Magnetic state of an unusual bacterial heme protein

The unusual magnetic properties of the bacterial heme proteins known as the ferricytochomes c′ are explained as arising from ground state heme-iron electronic configurations consisting of quantum mechanical admixtures of an intermediate S =(3/2) spin state and a high S =(5/2) spin state, coupled via the spin-orbit interaction. The general conditions under which quantum mechanical mixing will occur in ferric heme complexes are discussed, as well as the specific magnetic properties of the ferricytochromes c′. The published magnetic data for the proteins are compared with the magnetic parameters predicted from theory. Ligand field considerations are used to infer a heme-iron configuration at physiological pH which is structurally hybrid between the high and low spin configurations.