Magnetic circular dichroism studies on the electronic configuration of catalase compounds I and II

Absorption and magnetic circular dichroism spectra of native catalase, compound I and compound II have been measured and the data compared with that observed previously for horseradish peroxidase. The native catalase data at pH 6.9 are characteristic of a high-spin ferric porphyrin and are similar to the data reported for the ferric myoglobin and ferric horseradish peroxidase at pH 7. Oxidation of native catalase by peroxoacetic acid forms the compound I species that is identified by its low absorbance in the 400 nm Soret region and a series of overlapping bands between 450 nm and 680 nm. The magnetic circular dichroism spectra of compound I of catalase closely resembles that previously obtained for horseradish peroxidase compound I. These results indicate that the ground state of the heme pi-system is the same in both catalase and horseradish peroxidase compound I species. The compound II data show that the ratio of the magnetic circular dichroism intensity for the Soret to alpha A terms is 0.5 which means that there is a redistribution of angular momentum between the pi* excited states that give rise to the Soret and alpha-bands compared with the horseradish peroxidase compound II data where the ratio of the analogous A term intensities has a value of about 3. In addition, the magnetic CD spectra of both the catalase and horseradish peroxidase compound II species are reminiscent of typical metalloporphyrin spectra which lack charge-transfer transitions and where the metal d-orbitals are decoupled from the porphyrin pi-system.