title: The temperature dependence of the MCD spectrum of horseradish peroxidase compound I.
abstract: The magnetic circular dichroism spectrum of the compound I species of horseradish peroxidase, which contains an iron (IV) porphyrin pi-cation radical complex, has been measured between 273 K and 4.2 K. The spectrum is temperature independent between 273 K and 30 K. However, very strong temperature dependence is observed below 30 K. These data do not appear to fit the temperature dependence expected for the presence of a simple MCD C term, or combination of C terms, but suggest that an increase in the coupling between the S = 1 iron (IV), and the S = 1/2 porphyrin pi-cation radical occurs forming a degenerate ground state. This increase in coupling below 30 K may be the result of a phase change in the protein which in turn affects the electronic structure of the heme group.
mesh_terms: D055598:Chemical Phenomena; D002621:Chemistry; D002942:Circular Dichroism; D006418:Heme; D006735:Horseradish Peroxidase / Q000032:analysis*; D010544:Peroxidases / Q000032:analysis*; D013696:Temperature
chemical_list: D006418:Heme; D006735:Horseradish Peroxidase; D010544:Peroxidases