Relaxation spectra of the iron spin transition in methemoglobin

Temperature-jump relaxation spectra of methemoglobin have been monitored in the spin-sensitive region of the absorption spectrum at pH 6. A single relaxation process is observed, the amplitude of which correlates exactly with that expected for spin state changes. The time-constant is of the order of 1 to 10 ms at 13 °C. Quaternary structural effects perturb the spin dynamics, as evidenced by a slower relaxation in the αβ dimer as opposed to the tetramer. On the other hand, the spin dynamics of the tetramer are not greatly affected by binding saturating amounts of inositol hexaphosphate. This is partly a reflection of the fact that the relative perturbation, caused by inositol hexaphosphate binding, of the equilibrium between high and low-spin species is small, under the conditions studied. In addition, it means that under these conditions, inositol hexaphosphate does not significantly perturb the flexibility of the irons in the heme groups.