synchrotron-based spectroscopic and computational studies of bioinorganic and organometallic systems

SzilagyiMontana.EDU

Modern density functional theory is one of the most popular methods in computational chemistry. However, the commonly used functionals generally overestimate the bonding in inorganic complexes, such as the Cu, Ni, and Fe thiolates. Using K- and L-edge X-ray absorption spectroscopies, the composition of low-lying, unoccupied orbitals can be determined. These experimental wave function properties are unique in the sense that they can be compared without scaling/shifting to computed wave function properties by DFT theories. I use experimental ground and excited state wave functions to define a novel family of density functionals providing benefits beyond inorganic chemical applications.

Selected Publications

Szilagyi RK, Lim BS, Glaser T, Holm RH, Hedman B, Hodgson KO, Solomon EI:
Description of the ground state wave function of Ni-dithiolenes using sulfur K-edge X-ray absorption spectroscopy
J. Am. Chem. Soc. 125 9158-9169 (2003)

Szilagyi RK, Metz M, Solomon EI:
Spectroscopic calibration of modern density functional theory using [CuCl₄]^2-
J. Chem. Phys. A 106 2994-3007 (2002)

Keywords:
Computational, Inorganic, Physical, Spectroscopy, Structure

My research interest covers three highly interdisciplinary fields of chemistry: bioinorganic structure and reactivity (see A below), computational method development (see B below), and physical organometallic chemistry and catalysis (see C below). The key aspect of my research philosophy is the close correlation of experiment and theory. Therefore, I use spectroscopic techniques (mainly near-edge X-ray absorption spectroscopy (XAS) and Extended X-ray Absorption Fine Structure (EXAFS) to directly probe electronic and geometric structures of inorganic and organometallic systems. These XAS measurements are carried out at the beamlines of Stanford Synchrotron Radiation Laboratory, Menlo Park, CA and Advanced Light Source, Berkeley, CA. In addition, I employ a broad range of computational chemical methods, including forcefields, semi-empirical Hamiltonians, ab initio molecular orbital and density functional calculations to support and interpret experimental findings.

One of the most tantalizing challenges at the interface of biology and chemistry is the understanding of complex biochemical processes in which inert molecules, such as H2, N2 and CO2, are transformed into more reactive forms, such as protons, electrons, ammonia, and methane. These processes occur at ambient temperature and pressure; therefore their technological implementations would be valuable. These novel \'green chemical\' transformations developed by learning from Nature, would also be environmentally sound, hazardous waste-free technologies.
N-cycle: The active-site of the nitrogenase metalloenzyme is the so-called iron-molybdenum-cofactor (FeMo-co), where the N2 binding and reduction occur. The FeMo-co has a rich activation chemistry, since it is capable of reducing N-N, C-N, C-C triple and double bonds such as in N2, cyanides, imines, alkines and alkenes. Inhibition studies indicate multiple substrate binding sites. Side-directed point-mutations reveal an intricate role of the protein environment. Other studies show the critical role of the homocitrate ligand. I am investigating the molecular basis of these environmental effects and the mechanism of the N-cycle.
H-cycle: The hydrogenases are responsible for H2 reduction and oxidation reactions. In the microorganism Clostridium pasteurianum, the H-cycle is coupled to the N-cycle. The evolved H2 as a sideproduct is recycled by hydrogenases. The active-site structure has revealed a biochemically unusual coordination of Fe ions with cyanide and carbonyl ligands. The [4Fe-4S] cluster, which is assumed to be the electron storage for the redox chemistry, can donate or accept electrons formed in H2 evolution or uptake, respectively. The H-cluster is the site where the bond cleavage/formation occurs. The roles of the iron atoms, the bridging dithiolate, and the CO/CN ligands are the focus of my research.

Selected Publications

Szilagyi RK, Bryngelson PA, Maroney MJ, Hedman B, Hodgson KO, Solomon EI:
S K-Edge X-ray Absorption Spectroscopic Investigation of the Ni-Containing Superoxide Dismutase Active Site: New Structural Insight into the Mechanism
J. Am. Chem. Soc. 126 3018-3019 (2004)

Frank P, Benfatto M, Szilagyi RK, De\'Angelo P, Della Longa S, Hodgson K:
The Solution Structure of [Cu(aq)]²⁺ and Its Implications for Rack-Induced Bonding in Blue Copper Protein Active Sites
Inorg. Chem. 44 1922-1933 (2005)

Szilagyi RK, Schwab DE :
Sulfur K-edge X-ray absorption spectroscopy as an experimental probe for S-nitroso proteins
Biochem. Biophys. Res. Commun. 330 60-64 (2005)

Keywords:
Bioinorganic, Biophysical, Computational, Inorganic, Mechanism, Physical, Spectroscopy, Structure

The importance of organic phosphines in organometallic chemistry is indisputable. The extention of current spectroscopic techniques with XAS is promising in studying phosphorous compounds. The P K-edge XAS directly probes the phosphorous-based orbitals, i.e. the metal-phosphorous bond. In addition, structural information can be obtained by analyzing the post-edge EXAFS region of the XAS spectrum. By introducing the P K-edge XAS technique, my research aims to provide complimentary ground state information to NMR spectroscopy. This can give additional insight into coordination chemical and catalytic problems directly at the level of molecular orbitals.

Selected Publications

Szabo M, Szilagyi RK, Bencze L:
Density functional studies of [(alkoxy-carbonyl)methyl]cobalt tricarbonyl triphenylphosphine complexes: an alpha-ester hapto³-coordination
Inorg. Chim. Acta 344 158-168 (2003)

Solomon EI, Hedman B, Hodgson KO, Dey A, Szilagyi RK:
Ligand K-edge X-ray absorption spectroscopy: covalency of ligand-metal bonds
Coord. Chem. Rev. 249 97-129 (2005)

Keywords:
Computational, Inorganic, Physical, Spectroscopy, Structure

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