Spin-lattice relaxation of denatured nitrosyl hemoproteins.

The temperature dependence of the spin-lattice relaxation of denatured nitrosyl hemoglobin (HbNO), nitrosyl myoglobin, powdered HbNO, and hematin-NO was studied between 4 and 70 K. The results were fitted with both Tn and e-delta/tau models. In the first model, the relaxation is mediated by tunneling modes of a two-level system. A correlation between the n values and the functional state of the protein was observed. The striking coincidence of the range of the low-lying energy level and the temperature range where EPR spectra change suggests the existence of two conformations of the bound heme. The importance of the presence and structure of the globin is revealed in the difference between relaxation parameters of native proteins, denatured proteins, and hematin.