Department of Chemistry, University of Isfahan, 81746-73441 Isfahan, Iran.
Phys Chem Chem Phys. 2019 Jan 23;21(4):1750-1760. doi: 10.1039/c8cp06210b.
The binding energy of CO, O2 and NO to isolated ferric heme, [FeIIIP]+, was studied in the presence and absence of a σ donor (N-methylimidazole and histidine) as the trans axial ligand. This study combines the experimental determination of binding enthalpies by equilibrium measurements in a low temperature ion trap using the van't Hoff equation and high level DFT calculations. It was found that the presence of N-methylimidazole as the axial ligand on the [FeIIIP]+ porphyrin dramatically weakens the [FeIIIP-ligand]+ bond with an up to sevenfold decrease in binding energy owing to the σ donation by N-methylimidazole to the FeIII(3d) orbitals. This trans σ donor effect is characteristic of ligation to iron in hemes in both ferrous and ferric redox forms; however, to date, this has not been observed for ferric heme.
在存在和不存在 σ 供体（N-甲基咪唑和组氨酸）作为轴向配体的情况下，研究了 CO、O2 和 NO 与游离高铁血红素[FeIIIP]+的结合能。本研究结合了低温离子阱中平衡测量的实验确定的结合焓，使用范特霍夫方程和高精度 DFT 计算。结果发现，N-甲基咪唑作为轴向配体存在于[FeIIIP]+卟啉上，由于 N-甲基咪唑向 FeIII(3d)轨道的σ 供体，[FeIIIP-配体]+键显著减弱，结合能降低了多达七倍。这种反 σ 供体效应是亚铁和高铁氧化还原形式中血红素中铁的配位的特征；然而，迄今为止，尚未在高铁血红素中观察到这种情况。
