Journal
CHEMICAL SCIENCE
Volume 10, Issue 10, Pages 2893-2905Publisher
ROYAL SOC CHEMISTRY
DOI: 10.1039/c8sc05165h
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Funding
- U.S. National Institutes of Health [GM60353, DK031450]
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Dioxygen reduction by heme-copper oxidases is a critical biochemical process, wherein hydrogen bonding is hypothesized to participate in the critical step involving the active-site reductive cleavage of the O-O bond. Sixteen novel synthetic heme-(mu-O-2(2-))-Cu((X)TMPA) complexes, whose design is inspired by the cytochrome c oxidase active site structure, were generated in an attempt to form the first intramolecular H-bonded complexes. Derivatives of the parent ligand (XTMPA, TMPA = (tris((2-pyridyl) methyl) amine)) possessing one or two amine pendants preferentially form an H-bond with the copper-bound O-atom of the peroxide bridge. This is evidenced by a characteristic blue shift in the ligand-to-metal charge transfer (LMCT) bands observed in UV-vis spectroscopy (consistent with lowering of the peroxo p* relative to the iron orbitals) and a weakening of the O-O bond determined by resonance Raman spectroscopy (rR), with support from Density Functional Theory (DFT) calculations. Remarkably, with the TMPA-based infrastructure (versus similar heme-peroxo-copper complexes with different copper ligands), the typically undetected Cu-O stretch for these complexes was observed via rR, affording critical insights into the nature of the O-O peroxo core for the complexes studied. While amido functionalities have been shown to have greater H-bonding capabilities than their amino counterparts, in these heme-peroxo-copper complexes amido substituents distort the local geometry such that Hbonding with the peroxo core only imparts a weak electronic effect; optimal H-bonding interactions are observed by employing two amino groups on the copper ligand. The amino-substituted systems presented in this work reveal a key orientational anisotropy in H-bonding to the peroxo core for activating the O-O bond, offering critical insights into effective O-O cleavage chemistry. These findings indirectly support computational and protein structural studies suggesting the presence of an interstitial H-bonding water molecule in the CcO active site, which is critical for the desired reactivity. The results are evaluated with appropriate controls and discussed with respect to potential O2-reduction capabilities.
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