The temperature dependence of the spin-lattice relaxation of qualitatively by a relaxation process which involves hemedenatured nitrosyl hemoglobin (HbNO), nitrosyl myoglobin, powheme magnetic dipolar interactions between both kinds of dered HbNO, and hematin-NO was studied between 4 and 70 K. subunits (3). On the other hand, electron paramagnetic reso-The results were fitted with both T n and e 0 D / T models. In the first nance (EPR) spectra of HbNO solutions studied in the temmodel, the relaxation is mediated by tunneling modes of a twoperature range from 7.5 to 104 K have been shown to be level system. A correlation between the n values and the functional composed of at least three components (A, B, and C) whose state of the protein was observed. The striking coincidence of the intensities have different dependences on temperature and range of the low-lying energy level and the temperature range microwave power level (6). Their relaxation behavior was where EPR spectra change suggests the existence of two conformastudied by the continuous saturation method, and the tempertions of the bound heme. The importance of the presence and ature dependence of T 1 of A, the low-temperature and lowstructure of the globin is revealed in the difference between relaxation parameters of native proteins, denatured proteins, and hema-power-level component, was shown to follow an Orbachtin.