Structural Disorder of Native Horseradish Peroxidase C Probed by Resonance Raman and Low-Temperature Optical Absorption Spectroscopy

Abstract

We have measured the low-temperature absorption and the depolarization ratio dispersion of various intense resonance Raman lines of horseradish peroxidase C. The absorption spectra reveal significant splitting of the Q and charge-transfer bands whereas the Soret band solely exhibits a narrowing of the band profile. The depolarization ratios of all Raman lines investigated are different from expectation values in D4h symmetry and show significant dispersion in the preresonance and resonance region of the Qv band. All these data indicate symmetry lowering distortions of the heme macrocycle. An analysis of the depolarization ratios shows that in-plane B1g- and B2g-type perturbations distort the heme along the N−Fe−N and Cm−Fe−Cm line of the heme. The B1g perturbation gives rise to the band splitting in the optical spectrum and to a rhombicity of the iron's ligand field detected by EPR experiments. On the basis of group theoretical arguments we propose that particularly the strong B2g distortion gives rise to the quantum mixed spin state characteristic of class III peroxidases.