Temperature-dependent spin-state equilibrium in aquo and hydroxo ferric heme octapeptide complexes. Model systems for the spin equilibrium of ferric hemoproteins

The temperature dependencies of the magnetic susceptibilities of aquo and hydroxo ferric heme octapeptide complexes were investigated as models for the temperature-dependent spin-state equilibria of ferric hemoproteins. The shifts in proton NMR signal of the sodium 4,4-dimethy1-4-silapentanesulfonate (DSS) caused by the heme octapeptide were measured from below room temperature to well above room temperature in aqueous and aqueous-ethylene glycol solutions. The thermodynamic data for these model systems are compared with those of hemoproteins. The results indicate that while suitable axial ligands to the heme iron give rise to spin-state equilibria in model systems, the thermodynamic values for hemoproteins are determined by interactions of the coordination center with the protein.