Magnetic Circular Dichroism of Cytochrome c

We have measured the magnetic circular dichroism of ferrocytochrome c from 350 to 1000 nm and the MCD of ferricytochrome c from 350 to 750 nm. We present expressions for the shapes of MCD and absorption spectra in the presence of a magnetic field of randomly oriented molecules for the general case in which the absorption band is split in zero field, and the two components of the split band have different intensities. After correcting for the effects of unresolved zero‐field splitting, we find that the ``angular momentum'' of the Q₀₀ band of Fe(II) cyt c is 7.9 _μB. We show that the 23‐fold decrease in the MCD of the Q₀₀ band upon oxidation of Fe(II) to Fe(III) is due to about a fivefold increased bandwidth while the oscillator strength and angular momentum remain essentially unchanged. The increase in bandwidth may reflect greater mixing of the porphyrin orbitals with the d levels of the iron ion in the oxidized state. The MCD of Fe(II) cyt c in the near infrared (600–1000 nm) can be decomposed using essentially the same components previously used to decompose the CD if some components are presumed degenerate. The angular momenta of the degenerate components are in the range expected for $\mathrm{d\to \; d}$ transitions. The CD and MCD of the 695 nm band of Fe(III) cyt c are both relatively weak and suggest, respectively, that the transition is electric dipole allowed and nondegenerate.