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

Paramagnetic susceptibilities and absorption spectra of ferrihemoglobin complexes (Hb(Fe3+) · CN-, ·N3-, · OH-, · OCN- and · H2O) have been measured between liquid N2 and room temperatures. It has been found that temperature-dependent transitions exist in squares of effective Bohr magneton numbers for these complexes except ferrihemoglobin cyanide, as seen in the case of ferrimyoglobin complexes, These transitions are explained on the basis of thermal equilibria between high-spin and low-spin states of Fe3+ in hemes. In the case of acid ferrihemoglobin (pH 5.9) a strange behavior has been observed near the freezing point of solution. From the analysis of temperature dependence of the equilibrium constant in high spin ⇌ low spin states, energy and entropy differences between the two states are estimated. Thermodynamic quantities ΔH° and ΔS° are calculated by use of ε and γ values for these complexes. Compensation phenomena between ΔH° and ΔS° are also described together with the results of ferric myoglobin and cytochrome c peroxidase complexes. The data of three hemoproteins are compared. This report is the fourth in our series of reports on thermal equilibrium between high-spin and low-spin states. © 1969.