George Cutsail Iii

Chalcogenide radical cations are typically transient species. By one-electron oxidation of kinetically-stabilised arylchalcogenyl iodides, three stable representatives could be isolated and fully characterised.

Biocatalytic metal-containing clusters are nanometer-sized chemical reactors that enable enzymes to perform chemistry far beyond what would be possible with amino acids alone ^1,2 . These clusters display a remarkable variability ³ and elucidating their exact mechanisms is often difficult to determine, requiring input from a multitude of techniques ranging from spectroscopic to structural and theoretical methods. Thus, for unknown clusters, it is essential to accumulate, collate, and interpret as much information as possible from every relevant technique available. Here we report the discovery and in-depth, interdisciplinary characterization of an entirely novel, [Cu-4Fe-4S] cluster in the protein acetol dehydrogenase (AceDH). AceDH, isolated directly from Methylacidiphilum fumariolicum SolV cells, catalyzed the oxidation of acetol to methylglyoxal, proving its role in the 2-propanol/acetone metabolism of methanotrophs ^4–6 . Structural, spectroscopic and electrochemical analyses reveal the [Cu-4Fe-4S] cluster has a unique three-dimensional- and electronic structure involving electronic coupling between the copper and one of the iron atoms, likely contributing to its high, +275 mV, redox potential. The binding site for the novel cluster is composed of two protein subunits and involves a novel motif. These findings expand the known repertoire of biological metal cofactors and provide insight into how heterometallic clusters are adapted for biological processes.

A chemically versatile, low-coordinate imido iron complex is reported.

The nominally trigonal, pseudo-Jahn-Teller (PJT)-active, S = 1/2 N₂-bound complexes, P₃^EM(N₂), M = Fe, Co, with three in-plane phosphine ligands and axial donors, E = Si, B, C, include functional nitrogenase models that catalyze the reduction of N₂ to NH₃. We applied EPR, ³¹P ENDOR spectroscopy, and DFT computations to characterize the PJT-induced distortions of four selected P₃^EM(N₂), revealing how the metal ion and axial ligand E together tune both PJT dynamics, as revealed by ³¹P ENDOR and N₂ activation, as indicated by a decrease in N≡N stretching frequency, ν(N≡N). P₃^SiFe(N₂), P₃^BFe(N₂)^-, and P₃^BCo(N₂) each exhibit a single ³¹P isotropic hyperfine coupling, revealing dynamic pseudorotation of the PJT distortion, producing averaged C₃ symmetry with equivalent phosphine ligands. Conversely, P₃^CCo(N₂)⁺ exhibits a static PJT distortion directed toward a phosphine bonded to a d_x2-y2 SOMO lobe, leading to the exceptional isotropic ³¹P coupling a_iso(³¹P) = +250 MHz, leaving two phosphines, a_iso(³¹P) ≈ -20 MHz. Importantly, minimization of metal 'doming' out of the P₃^CCo(N₂)⁺ phosphine plane toward N₂ imposed by C-Co bond 'inelasticity' yields the longest M-N₂ bond and the least-activated N₂, as measured by ν(N≡N). Comparisons reveal an unrecognized correlation among PJT distortion, M-E bond elasticity, and N₂ activation, providing guidelines for designing bioinspired N₂-reduction catalysts.

Chalcogenide radical cations [R₂E]^•+ (E = S, Se, Te) are commonly short-lived intermediates of fundamental interest. Sulfide radical cations in particular are associated in vivo with oxidative stress and neuropathological processes. Having succeeded in the preparation of meta-terphenyl-based dichalcogenide radical cations [R₂E₂]^•+ (E = S, Se, Te), and a telluride analogue [R₂Te]^•+ in the past, we aimed to complete the series regarding sulfur and selenium. Here we report on the single-electron oxidation of diarylchalcogenides M^SFluindPhE (E = S, Se, Te; M^SFluind = dispiro[fluorene-9,3’-(1’,1’,7’,7’-tetramethyl-s-hydrindacen-4’-yl)-5’,9”-fluorene]) using XeF₂ in the presence of K[B(C₆F₅)₄], which afforded deeply coloured and isolable radical cation salts [M^SFluindPhE][B(C₆F₅)₄] (E = S, Se, Te). Structural and electronic properties were characterised by electron paramagnetic spectroscopy, cyclic voltammetry, optical absorption spectroscopy and single crystal X-ray diffraction (E = Se, Te), combined with extensive quantum mechanical computations.

Iron-sulfur clusters fulfill numerous roles throughout biology. The reduced [2Fe-2S]⁺ cluster offers unique electronic and magnetic properties due to its mixed-valent nature and can serve as an essential model for understanding electron transfer, electron delocalization, and accessible spin states not only in mixed-valent dimers, but potentially larger iron sulfur clusters. Recently a series of mixed-valent diiron dichalcogenide complexes [L₂Fe₂Q₂]⁻ (Q = S (1), Se (2), Te (3), L = 2,6-diisopropylphenyl β-diketiminate ligand) were synthesized and characterized, where complex 1 showed a typical S = 1/2 spin state, while complexes 2 and 3 exhibited intermediate S = 3/2 spin states, potentially enabled by the minimization of vibronic coupling. Here we studied the vibrational dynamics of the Fe and Te centers in these complexes using ⁵⁷Fe and ¹²⁵Te nuclear resonance vibrational spectroscopy (NRVS), coupled with DFT calculations. The findings suggest that heavy character of larger chalcogen atoms results in decreased vibronic coupling. The observation of an intermediate spin state is shown to be unattainable for lighter Fe₂Q₂ cores. This highlights the crucial role of vibronic coupling in modulating the electronic structure of mixed-valence systems and should enhance understanding of the electronic structure in more complex biological Fe-S clusters.

Lytic Polysaccharide Monooxygenases (LPMOs) catalyze the oxidative depolymerization of polysaccharides at a monocopper active site, that is coordinated by the so-called histidine brace. In the past, this motif has sparked considerable interest, mostly due to its ability to generate and stabilize highly oxidizing intermediates during catalysis. We used a variety of advanced EPR techniques, including Electron Nuclear Double Resonance (ENDOR), Electron Spin Echo Envelope Modulation (ESEEM) and Hyperfine Sublevel Correlation (HYSCORE) spectroscopy in combination with isotopic labelling (¹⁵N, ²H) to characterize the active site of the bacterial LPMO SmAA10A over a wide pH range (pH 4.0–pH 12.5). At elevated pH values, several ligand modifications are observed, including changes in the H_xO ligand coordination, but also regarding the protonation state of the histidine brace. At pH > 11.5, the deprotonation of the two remote nitrogen nuclei of the imidazole moieties and of the terminal amine is observed. These deprotonations are associated with major electronic changes, including increased σ-donor capabilities of the imidazolates and an overall reduced interaction of the deprotonated amine function. This observation highlights a potentially more significant role of the imidazole ligands, particularly for the stabilization of potent oxidants during turnover. The presented study demonstrates the application of advanced EPR techniques for a thorough characterization of the active site in LPMOs, which ultimately sets a foundation for and affords an outlook on future applications characterizing reaction intermediates.

EPR and ENDOR spectroscopy of ¹⁷O labelled copper complexes yields insights into Cu–O bond strengths depending on H_xO speciation and coordination positions.

Lytic polysaccharide monooxygenases (LPMOs) are enzymes that catalyze the oxidative depolymerization of polysaccharides through activation of strong C-H bonds. Their active site is composed of a copper ion that is coordinated by the so-called histidine brace, consisting of two histidine residues that coordinate the metal ion in a T-shaped arrangement. The unique structure of this motif and its suggested ability to generate and stabilize highly oxidizing intermediates has sparked considerable interest, leading to various hypotheses on its role for LPMOs and LPMO-inspired biomimetic complexes. To gain detailed insights into the electronic and geometric structure of the histidine brace and test its chemical variability, we have used advanced electron paramagnetic resonance (EPR) techniques, in combination with isotopic labelling (15N, 2H) of the AA10 LPMO SmAA10A over a wide pH range (pH 4.0 - pH 12.5), in which the monocopper site remains intact and the protein does not exhibit permanent damage. Between pH 4 and 11.5, Electron Nuclear Double Resonance (ENDOR) spectroscopy identifies and discriminates between coordinating waters and hydroxo ions (pKa1 = 9.65) at an open-site of the histidine brace. Above pH 11.5, deprotonation of the two remote nitrogen nuclei of the coordinating imidazole moieties and of the coordinating N-terminal amine function was observed via ENDOR, Electron Spin Echo Envelope Modulation (ESEEM) and Hyperfine Sublevel Correlation (HYSCORE) spectroscopies. This is associated with major electronic changes in the histidine brace, including increased σ-donor capabilities of the imidazolates and an overall reduced interaction of the deprotonated amine function with the copper center. This observation highlights the possible larger role of the imidazole moieties, potentially stabilizing potent oxidants during turnover. The associated spectroscopic and electronic changes of the LPMO are discussed in reference with functional biomimetic complexes. The presented study demonstrates the application of advanced EPR techniques for a thorough characterization of the active site in LPMOs, which ultimately sets a foundation for and affords an outlook on future applications characterizing reaction intermediates.

Oxidoreductases have evolved tyrosine/tryptophan pathways that channel highly oxidizing holes away from the active site to avoid damage. Here we dissect such a pathway in a bacterial LPMO, member of a widespread family of C-H bond activating enzymes with outstanding industrial potential. We show that a strictly conserved tryptophan is critical for radical formation and hole transference and that holes traverse the protein to reach a tyrosine-histidine pair in the protein’s surface. Real-time monitoring of radical formation reveals a clear correlation between the efficiency of hole transference and enzyme performance under oxidative stress. Residues involved in this pathway vary considerably between natural LPMOs, which could reflect adaptation to different ecological niches. Importantly, we show that enzyme activity is increased in a variant with slower radical transference, providing experimental evidence for a previously postulated trade-off between activity and redox robustness.