Zero-Field Splitting Parameters of Hemin Investigated by High-Frequency and High-Pressure Electron Paramagnetic Resonanc
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Applied Magnetic Resonance
REVIEW
Zero‑Field Splitting Parameters of Hemin Investigated by High‑Frequency and High‑Pressure Electron Paramagnetic Resonance Spectroscopy Eiji Ohmichi1 · Tsubasa Okamoto1 · Takahiro Sakurai2 · Hideyuki Takahashi3 · Susumu Okubo3 · Hitoshi Ohta3 Received: 6 July 2020 / Revised: 24 July 2020 © Springer-Verlag GmbH Austria, part of Springer Nature 2020
Abstract We systematically studied the zero-field splitting (ZFS) parameters of Fe(III) protoporphyrin IX chloride, or hemin, using the terahertz electron paramagnetic resonance (EPR) spectroscopy technique at ambient and high pressures. Although hemin is known as a model substance of hemoproteins, the pressure effect on the electronic structure has not yet been explored owing to the large ZFS. In this study, high-field and high-frequency EPR measurements were carried out in the frequency range up to 700 GHz and at hydrostatic pressures up to 2 GPa. At ambient pressure, multiple EPR branches were clearly observed, and the axial and rhombic components of ZFS were determined as D = 6.90 ± 0.01 cm−1 and E = 0.055 ± 0.005 cm−1 , respectively. Upon pressure application, we observed a systematic shift of the resonance field, indicating a monotonous increase of the axial component from D = 6.9 to 7.9 cm−1 at 2 GPa. The origin of this unusually large shift was discussed from a microscopic viewpoint of the electronic structure of iron under pressure.
* Hitoshi Ohta hohta@kobe‑u.ac.jp Eiji Ohmichi [email protected]‑u.ac.jp 1
Graduate School of Science, Kobe University, 1‑1 Rokkodai‑cho, Nada, Kobe 657‑8501, Japan
2
Research Facility Center for Science and Technology, Kobe University, 1‑1 Rokkoadai‑cho, Nada, Kobe 657‑8501, Japan
3
Molecular Photoscience Research Center, Kobe University, 1‑1 Rokkodai‑cho, Nada, Kobe 657‑8501, Japan
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1 Introduction Porphyrin complexes play important roles in biochemistry [1, 2]. In particular, metalloporphyrins [3], in which a metal ion is located at the center of a porphyrin ring, are widely found in numerous proteins and enzymes, such as chlorophyll and hemoproteins. The metal ions in these complexes play functional roles in biological processes such as redox, catalysis, and electron transfer. In this sense, the electronic structures of metal ions in porphyrin complexes are crucial for understanding the functional roles of metalloproteins and metalloenzymes from a microscopic viewpoint. It is known that hemoproteins are among the most important metalloproteins in various biological systems [4, 5]. For example, hemoglobin transports oxygen molecules in blood, and its functional center is iron protoporphyrin IX, which is also known as heme. Hemoglobin controls whether the oxygen molecule is captured or released at the iron site, depending on subtle differences in the environment. During this process, the molecular structure of heme changes accordingly; thus, the microscopic study of the electronic structure of heme, especially around an iron atom, is very impo
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