Professor Julie Kovacs
Inorganic Synthesis and Mechanisms
We all benefit to learn from catalyst design principles perfected by Nature over billions of years. It inspires us! The Kovacs Group toolbox includes a variety of spectroscopic techniques, including EPR, Mössbauer, low T electronic absorption spectroscopy, and rR, as well as stopped-flow kinetics, electrochemistry, X-ray crystallography, inorganic and organic synthesis, and theoretical (DFT and TD-DFT) calculations. Specifically, we have a long-standing interest in determining how cysteinates influence the function of non-heme iron enzymes. Cysteinate-ligated non-heme iron enzymes facilitate tumor suppression, the biosynthesis of antibiotics, and the degradation of reactive oxygen species. The mechanisms by which these reactions are carried out are not well-understood.
Cysteinate-ligated non-heme iron superoxo intermediates (RS-Fe-O2•–) are proposed to play a key role1 in the biosynthesis of β-lactam antibiotics by isopenicillin N synthase (IPNS), as well as the regulation of cysteine by cysteine dioxygenase (CDO), toxic levels of which can lead to neurological disorders, or metastases of cancerous tumors. Dioxygen is proposed to bind cis to the cysteinate sulfur and trans to an aspartate oxygen to afford a ferric superoxo (Fe(III)-O2•–) intermediate. The putative IPNS Fe(III)-O2•– is only observable if the β-hydrogens adjacent to the sulfur are deuterated. The IPNS Fe(III)-O2•– intermediate is proposed to cleave a strong cysteine β C-H bond (93 kcal/mol) by to afford a putative unobserved Fe(III)-OOH intermediate and a carbon centered radical.
Very few well-characterized iron superoxo compounds have been reported, and of those reported only two incorporate a thiolate in the coordination sphere. We recently showed that alkyl thiolate-ligated 1 reacts with O2 to form an unprecedented example of a aliphatic thiolate-ligated iron superoxo intermediate, 2, which is capable of cleaving strong C-H bonds.
Spectroscopic evidence to support the formation of superoxo 2 includes electronic absorption and resonance Raman spectroscopy, low temperature 1H NMR, and ESI-MS. The calibrated density functional theory (DFT) calculated structure of 2 contains an O2 moiety cis to one of the thiolate sulfurs, with bond lengths (O-O= 1.289 Å, Fe-O= 1.947 Å, Fe-S= 2.203 Å), and a calculated 𝝂O-O stretch (1154 cm-1), consistent with a ferric superoxo The
O2•– radical of 2 was shown to be strongly coupled to a low-spin S=1/2 Fe(III) ion.
Superoxo 2 converts to a metastable low-spin S= 1/2 FeIII-OOH, at –73 ˚C in THF, at rates that are dependent on the C-H bond strength of the H-atom donor. A deuterium isotope effect kH/kD = 4.8 comparable to that of IPNS (kH/kD = 5.6) is observed if the reaction is carried out in d8 THF solvent. This indicates that superoxo 2 is capable of abstracting hydrogen atoms from the strong C-H bonds of THF (BDE= 92 kcal/mol) comparable to those cleaved by IPNS.