Zero-Field Splitting Parameters of Hemin Investigated by High-Frequency and High-Pressure Electron Paramagnetic Resonance Spectroscopy

• Published in: Applied magnetic resonance 2020-10, Vol.51 (9-10), p.1103-1115
• Main Authors: Ohmichi, Eiji, Okamoto, Tsubasa, Sakurai, Takahiro, Takahashi, Hideyuki, Okubo, Susumu, Ohta, Hitoshi
• Format: Article
• Language: English
• Subjects: Atoms and Molecules in Strong Fields; Electron paramagnetic resonance; Electronic structure; Electrons; Energy; Frequency ranges; Hemoglobin; High pressure; Iron; Laser Matter Interaction; Ligands; Molecular structure; Neutrons; Organic Chemistry; Parameters; Physical Chemistry; Physics; Physics and Astronomy; Pressure effects; Review; Solid State Physics; Spectroscopy/Spectrometry; Spectrum analysis; Splitting; Symmetry; Yakov S. Lebedev: on the Occasion of His 85th Anniversary
• ISSN: 0937-9347; 1613-7507
• DOI: 10.1007/s00723-020-01239-5

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.