Electron spin-echo studies of relaxation processes in high-spin ferrimyoglobin

The spin-echo technique is used to study the relaxation processes associated with the trivalent iron ion in horse heart myoglobin at 1.2°K. The destruction of transverse and longitudinal phase memory, as, respectively, measured by the two-pulse (T2p) and three-pulse (T3p) echo sequences, is attributed to a spectral diffusion process. Saturation-recovery data help to affirm this belief. The diffusion process is not dependent upon the concentration of myoglobin in a water solution, which means that it is not dependent upon the Fe3+ concentration (over the range 1017–1019 Fe3+ ions/cc). Moreover, values of T2p and T3p are the same for myoglobin powder as they are for hemoglobin powder. Both of these observations support the conclusion that interactions between the iron spins do not play a major role in the destruction of phase memory; but, rather, that the interaction between the iron magnetic dipoles with the surrounding nuclear moments (particularly the hydrogen nuclei of the water molecule at the sixth ligand and the water molecules of the solvent) is the dominant mechanism. Included in the experimental results are saturation-recovery spin–lattice relaxation measurements at 1.2°K as well as 4.2°K. The short spin–lattice time at 4.2°K (≦ 2μsec) makes it conceivable that the linewidth of paramagnetic centers in this and in similar biological specimens at temperatures of 77°K and above is largely determined by spin-lattice effects.