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Heme-Cu Binucleating Ligand Supports Heme/O2 and FeII-CuI/O2 Reactivity Providing High- and Low-Spin FeIII-Peroxo-CuII Complexes.

ABSTRACT: The focus of this study is in the description of synthetic heme/copper/O2 chemistry employing a heme-containing binucleating ligand which provides a tridentate chelate for copper ion binding. The addition of O2 (-80 °C, tetrahydrofuran (THF) solvent) to the reduced heme compound (PImH)FeII (1), gives the oxy-heme adduct, formally a heme-superoxide complex FeIII-(O2•-) (2) (resonance Raman spectroscopy (rR): ?O-O, 1171 cm-1 (?18O2, -61 cm-1); ?Fe-O, 575 cm-1 (?18O2, -24 cm-1)). Simple warming of 2 to room temperature regenerates reduced complex 1; this reaction is reversible, as followed by UV-vis spectroscopy. Complex 2 is electron paramagnetic resonance (EPR)-silent and exhibits upfield-shifted pyrrole resonances (? 9.12 ppm) in 2H NMR spectroscopy, indicative of a six-coordinate low-spin heme. The coordination of the tethered imidazolyl arm to the heme-superoxide complex as an axial base ligand is suggested. We also report the new fully reduced heme-copper complex [(PImH)FeIICuI]+ (3), where the copper ion is bound to the tethered tridentate portion of PImH. This reacts with O2 to give a distinctive low-temperature-stable, high-spin (S = 2, overall) peroxo-bridged complex [(PImH)FeIII-(O22-)-CuII]+ (3a): ?max, 420 (Soret), 545, 565 nm; ?pyrr, 93 ppm; ?O-O, 799 cm-1 (?18O2, -48 cm-1); ?Fe-O, 524 cm-1 (?18O2, -23 cm-1). To 3a, the addition of dicyclohexylimidazole (DCHIm), which serves as a heme axial base, leads to low-spin (S = 0 overall) species complex [(DCHIm)(PImH)FeIII-(O22-)-CuII]+ (3b): ?max, 425 (Soret), 538 nm; ?pyrr, 10.2 ppm; ?O-O, 817 cm-1 (?18O2, -55 cm-1); ?Fe-O, 610 cm-1 (?18O2, -26 cm-1). These investigations into the characterization of the O2-adducts from (PImH)FeII (1) with/without additional copper chelation advance our understanding of the dioxygen reactivity of heme-only and heme/Cu-ligand heterobinuclear system, thus potentially relevant to O2 reduction in heme-copper oxidases or fuel-cell chemistry.

SUBMITTER: Kim H

PROVIDER: S-EPMC6916717 | biostudies-literature | 2019 Nov

REPOSITORIES: biostudies-literature

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Heme-Cu Binucleating Ligand Supports Heme/O2 and FeII-CuI/O2 Reactivity Providing High- and Low-Spin FeIII-Peroxo-CuII Complexes.

Kim Hyun H Sharma Savita K SK Schaefer Andrew W AW Solomon Edward I EI Karlin Kenneth D KD

Inorganic chemistry 20191028 22

The focus of this study is in the description of synthetic heme/copper/O2 chemistry employing a heme-containing binucleating ligand which provides a tridentate chelate for copper ion binding. The addition of O2 (-80 °C, tetrahydrofuran (THF) solvent) to the reduced heme compound (PImH)FeII (1), gives the oxy-heme adduct, formally a heme-superoxide complex FeIII-(O2•-) (2) (resonance Raman spectroscopy (rR) ...[more]

PMID: 31657921

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Project description:Synthetic peroxo-bridged high-spin (HS) heme-(μ-η2:η1-O22-)-Cu(L) complexes incorporating (as part of the copper ligand) intramolecular hydrogen-bond (H-bond) capabilities and/or steric effects are herein demonstrated to affect the complex's electronic and geometric structure, notably impacting the spin state. An H-bonding interaction with the peroxo core favors a low-spin (LS) heme-(μ-η1:η1-O22-)-Cu(L) structure, resulting in a reversible temperature-dependent interconversion of spin state (5 coordinate HS to 6 coordinate LS). The LS state dominates at low temperatures, even in the absence of a strong trans-axial heme ligand. Lewis base addition inhibits the H-bond facilitated spin interconversion by competition for the H-bond donor, illustrating the precise H-bonding interaction required to induce spin-crossover (SCO). Resonance Raman spectroscopy (rR) shows that the H-bonding pendant interacts with the bridging peroxide ligand to stabilize the LS but not the HS state. The H-bond (to the Cu-bound O atom) acts to weaken the O-O bond and strengthen the Fe-O bond, exhibiting ν(M-O) and ν(O-O) values comparable to analogous known LS complexes with a strong donating trans-axial ligand, 1,5-dicyclohexylimidazole, (DCHIm)heme-(μ-η1:η1-O22-)-Cu(L). Variable-temperature (-90 to -130 °C) UV-vis and 2H NMR spectroscopies confirm the SCO process and implicate the involvement of solvent binding. Examining a case of solvent binding without SCO, thermodynamic parameters were obtained from a van't Hoff analysis, accounting for its contribution in SCO. Taken together, these data provide evidence for the H-bond group facilitating a core geometry change and allowing solvent to bind, stabilizing a LS state. The rR data, complemented by DFT analysis, reveal a stronger H-bonding interaction with the peroxo core in the LS compared to the HS complexes, which enthalpically favors the LS state. These insights enhance our fundamental understanding of secondary coordination sphere influences in metalloenzymes.

Project description: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-(μ-O2 2-)-Cu(XTMPA) 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 π* 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 H-bonding 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.

Dataset Information

Heme-Cu Binucleating Ligand Supports Heme/O2 and FeII-CuI/O2 Reactivity Providing High- and Low-Spin FeIII-Peroxo-CuII Complexes.

Publications

Heme-Cu Binucleating Ligand Supports Heme/O<sub>2</sub> and Fe<sup>II</sup>-Cu<sup>I</sup>/O<sub>2</sub> Reactivity Providing High- and Low-Spin Fe<sup>III</sup>-Peroxo-Cu<sup>II</sup> Complexes.

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