| Article ID | Journal | Published Year | Pages | File Type |
|---|---|---|---|---|
| 1317878 | Journal of Inorganic Biochemistry | 2011 | 7 Pages |
The low-lying electronic states of the ferrous high-spin heme in deoxy-myoglobin (deoxy-Mb) and deoxy-hemoglobin (deoxy-Hb) were probed by multi-frequency electron paramagnetic resonance (MFEPR) spectroscopy. An unexpected broad EPR signal was measured at the zero magnetic field using cavity resonators at 34–122 GHz that could not be simulated using any parameter sets for the S = 2 spin Hamiltonian assuming spin quintet states in the 5B2 ground state. Furthermore, we have observed novel, broad EPR signals measured at 70–220 GHz and 1.5 K using a single pass transmission probe. These signals are attributed to the ferrous high-spin heme in deoxy-Mb and deoxy-Hb. The resonant peaks shifted to a higher magnetic field with increasing frequency. The energy level separation between the ground singlet and the first excited state at the zero magnetic field was directly estimated to be 3.5 cm− 1 for deoxy-Hb. For deoxy-Mb, the first two excited singlet states are separated by 3.3 cm− 1 and 6.5 cm− 1, respectively, from the ground state. The energy gap at the zero magnetic field is directly derived from our MFEPR for deoxy-Mb and deoxy-Hb and strongly supports the theoretical analyses based on the Mössbauer and magnetic circular dichroism experiments.
Graphical abstractThe low-lying electronic states of the ferrous high spin (S = 2) heme in deoxy-Mb and deoxy-Hb were probed by a multi-frequency EPR spectroscopy at 1.5 K. From the frequency versus resonance field diagrams, the energy level separations between the ground singlet and the excited states were directly evaluated.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► We report novel EPR signals of deoxy-Hb and deoxy-Mb by multi-frequency EPR at 1.5 K. ► The resonant peak shifts to higher magnetic field upon increasing frequency. ► Energy gaps between the ground singlet and the excited ones are directly evaluated. ► The observed EPR absorptions can't be interpreted by a spin Hamiltonian formalism. ► Our EPR results strongly support the theoretical analyses for Mössbauer and MCD.
