Fe L-Edge X-ray Absorption Spectroscopy of Low-Spin Heme Relative to Non-heme Fe Complexes: Delocalization of Fe d-Electrons into the Porphyrin Ligand

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• 作者：Rosalie K. Hocking ; Erik C. Wasinger ; Yi-Long Yan ; Frank M. F. deGroot ; F. Ann Walker ; Keith O. Hodgson ; Britt Hedman ; Edward I. Solomon
• 刊名：Journal of the American Chemical Society
• 出版年：2007
• 出版时间：January 10, 2007
• 年：2007
• 卷：129
• 期：1
• 页码：113 - 125
• 全文大小：530K
• 年卷期：v.129,no.1(January 10, 2007)
• ISSN：1520-5126

文摘

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 theporphyrin ring and its effect on the redox chemistry and reactivity of these systems has been difficult tostudy by optical spectroscopies due to the dominant porphyrin mages/gifchars/pi.gif" BORDER=0 >mages/entities/rarr.gif">mages/gifchars/pi.gif" BORDER=0 >* transitions, which obscure the metalcenter. Recently, we have developed a methodology that allows for the interpretation of the multiplet structureof Fe L-edges in terms of differential orbital covalency (i.e., differences in mixing of the d-orbitals withligand orbitals) using a valence bond configuration interaction (VBCI) model. Applied to low-spin hemesystems, this methodology allows experimental determination of the delocalization of the Fe d-electronsinto the porphyrin (P) ring in terms of both Pmages/entities/rarr.gif">Fe mages/gifchars/sigma.gif" BORDER=0 > and mages/gifchars/pi.gif" BORDER=0 >-donation and Femages/entities/rarr.gif">P mages/gifchars/pi.gif" BORDER=0 > back-bonding. We findthat mages/gifchars/pi.gif" BORDER=0 >-donation to Fe(III) is much larger than mages/gifchars/pi.gif" BORDER=0 > back-bonding from Fe(II), indicating that a hole superexchangepathway dominates electron transfer. The implications of the results are also discussed in terms of thedifferences between heme and non-heme oxygen activation chemistry.