title: 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.
abstract: 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.
authors.affiliation: Department of Chemistry, Stanford University, Stanford, California 94305, USA.
authors.forename: Rosalie K
authors.initials: RK
authors.lastname: Hocking
authors.forename: Erik C
authors.initials: EC
authors.lastname: Wasinger
authors.forename: Yi-Long
authors.initials: YL
authors.lastname: Yan
authors.forename: Frank M F
authors.initials: FM
authors.lastname: Degroot
authors.forename: F Ann
authors.initials: FA
authors.lastname: Walker
authors.forename: Keith O
authors.initials: KO
authors.lastname: Hodgson
authors.forename: Britt
authors.initials: B
authors.lastname: Hedman
authors.forename: Edward I
authors.initials: EI
authors.lastname: Solomon
mesh_terms: D004583:Electrons; D005290:Ferric Compounds / Q000737:chemistry*; D005296:Ferrous Compounds / Q000737:chemistry*; D006418:Heme / Q000737:chemistry*; D008024:Ligands; D019159:Nonheme Iron Proteins / Q000737:chemistry; D011166:Porphyrins / Q000737:chemistry*; D013057:Spectrum Analysis; D014965:X-Rays
chemical_list: D005290:Ferric Compounds; D005296:Ferrous Compounds; D008024:Ligands; D019159:Nonheme Iron Proteins; D011166:Porphyrins; D006418:Heme