Electron-nuclear double resonance of horseradish peroxidase compound I. Detection of the porphyrin pi-cation radical.

Electron-nuclear double resonance spectra of horseradish peroxidase Compound I exhibit resonances from 14N and beta-protons, and most probably alpha-protons as well. The measurements prove that the oxyferryl iron of this enzymic intermediate is spin-coupled to a porphyrin radical. Comparison of experimentally obtained average pi-electron spin densities with theoretical predictions for a 4-fold symmetric porphyrin pi-cation radical is supportive of a 2A2u assignment for the radical's electronic state.