EPR studies of protein state transitions in Chromatium ferricytochrome c′

Recent magnetic studies have shown that in the pH range 1 to 11 the bacterial heme protein ferricytochrome c′ can undergo reversible transitions between various pure high-spin and quantum mechanically mixed (intermediate and high)-spin protein states. The EPR data presented here extend the recent work to high alkaline pH and show that above pH 11.6 reversible transitions occur between various pure high-spin and pure low-spin protein states. The new data and the previous magnetic studies of the protein are discussed in the context of the entatic nature of the iron-porphyrin complex in ferricytochrome c′. EPR data are also provided concerning an anomalous, but reversible, transition to a protein state with reduced heme iron, triggered by freezing and thawing at physiological pH. © 1975.