Low-temperature magnetic circular dichroism studies of the photoreaction of horseradish peroxidase compound I

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Abstract
Horseradish peroxidase (HRP) compound I is photolabile at all temperatures between room temperature and 4 K. The photoredox reaction has been studied in frozen glassy solutions by using optical absorption and magnetic circular dichroism spectra following photolysis of HRP compound I with visible-wavelengh light at 4.2 and 77 K. The photochemical process is characterized as a concerted two-electron transfer reaction which results in the conversion of the Fe(IV) heme .pi.-cation radical species of HRP compound I into a low-spin Fe(III) heme species. This reaction occurs even when photolysis is carried out at 4.2 K. Spectra recorded between 4.2 and 80 K for the low-spin ferric hydroxide complex of HRP closely resemble the data measured for the photochemical product. A proposed mechanism for the photoreaction is presented. No evidence is found for the formation of an Fe(II) heme at these temperatures.