Analysis of thermal equilibria between high spin and low spin states in mesohemoproteins.

The protoheme group of myoglobin, hemoglobin, and cytochrome c peroxidase was chemically replaced by the mesoheme group. The magnetic susceptibilities and optical absorption spectra of these mesohemoproteins and their derivatives in ferric states were measured as a function of temperature between 77” K and 296” K in order to examine the effect of chemical modification of the heme group on the heme group on the heme-apoprotein interaction. Ferric mesohemoproteins and their derivatives showed thermal excitation characteristics of spin states similar to those of the corresponding protohemoproteins. The compensation temperature (T,), which is the ratio of the enthalpy difference (AHO) and the entropy difference (ASO) between high and low spin states and at which the concentration of the high and low spin states are equal, was found to be 50-100” higher in mesohemoproteins than the corresponding protohemoproteins. On the basis of the present results it was concluded that the Van der Waals contacts between porphyrin and apoprotein moieties are essential for the thermal equilibria of spin states in ferric hemoproteins.