title: Zero-field splitting of Fe3+ in horseradish peroxidase and of Fe4+ in horseradish peroxidase compound I from electron spin relaxation data.
abstract: From the temperature dependence of the Orbach relaxation rate of the paramagnetic center in horseradish peroxidase (HRP), we deduce an excited-state energy of 40.9 +/- 1.1 K. Similar studies on the broad EPR signal of HRP compound I indicate a much weaker Orbach relaxation process involving an excited state at 36.8 +/- 2.5 K. The strength of the Orbach process in HRP-I is weaker than one would normally estimate by 2-4 orders of magnitude. This fact lends support to the model of HRP-I involving a spin 1/2 free radical coupled to a spin 1 Fe4+ heme iron via a weak exchange interaction. Such a system should exhibit an Orbach relaxation process involving delta E, the excited state of the Fe4+ ion, but reduced in strength by (Jyy/delta E)2, where Jyy is related to the strength of the exchange interaction between the two spin systems.
mesh_terms: D004578:Electron Spin Resonance Spectroscopy; D005290:Ferric Compounds / Q000378:metabolism*; D006735:Horseradish Peroxidase / Q000378:metabolism*; D007501:Iron / Q000378:metabolism*; D008433:Mathematics; D010544:Peroxidases / Q000378:metabolism*
chemical_list: D005290:Ferric Compounds; D007501:Iron; D006735:Horseradish Peroxidase; D010544:Peroxidases