Matthew T. Kieber-Emmons

The mitochondrial electron transport chain (ETC) of Plasmodium malaria parasites is a major antimalarial drug target, but critical cytochrome (cyt) functions remain unstudied and enigmatic. Parasites express two distinct cyt c homologs ( c and c -2) with unusually sparse sequence identity and uncertain fitness contributions. P. falciparum cyt c -2 is the most divergent eukaryotic cyt c homolog currently known and has sequence features predicted to be incompatible with canonical ETC function. We tagged both cyt c homologs and the related cyt c ₁ for inducible knockdown. Translational repression of cyt c and cyt c ₁ was lethal to parasites, which died from ETC dysfunction and impaired ubiquinone recycling. In contrast, cyt c -2 knockdown or knockout had little impact on blood-stage growth, indicating that parasites rely fully on the more conserved cyt c for ETC function. Biochemical and structural studies revealed that both cyt c and c -2 are hemylated by holocytochrome c synthase, but UV-vis absorbance and EPR spectra strongly suggest that cyt c -2 has an unusually open active site in which heme is stably coordinated by only a single axial amino acid ligand and can bind exogenous small molecules. These studies provide a direct dissection of cytochrome functions in the ETC of malaria parasites and identify a highly divergent Plasmodium cytochrome c with molecular adaptations that defy a conserved role in eukaryotic evolution.

The mitochondrial electron transport chain (ETC) of Plasmodium malaria parasites is a major antimalarial drug target, but critical cytochrome functions remain unstudied and enigmatic. Parasites express two distinct cyt c homologs ( c and c -2) with unusually sparse sequence identity and uncertain fitness contributions. P. falciparum cyt c -2 is the most divergent eukaryotic cyt c homolog currently known and has sequence features predicted to be incompatible with canonical ETC function. We tagged both cyt c homologs and the related cyt c ₁ for inducible knockdown. Translational repression of cyt c and cyt c ₁ was lethal to parasites, which died from ETC dysfunction and impaired ubiquinone recycling. In contrast, cyt c -2 knockdown or knock-out had little impact on blood-stage growth, indicating that parasites rely fully on the more conserved cyt c for ETC function. Biochemical and structural studies revealed that both cyt c and c -2 are hemylated by holocytochrome c synthase, but UV-vis absorbance and EPR spectra strongly suggest that cyt c -2 has an unusually open active site in which heme is stably coordinated by only a single axial amino-acid ligand and can bind exogenous small molecules. These studies provide a direct dissection of cytochrome functions in the ETC of malaria parasites and identify a highly divergent Plasmodium cytochrome c with molecular adaptations that defy a conserved role in eukaryotic evolution. SIGNIFICANCE STATEMENT: Mitochondria are critical organelles in eukaryotic cells that drive oxidative metabolism. The mitochondrion of Plasmodium malaria parasites is a major drug target that has many differences from human cells and remains poorly studied. One key difference from humans is that malaria parasites express two cytochrome c proteins that differ significantly from each other and play untested and uncertain roles in the mitochondrial electron transport chain (ETC). Our study revealed that one cyt c is essential for ETC function and parasite viability while the second, more divergent protein has unusual structural and biochemical properties and is not required for growth of blood-stage parasites. This work elucidates key biochemical properties and evolutionary differences in the mitochondrial ETC of malaria parasites.

Ribosomally synthesized post-translationally modified peptides (RiPPs) are ubiquitous and represent a structurally diverse class of natural products. The ribosomally encoded precursor polypeptides are often extensively modified post-translationally by enzymes that are encoded by coclustered genes. Radical S-adenosyl-l-methionine (SAM) enzymes catalyze numerous chemically challenging transformations. In RiPP biosynthetic pathways, these transformations include the formation of C–H, C–C, C–S, and C–O linkages. In this paper, we show that the Geobacter lovleyisbtM gene encodes a radical SAM protein, SbtM, which catalyzes the cyclization of a Cys/SeCys residue in a minimal peptide substrate. Biochemical studies of this transformation support a mechanism involving H-atom abstraction at the C-3 of the substrate Cys to initiate the chemistry. Several possible cyclization products were considered. The collective biochemical, spectroscopic, mass spectral, and computational observations point to a thiooxazole as the product of the SbtM-catalyzed modification. To our knowledge, this is the first example of a radical SAM enzyme that catalyzes a transformation involving a SeCys-containing peptide and represents a new paradigm for formation of oxazole-containing RiPP natural products.

Incorporating radical ligands into metal complexes is one of the emerging trends in the design of single-molecule magnets (SMMs). While significant effort has been expended to generate multinuclear transition metal-based SMMs with bridging radical ligands, less attention has been paid to mononuclear transition metal–radical SMMs. Herein, we describe the first α-diiminato radical-containing mononuclear transition metal SMM, namely, [κ²-PhTt^tBu]Fe(AdNCHCHNAd) (1), and its analogue [κ²-PhTt^tBu]Fe(CyNCHCHNCy) (2) (PhTt^tBu = phenyltris(tert-butylthiomethyl)borate, Ad = adamantyl, and Cy = cyclohexyl). 1 and 2 feature nearly identical geometric and electronic structures, as shown by X-ray crystallography and electronic absorption spectroscopy. A more detailed description of the electronic structure of 1 was obtained through EPR and Mössbauer spectroscopies, SQUID magnetometry, and DFT, TD-DFT, and CAS calculations. 1 and 2 are best described as high-spin iron(II) complexes with antiferromagnetically coupled α-diiminato radical ligands. A strong magnetic exchange coupling between the iron(II) ion and the ligand radical was confirmed in 1, with an estimated coupling constant J < −250 cm^–1 (J = −657 cm^–1, DFT). Calibrated CAS calculations revealed that the ground-state Fe(II)−α-diiminato radical configuration has significant ionic contributions, which are weighted specifically toward the Fe(I)-neutral α-diimine species. Experimental data and theoretical calculations also suggest that 1 possesses an easy-axis anisotropy, with an axial zero-field splitting parameter D in the range from −4 to–1 cm^–1. Finally, dynamic magnetic studies show that 1 exhibits slow magnetic relaxation behavior with an energy barrier close to the theoretical maximum, 2|D|. These results demonstrate that incorporating strongly coupled α-diiminato radicals into mononuclear transition metal complexes can be an effective strategy to prepare SMMs.

The strength of the relevant bonds in bond-making and bond-breaking processes can directly affect the overall efficiency of the process. Copper-oxygen sites are known to catalyze reactions with some of the most recalcitrant C-H bonds found in nature as quantified by bond dissociation free energy (BDFE), yet only a handful of copper-bound O-H bond strengths have been defined. Equally important in the design of synthetic catalysts is an understanding of the geometric and electronic structure origins of these thermodynamic parameters. In this report, the BDFE(OH) of two dicopper-hydroxo complexes, {[LCu]2-(µ-OH)}3+ and {[LCu]2-(µ-OH)}4+ (L = tris(2-pyridylmethyl)amine), were measured. Two key observations were made: i) the BDFE(OH) of these complexes were exceptionally high at 103.4 kcal/mol and 91.7 kcal/mol respectively, which are the highest condensed phase MO-H BDFE to date; and ii) that the higher oxidation state had a lower BDFE(OH), which is counter to expectations based on known mononuclear BDFE(OH) which increase with oxidation state. To understand the origin of these thermodynamic values, BDFE(OH) were measured and analyzed for the mononuclear complexes [LCu(OH2)]1+ and [LCu(OH2)]2+ in the same ligand environment. This treatment revealed “dinuclear effects” that include contributions from rehybridization of the oxygen, mixed-valency of the metals, magnetic exchange between the metals, and differences in solvation, and which are general with respect to [M]2-OH complexes to varying degrees. These analyses are important because they provide a starting point for rationally tuning the thermodynamics of catalytic intermediates broadly and for understanding how copper active-sites achieve activation of strong C-H bonds.

A flexible synthesis for highly substituted benzothietes that does not require flash-vacuum pyrolysis was developed. This allows for the use of a number of functional groups and nonvaporizable molecules. Highly stabilized derivatives were isolated. The molecular orbital properties of various benzothietes were evaluated by density functional methods. The mechanism of the cycloreversion of the four-membered ring was compared to that of the oxygen-containing analogues.

Methylammonium lead halide perovskite‐based solar cells have demonstrated efficiencies as high as 24.2 %, highlighting their potential as inexpensive and solution‐processable alternatives to silicon solar cell technologies. Poor stability towards moisture, ultraviolet irradiation, heat, and a bias voltage of the perovskite layer and its various device interfaces limits the commercial feasibility of this material for outdoor applications. Herein, we investigate the role of hydrogen bonding interactions induced when metal halide perovskite crystals are crosslinked with alkyl or π‐conjugated boronic acid small molecules (‐B(OH)₂). The crosslinked perovskite crystals are investigated under continuous light irradiation and moisture exposure. These studies demonstrate that the origin of the interaction between the alkyl or π‐conjugated crosslinking molecules is due to hydrogen bonding between the ‐B(OH)₂ terminal group of the crosslinker and the I of the [PbI₆]⁴⁻ octahedra of the perovskite layer. Also, this interaction influences the stability of the perovskite layer towards moisture and ultraviolet light irradiation. Morphology and structural analyses, as well as IR studies as a function of aging under both dark and light conditions show that π‐conjugated boronic acid molecules are more effective crosslinkers of the perovskite crystals than their alkyl counterparts thus imparting better stability towards light and moisture degradation.

Methylammonium lead halide perovskite‐based solar cells have demonstrated efficiencies as high as 24.2 %, highlighting their potential as inexpensive and solution‐processable alternatives to silicon solar cell technologies. Poor stability towards moisture, ultraviolet irradiation, heat, and a bias voltage of the perovskite layer and its various device interfaces limits the commercial feasibility of this material for outdoor applications. Herein, we investigate the role of hydrogen bonding interactions induced when metal halide perovskite crystals are crosslinked with alkyl or π‐conjugated boronic acid small molecules (‐B(OH)₂). The crosslinked perovskite crystals are investigated under continuous light irradiation and moisture exposure. These studies demonstrate that the origin of the interaction between the alkyl or π‐conjugated crosslinking molecules is due to hydrogen bonding between the ‐B(OH)₂ terminal group of the crosslinker and the I of the [PbI₆]⁴⁻ octahedra of the perovskite layer. Also, this interaction influences the stability of the perovskite layer towards moisture and ultraviolet light irradiation. Morphology and structural analyses, as well as IR studies as a function of aging under both dark and light conditions show that π‐conjugated boronic acid molecules are more effective crosslinkers of the perovskite crystals than their alkyl counterparts thus imparting better stability towards light and moisture degradation.

Oxygenation of a tricopper(I) cyclophanate (1) affords reactive transients competent for C–H bond activation and O atom transfer to various substrates (including toluene, dihydroanthracene, and ethylmethylsulfide) based on ¹H NMR, gas chromatography/mass spectrometry (MS), and electrospray ionization (ESI)/MS data. Low product yields (1H NMR, and density functional theory (DFT) results for reaction of 1 with O₂ are consistent with transient peroxo- and di(oxo)-bridged intermediates. DFT calculations elucidate a concerted proton-coupled electron transfer from toluene to the di(μ-oxo) intermediate and subsequent radical rebound as the C–H activation mechanism. Our results support a multicopper oxidase-like mechanism for O₂ activation by 1, traversing species similar to the coplanar Cu₃O₂ unit in the peroxy and native intermediates.