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A Thioether-Ligated Cupric Superoxide Model with Hydrogen Atom Abstraction Reactivity
journal contribution
posted on 2021-03-08, 22:29 authored by Mayukh Bhadra, Wesley J. Transue, Hyeongtaek Lim, Ryan E. Cowley, Jung Yoon C. Lee, Maxime A. Siegler, Patrick Josephs, Gerald Henkel, Markus Lerch, Siegfried Schindler, Adam Neuba, Keith O. Hodgson, Britt Hedman, Edward I. Solomon, Kenneth D. KarlinThe central role of cupric superoxide
intermediates proposed in
hormone and neurotransmitter biosynthesis by noncoupled binuclear
copper monooxygenases like dopamine-β-monooxygenase has drawn
significant attention to the unusual methionine ligation of the CuM (“CuB”) active site characteristic
of this class of enzymes. The copper–sulfur interaction has
proven critical for turnover, raising still-unresolved questions concerning
Nature’s selection of an oxidizable Met residue to facilitate
C–H oxygenation. We describe herein a model for CuM, [(TMGN3S)CuI]+ ([1]+), and its O2-bound analog [(TMGN3S)CuII(O2•–)]+ ([1·O2]+).
The latter is the first reported cupric superoxide with an experimentally
proven Cu–S bond which also possesses demonstrated hydrogen
atom abstraction (HAA) reactivity. Introduction of O2 to
a precooled solution of the cuprous precursor [1]B(C6F5)4 (−135 °C, 2-methyltetrahydrofuran
(2-MeTHF)) reversibly forms [1·O2]B(C6F5)4 (UV/vis spectroscopy: λmax 442, 642, 742 nm). Resonance Raman studies (413 nm) using 16O2 [18O2] corroborated the
identity of [1·O2]+ by revealing
Cu–O (446 [425] cm–1) and O–O (1105
[1042] cm–1) stretches, and extended X-ray absorption
fine structure (EXAFS) spectroscopy showed a Cu–S interatomic
distance of 2.55 Å. HAA reactivity between [1·O2]+ and TEMPO–H proceeds rapidly (1.28 ×
10–1 M–1 s–1, −135 °C, 2-MeTHF) with a primary kinetic isotope effect
of kH/kD =
5.4. Comparisons of the O2-binding behavior and redox activity
of [1]+ vs [2]+, the
latter a close analog of [1]+ but with all
N atom ligation (i.e., N3S vs N4), are presented.