Understanding How Thiolates Promote Dioxygen Chemistry

Our research program is aimed at determining how cysteinates influence function in non-heme iron enzymes. Non-heme iron enzymes promote important biological reactions, including tumor suppression, the biosynthesis of antibiotics, scavenge reactive oxygen species, and detoxification of heavy metals. The mechanisms by which these reactions are carried out are not well understood.

In metalloenzymes, key details such as bond lengths, the presence or absence of protons, or the identity of intermediates, can be lacking, due to limitations imposed by either the biological system, or physical technique. Quite often these details can be revealed via small molecules analogues which can be synthetically tuned. We aim to spectroscopically characterize meta-stable cis-thiolate ligated Fe-dioxygen(O₂), Fe-superoxo (O^•-₂), Fe-peroxo (OOR) and Fe-oxo (O) intermediates. There are few examples of these species with thiolates in the coordination sphere. Our lab utilizes a wide range of techniques to characterize and elucidate the mechanisms of these dioxygen intermediates. Characterization of synthetic analogs is performed using X-ray crystallography, NMR, EPR, mass spectroscopies and cyclic voltammetry. The formation of meta-stable intermediates and their reactivity is monitored by low temperature UV-Vis spectroscopy. Resonance Raman spectroscopy is used to gain insights into the electron transitions and binding mode geometries of the dioxygen intermediates. To gain further insights into these systems, TD-DFT computations are carried out and calibrated to experimental spectra. This wide range of techniques provide benchmark parameters that aid in the better understanding of how thiolates influence reactivity and thus metalloenzyme function.

Understanding the Mechanism of the Oxygen-Evolving Complex

One of the biggest problems in chemistry today is finding an efficient way to convert solar energy into storable fuels. It’s also something that Nature has been doing well for millions of years by using photosynthesis to store energy in chemical bonds. The oxygen-evolving complex, where Nature accomplishes this, has been studied extensively in situ, however, the mechanism of the oxygen-oxygen bond forming event has yet to be determined.

Our lab hopes to elucidate the mechanism of oxygen-oxygen bond formation by creating synthetically tunable small molecule analogues to investigate the most prominent theories: the radical coupling (RC) mechanism wherein a Mn^IV-oxyl radical attacks a bridging oxo; and the nucleophilic attack (AB) mechanism, wherein a hydroxyl group attached to the OEC’s calcium atom attacks a Mn^V-oxo. We aim to spectroscopically characterize the intermediates formed in these reactions through a variety of methods, including X-ray crystallography, NMR, EPR, mass spectroscopy, X-ray absorption spectroscopies, resonance Raman spectroscopy, and cyclic voltammetry. The insights gained from studying these small molecule analogues will allow better study of the OEC itself, as well as provide information for creation of more effective artificial water oxidizing systems.