The unusual magnetic properties of the bacterial heme proteins known as the ferricytochomescprime;are explained as arising from ground state hemehyphen;iron electronic configurations consisting ofquantum mechanicaladmixtures of an intermediate Sequals;(3sol;2) spin state and a high Sequals;(5sol;2) spin state, coupled via the spinhyphen;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 ferricytochromescprime;. The published magnetic data for the proteins are compared with the magnetic parameters predicted from theory. Ligand field considerations are used to infer a hemehyphen;iron configuration at physiologicalpH which is structurally hybrid between the high and low spin configurations.
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