We experimentally studied the pressure dependence of the zero-field splitting (ZFS) parameter of hemin (iron(III) protoporphyrin IX chloride), which is a model complex of hemoproteins, via high-frequency and high-field electron paramagnetic resonance (HFEPR) under pressure. Owing to the large ZFS, the pressure effect on the electronic structure of iron–porphyrin complexes has not yet been explored using EPR. Therefore, we systematically studied this effect using our newly developed sub-terahertz EPR spectroscopy system in the frequency range of 80–515 GHz, under magnetic fields up to 10 T and pressure up to 2 GPa. We observed a systematic shift of the resonance fields of hemin upon pressure application, from which the axial component of the ZFS parameter was found to increase from D = 6.9 to 7.9 cm^–1 at 2 GPa. In contrast to the previous methods used to study proteins under pressure, which mainly focused on conformational changes, our HFEPR technique can obtain more microscopic insights into the electronic structures of metal ions under pressure. In this sense, our technique provides novel opportunities to study the pressure effects on biofunctional active centers of versatile metalloproteins.

中文翻译：

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压力对血红素零场分裂参数的影响：压力下血红蛋白的模型实例

我们通过高频和高场电子顺磁共振（HFEPR）实验研究了血红素的模型复合物-血红素（铁（III）原卟啉IX氯化物）的零场分裂（ZFS）参数的压力依赖性在压力之下。由于ZFS较大，因此尚未使用EPR探索压力对铁-卟啉配合物电子结构的影响。因此，我们使用新开发的亚太赫兹EPR光谱系统在80-515 GHz的频率范围内，在磁场强度最高为10 T且压力压力最高为2 GPa的情况下，系统地研究了这种效应。我们观察到施加压力时血红素共振场的系统位移，从中发现ZFS参数的轴向分量从D = 6.9增加到7.9 cm在2 GPa时为^–1。与以前用于研究压力下蛋白质的方法（主要侧重于构象变化）相反，我们的HFEPR技术可以对压力下金属离子的电子结构获得更多的微观见解。从这个意义上讲，我们的技术为研究多功能金属蛋白对生物功能活性中心的压力作用提供了新的机会。