Magnetic studies on the ferri-haem undecapeptide of cytochrome c

The pH-dependence of the magnetic moment of a ferri-haem undecapeptide, produced by peptic digestion of cytochrome c, has been measured in aqueous solution using a nuclear magnetic resonance method. Below pH 3 the magnetic moment is consistent with the presence of hydroxo-bridged dimers of high-spin iron(III). Above pH 7 the iron(III) is low-spin, the spin crossover conforming to a simple titration curve with pK 6.3 (n=1). This transition is discussed with reference to spectrophotometric studies of ligand binding at the haem.