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In addition to my curiosity in bioinorganic chemistry and systematic approach to computational chemistry, synchrotron science truly has become my passion. Just thinking about the uncharted areas of the Periodic Table waiting to be explored by various synchrotron-enabled absorption and emission spectroscopic and scattering techniques gives me chills. Thus, in the past few years I have focused more and more on accessing various energy ranges at different synchrotron facilities and beamlines to be able to collect as much experimental information as technically feasible about the electronic structure of catalytically important inorganic and organometallic compounds. While at first this may have looked like a fishing expedition, with four publications we demonstrated a converging effort to illustrate the power of the multi-edge X-ray absorption spectroscopic (meXAS) technique. In collaboration with the Peters at Caltech and Mindiola groups at Indiana University, we published three papers in JACS, where the meXAS technique was critical to unambiguously demonstrate experimentally the previously undocumented non-innocent nature of the PPP and PNP pincer-type ligands for dinuclear Cu [1] and mononuclear Ni [2]. as well as aminyl ligands for mononuclear Cu [3] complexes, respectively. Their complexes gave me the opportunity to have demonstrative examples for the information content of the meXAS technique in addition to understanding the fundamental electronic structures of some catalytically important complexes.
Importantly with these two papers, we have not yet exhausted the possibility of future publications as we are gearing up for extending the earlier measurements to other metals and ligand derivatives. In a more methodological paper published in a special issue of Inorganica Chimica Acta [4], we laid the foundation of a new spectroscopic technique developed for experimentally mapping the catalytically important intermediates in palladium catalyzed organic and organometallic transformations. Using simple chloropalladium complexes we showed that transition dipole integrals for palladium L-shell core electron excitations can be empirically derived using already established dipole integrals for chloride K-shell excitation. Furthermore, we pointed out that caution is needed when comparing quantitative meXAS obtained on different beamlines at different synchrotron facilities. This has not been addressed previously in the literature. By developing the data normalization and quantitative analysis protocols for the palladium L-edge, we are in the position to continue this method development work for phosphorous, carbon, nitrogen, and oxygen-containing ligands.
An inspiring Department seminar triggered the idea of a sulfur K-edge XAS study on S-nitroso compounds that resulted in a BBRC publication [5]. Already from a ‘quick/dirty’ electronic structure analysis carried out immediately after the seminar we saw that there are at least three unique molecular orbitals in S-nitroso thiolates that could be used for detecting biochemically important S-nitrosated proteins. Importantly our experimental measurements further supported this assumption and we were even able to predict a possible spectrum for an S-nitrosated hemoglobin sample. These measurements provided solid preliminary results for a section of an institutional NIH COBRE grant. While our proposal for using XAS as a detection tool is still valid, unfortunately the orders of magnitude detection limit of XAS (milimolar) and the biologically relevant S-nitroso compound concentrations (micromolar) prohibits the use of this particular experimental technique. Recent development in high brightness synchrotron storage rings and beamlines soon will open up the possibility for these measurements.
An new direction for our research group grew out of research funded by the local node (ABRC center)of the NASA Astrobiology Institute. We teamed up the Minton group from the Department in creating and then characterizing modified Fe-S mineral surfaces. A recent publication [6] revealed the presence of a unique reduced Fe surface dubbed as Fe(I)2S that was formed from Fe(II)S2 pyrite upon exposure to beam of hydrogen atoms. This reduced Fe-S surface shows electronic structural similarity to the catalytically active cluster of hydrogenase and likely the nitrogenase (see Bioinorganic Structure/Function project).

Publications

  1. Harkins S.B., Mankad N.P., Miller A.J.M., Szilagyi R.K., Peters J.C. , "Probing the Electronic Structures of [Cu2(mu-XR2)]n+ Cores as a Function of the Bridging X Atom (X=N/P) and Charge (n=0/1/2)." Journal of the American Chemical Society, 2008, 130(11), 3478-3485
  2. Adhikari D., Mossin S., Basuli F., Huffman J.C., Szilagyi R.K., Meyer K., Mindiola D.J., "Structural, Spectroscopic, and Theoretical Elucidation of a Redox-Active Pincer-Type Ancillary Applied in Catalysis." Journal of the American Chemical Society, 2008, 130(11), 3676-3682
  3. Mankad N.P., Antholine W.E., Szilagyi R.K., Peters J.C., "Three-Coordinate Copper(I) Amido and Aminyl Radical Complexes." Journal of the American Chemical Society, 2009, 131(11), 3878-3881
  4. Boysen R.B., Szilagyi R.K.: , "Development of Palladium L-Edge X-Ray Absorption Spectroscopy and its Application for Chloropalladium Complexes." Inorganica Chimica Acta, 2008, 361(4), 1047-1058
  5. Szilagyi R.K., Schwab D.E., "Sulfur K-edge X-ray absorption spectroscopy as an experimental probe for S-nitroso proteins." Biochemical Biophysical Research Communication 330 60-64 (2005)
  6. Che Li, Gardenghi D.J., Szilagyi R.K., Minton T.K. , "Production of a Biomimetic Fe(I)-S Phase on Pyrite by Atomic Hydrogen Beam Surface Reactive Scattering ." Langmuir, 2011, 27(11), 6814-6821

Personnel:

Keywords:

Structure, Spectroscopy, Physical, Inorganic, Biophysical, Analytical