ANALYSIS OF THERMAL EQUILIBRIUM BETWEEN HIGH-SPIN AND LOW-SPIN STATES IN FERRIMYOGLOBIN COMPLEXES

Paramagnetic susceptibilities of ferrimyoglobin hydrate, cyanate, azide and imidazole were measured in a temperature range from liquid nitrogen to room temperatures. Above a certain temperature, the susceptibility of each of these complexes was found to deviate from Curie's law, while obeying it rather closely below that temperature. There were two kinds of deviation from Curie's law, one downwards from the straight line corresponding to the law and the other upwards. The former appears in Mb(Fe3+·H2O and OCN-, and the latter in Mb(Fe3+)·N3- and imidazole. This phenomenon was explained as due to the coexistence of high-spin and low-spin states in thermal equilibrium. From the analysis of the temperature dependence of the equilibrium constants, energy differences between high-spin and low-spin states and the relative weights of these states were determined. From these data thermodynamic quantities, ΔH° and ΔS°, were calculated. The entropy differences between the two states are largely different from - Rln3 expected from weight differences of these spin states only and the possible origin of this discrepancy is suggested. The relation between ΔH° and ΔS° is also discussed. © 1969.