The detection of two electron paramagnetic resonance radical signals associated with chloroperoxidase compound I

The electron paramagnetic resonance spectra of chloroperoxidase Compound I and native enzyme are compared. Upon the formation of Compound I, the g = 2.62, 2.26, and 1.82 signals associated with native enzyme disappear and are replaced by two new EPR signals, a sharp signal at g = 2.008 and a broad signal at g = 1.73. The g = 2.008 signal accounts for only 2% of the theoretical spins while the broad signal at g = 1.73 accounts for 60 to 70% of the theoretical spins in Compound I. The g = 1.73 broad signal is reminiscent of the broad EPR signal associated with horseradish peroxidase Compound I. however, the chloroperoxidase Compound I signal has a significantly different g value. The results suggest that the g = 1.73 signal represents a porphyrin pi cation radical which has a stronger coupling to the heme ferryl iron than is the case with horseradish peroxidase Compound I.