Untersuchung der elektronischen Struktur von Enzymzwischenzuständen mit Hilfe von hochaufgelöster resonanter inelastischer Röntgenstreuung: Eine experimentelle und theoretische Studie
Zusammenfassung der Projektergebnisse
We developed a method for a detailed interpretation of 1s2p resonant inelastic x-ray scattering (1s2p RIXS) data in terms of σ and π bonding character. While the soft X-ray Fe L- edge absorption spectra exhibit a single electron excitation 2p→3d process, 1s2p RIXS is a two-photon process initiated by a 1s→3d quadrupole excitation, followed by a 2p→1s decay. We worked out the different selection rules between direct Fe L-edge X-ray absorption data and "L-edge-like" cuts along the direction of energy transfer from the 1s2p hard X-ray RIXS plane as well as different intensity mechanisms. A theoretical simulation using a valencebond configuration interaction model completed the investigation. As a result, we find additional intensity at lower energies in systems with local cubic symmetry around the central Fe ion, such as our model complexes ferrous and ferric tacn and cyanide. The additional intensity results from dipole forbidden transitions that become allowed through the quadrupole allowed intermediate step. The strength of the splitting qualitatively reflects the amount of ligand character in the 3d orbital wave functions. In ferroand ferricyanide, systems that are known for their strong π back-bonding character, the intensity mechanisms of a spectral feature assigned to π back-bonding are very different for RIXS and L-edge XAS, which makes them complimentary in experimentally determining the electronic structure. When going to systems with a local tetragonal symmetry, the selection rules change again due to the splitting of the formerly degenerate eg and t2g orbitals. We investigated the heme protein cytochrome c (cyt c), an electron transfer metalloprotein that is a low-spin hexacoordinate heme with S(Met) and N(His) axial ligands. The strength and liability of the Fe-S(Met) bond is of key importance to the functions of cyt c: electron transfer and peroxidase activity in apoptosis. Using a combination of L-edge and 1s2p RIXS for the model complexes FeII/IIITPP(ImH)2 and only 1s2p RIXS for the reduced and oxidized cyt c protein, we could identify differences in the electronic structures. Compared to the ferrous model complex, the vertical RIXS cut for reduced cyt c shows only slight differences, emphasizing the validity of the chosen model. However, we were able to model those spectral differences using a charge transfer multiplet model and identified the Fe-S bond in red. cyt c to be stronger than the Fe-N bond. More pronounced are the differences in the oxidized Fe(III) complexes. The changes in intensity and energy when comparing the ferric model with Fe(II) cyt c indicate a smaller crystal field splitting together with an increased π donation and a lower local symmetry. Further theoretical modeling is currently under way and is coupled to reactivity calculations. The electronic structure of the Fe-O2 center in oxy-hemoglobin and oxy-myoglobin is a longstanding issue in the field of bioinorganic chemistry. Most spectroscopic studies of dioxygen bonding in heme systems have been complicated by the highly delocalized nature of the porphyrin, and calculations have required analysis of a multi-determinant description of the highly covalent Fe-O2 bond. We demonstrated both the limitations of the oxidation state formalisms for this highly covalent (both π and σ) Fe-O2 bond, and that iron L-edge XAS allows one to directly focus on the iron center in this highly covalent environment. The L-edge data of the dioxygen bond in the heme complex [Fe(pfp)(1-MeIm)O2] do not exhibit the low-energy feature of a hole in the dπ orbital of the iron, as is characteristic of all low-spin ferric complexes. The absence of this feature requires a strong π-interaction between the iron and the O2 that will limit the extent of spin polarization in the Fe-O2 bond.
Projektbezogene Publikationen (Auswahl)
- Spectroscopic Methods for Understanding Metals in Proteins. In Comprehensive Inorganic Chemistry II, Vol 3; Pecoraro, V.; Hambley, T.W.; Reedijk, J. Eds. in press, Elsevier (2012)
E.I. Solomon, J.W. Ginsbach, T. Kroll, L.V. Liu, E.M. Pierce, and M.F. Qayyum
- Iron L-edge X-ray Absorption Spectroscopy of Oxy-Picket Fence Porphyrin: Experimental Insight into Fe-O2, Journal of the American Chemical Society 135, 1124 (2013)
S. A. Wilson, T. Kroll, R.A. Decreau, R.K. Hocking, M. Lundberg, B. Hedman, K.O. Hodgson, and E.I. Solomon
