη′ = 5 MHz, and tξξ′ = −32 MHz. The principal axes of the g tensor do not coincide with these of the T tensor; the results were interpreted to give a bent Fe–N–O angle of 110°. The N–O axes of the ligands of one set lie in the ab plane; those of the others are inclined at an angle of about 80° to the ab plane. The electrostatic interaction of the oxygen atom with the imidazolium group of the distal histidine and the nonpolar evnironment in the vicinity of the heme probably contribute to the stabilization of this particular Fe–N–O bond.