Spectroscopic Elucidation of Electronic Structure Contributions to Electron Transfer and Oxo-Atom Transfer

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This award by the Chemical Structure, Dynamics, and Mechanisms (CSDM) Program of the Chemistry Division to Professor Edward I. Solomon of Stanford University supports the use of a wide range of spectroscopic methods coupled with electronic structure calculations to understand the unique spectral features of metalloprotein active sites involved in biological electron transfer (ET) and oxo atom transfer (OAT). Novel active site geometric and electronic structures can make major contributions to reactivity. New spectroscopic methods are developed as part of the program including: variable-temperature, variable-field magnetic circular dichroism (VTVH MCD) to assign electronic transitions and use excited states to probe ground states, sulfur K-edge x-ray absorption spectroscopy (XAS) to experimentally define the covalency of ligand-metal bonds, and metal L-edge XAS to quantify the delocalization of the different metal d orbitals in highly covalent environments. This research project defines the ET efficiencies of specific superexchange pathways in blue copper proteins and experimentally determines the key factors that tune their reduction potentials over hundreds of millivolts. The graduate students involved in the project extend recent results on protein control of the thioether-Cu bond in blue copper to the mixed-valent binuclear CuA center and to cytochrome c (cyt c) active sites. The iron-sulfur bond in cyt c, its contribution in ET, and its role in the peroxidase activity of cyt c in apoptosis is also under study. Solvent tuning of the covalency of iron-sulfur bonds in iron-sulfur proteins and the effects of the protein, DNA, S-adenosylmethionine, and ATP binding on the bonding are evaluated to determine how these structures contribute to the activation of the different ET functions of these clusters. Finally, the research group experimentally calibrates electronic structure calculations and uses experimentally-validated calculations to understand the oxo transfer process and proton coupled ET in the oxo-Mo transferases and in the non-innocent dithiolene ligation.This research is of fundamental importance in understanding electron transfer and oxygen atom transfer in biology and is relevant to human health (understanding and treating disease states, apoptosis and photodynamic therapy, and DNA repair), biotechnology (oxidoreductases and bio-fuel cells), and bioremediation (elimination of the greenhouse gas, N2O). New spectroscopic techniques developed as part of this research allow for the evaluation of computational methods that have applicability across many scientific disciplines. These methods impact the bioinorganic community through a wide range of collaborations. Professor Solomon promotes new and novel spectroscopic methods during his lectures, book chapters, and edited texts. There is also an organized effort to disseminate the laboratory's expertise through Wikipedia. Within the community, the Solomon group hosts Bay Area high school chemistry teachers in summer research and is involved in establishing an apprenticeship program in collaboration with a youth empowerment agency. This group promotes the scientific participation of minority middle school students.