Hocking Rosalie K, Wasinger Erik C, Yan Yi-Long, Degroot Frank M F, Walker F Ann, Hodgson Keith O, Hedman Britt, Solomon Edward I
Department of Chemistry, Stanford University, Stanford, California 94305, USA.
J Am Chem Soc. 2007 Jan 10;129(1):113-25. doi: 10.1021/ja065627h.
Hemes (iron porphyrins) are involved in a range of functions in biology, including electron transfer, small-molecule binding and transport, and O2 activation. The delocalization of the Fe d-electrons into the porphyrin ring and its effect on the redox chemistry and reactivity of these systems has been difficult to study by optical spectroscopies due to the dominant porphyrin pi-->pi(*) transitions, which obscure the metal center. Recently, we have developed a methodology that allows for the interpretation of the multiplet structure of Fe L-edges in terms of differential orbital covalency (i.e., differences in mixing of the d-orbitals with ligand orbitals) using a valence bond configuration interaction (VBCI) model. Applied to low-spin heme systems, this methodology allows experimental determination of the delocalization of the Fe d-electrons into the porphyrin (P) ring in terms of both P-->Fe sigma and pi-donation and Fe-->P pi back-bonding. We find that pi-donation to Fe(III) is much larger than pi back-bonding from Fe(II), indicating that a hole superexchange pathway dominates electron transfer. The implications of the results are also discussed in terms of the differences between heme and non-heme oxygen activation chemistry.
血红素（铁卟啉）参与生物学中的一系列功能，包括电子转移、小分子结合与运输以及氧气活化。由于占主导的卟啉π→π*跃迁会掩盖金属中心，通过光谱学研究铁d电子离域到卟啉环中及其对这些体系氧化还原化学和反应活性的影响一直很困难。最近，我们开发了一种方法，利用价键组态相互作用（VBCI）模型，根据微分轨道共价性（即d轨道与配体轨道混合的差异）来解释铁L边的多重结构。应用于低自旋血红素体系时，该方法能够根据卟啉（P）→铁σ和π给予以及铁→Pπ反馈键合，通过实验确定铁d电子离域到卟啉（P）环中的情况。我们发现，向Fe(III)的π给予远大于来自Fe(II)的π反馈键合，这表明空穴超交换途径主导电子转移。还根据血红素与非血红素氧活化化学之间的差异讨论了结果的意义。
