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1. Quinol-containing ligands enable high superoxide dismutase activity by modulating coordination number, charge, oxidation states and stability of manganese complexes throughout redox cycling

   https://doi.org/10.1039/D1SC02465E  

   Senft, Laura ; Moore, Jamonica L. ; Franke, Alicja ; Fisher, Katherine R. ; Scheitler, Andreas ; Zahl, Achim ; Puchta, Ralph ; Fehn, Dominik ; Ison, Sidney ; Sader, Safaa ; et al ( January 2021 , Chemical Science)

   Reactivity assays previously suggested that two quinol-containing MRI contrast agent sensors for H 2 O 2 , [Mn( H2qp1 )(MeCN)] 2+ and [Mn( H4qp2 )Br 2 ], could also catalytically degrade superoxide. Subsequently, [Zn( H2qp1 )(OTf)] + was found to use the redox activity of the H2qp1 ligand to catalyze the conversion of O 2 ˙ − to O 2 and H 2 O 2 , raising the possibility that the organic ligand, rather than the metal, could serve as the redox partner for O 2 ˙ − in the manganese chemistry. Here, we use stopped-flow kinetics and cryospray-ionization mass spectrometry (CSI-MS) analysis of the direct reactions between the manganese-containing contrast agents and O 2 ˙ − to confirm the activity and elucidate the catalytic mechanism. The obtained data are consistent with the operation of multiple parallel catalytic cycles, with both the quinol groups and manganese cycling through different oxidation states during the reactions with superoxide. The choice of ligand impacts the overall charges of the intermediates and allows us to visualize complementary sets of intermediates within the catalytic cycles using CSI-MS. With the diquinolic H4qp2 , we detect Mn( iii )-superoxo intermediates with both reduced and oxidized forms ofmore »the ligand, a Mn( iii )-hydroperoxo compound, and what is formally a Mn( iv )-oxo species with the monoquinolate/mono- para -quinone form of H4qp2 . With the monoquinolic H2qp1 , we observe a Mn( ii )-superoxo ↔ Mn( iii )-peroxo intermediate with the oxidized para -quinone form of the ligand. The observation of these species suggests inner-sphere mechanisms for O 2 ˙ − oxidation and reduction that include both the ligand and manganese as redox partners. The higher positive charges of the complexes with the reduced and oxidized forms of H2qp1 compared to those with related forms of H4qp2 result in higher catalytic activity ( k cat ∼ 10 8 M −1 s −1 at pH 7.4) that rivals those of the most active superoxide dismutase (SOD) mimics. The manganese complex with H2qp1 is markedly more stable in water than other highly active non-porphyrin-based and even some Mn( ii ) porphyrin-based SOD mimics.« less
2. Heterometallic [Cu–O–M] ²⁺ active sites for methane C–H activation in zeolites: stability, reactivity, formation mechanism and relationship to other active sites

   https://doi.org/10.1039/d1cy00687h  

   Adeyiga, Olajumoke ; Panthi, Dipak ; Odoh, Samuel O. ( August 2021 , Catalysis Science & Technology)

   The formation and reactivities of [Cu–O–M] 2+ species (M = Ti–Cu, Zr–Mo and Ru–Ag) in metal-exchanged zeolites, as well as stabilities of these species towards autoreduction by O 2 elimination are investigated with density functional theory. These species were investigated in zeolite mordenite in search of insights into active site formation mechanisms, the relationship between stability and reactivity as well as discovery of heterometallic species useful for isothermal methane-to-methanol conversion (MMC). Several [Cu–O–M] 2+ species (M = Ti–Cr and Zr–Mo) are substantially more stable than [Cu 2 O] 2+ . Other [Cu–O–M] 2+ species, (M = Mn–Ni and Ru–Ag) have similar formation energies to [Cu 2 O] 2+ , to within ±10 kcal mol −1 . Interestingly, only [Cu–O–Ag] 2+ is more active for methane activation than [Cu 2 O] 2+ . [Cu–O–Ag] 2+ is however more susceptible to O 2 elimination. By considering the formation energies, autoreduction, cost and activity towards the methane C–H bond, we can only conclude that [Cu 2 O] 2+ is best suited for MMC. Formation of [Cu 2 O] 2+ is initiated by proton transfer from aquo ligands to the framework and proceeds mostly via dehydration steps. Its μ-oxo bridge is formed via water-assistedmore »condensation of two hydroxo groups. To evaluate the relationship between [Cu 2 O] 2+ and other active sites, we also examined the formation energies of other species. The formation energies follow the trend: isolated [Cu–OH] + < paired [Cu–OH] + < [Cu 2 O] 2+ < [Cu 3 O 3 ] 2+ . Inclusion of Gibbs free-energy corrections indicates activation temperatures of 257, 307 and 327 and 331 °C for isolated [Cu–OH] + , paired [Cu–OH] + , [Cu 2 O] 2+ and [Cu 3 O 3 ] 2+ , respectively. The provocative nature of the lower-than-expected activation temperature for isolated [Cu–OH] + species is discussed.« less
3. Reaction mechanism and dynamics for C8-hydroxylation of 9-methylguanine radical cation by water molecules

   https://doi.org/10.1039/d1cp03884b  

   Zhou, Wenjing ; Liu, Jianbo ( November 2021 , Physical Chemistry Chemical Physics)

   In contrast to their spontaneous deprotonation in aqueous solution, reactions of guanine and guanosine radical cations with water in the gas phase are exclusively initiated by hydration of the radical cations as reported in recent work (Y. Sun et al. , Phys. Chem. Chem. Phys. , 2018, 20 , 27510). As gas-phase hydration reactions closely mimic the actual scenario for guanine radical cations in double-stranded DNA, exploration of subsequent reactions within their water complexes can provide an insight into the resulting oxidative damage to nucleosides. Herein guided-ion beam mass spectrometry experiment and direct dynamics trajectory simulations were carried out to examine prototype complexes of the 9-methylguanine radical cation with one and two water ligands ( i.e. , 9MG˙ + ·(H 2 O) 1–2 ) in the gas phase, wherein the complexes were activated by collisional activation in the experiment and by thermal excitation at high temperatures in the simulations. Guided by mass spectroscopic measurements, trajectory results and reaction potential energy surface, three reaction pathways were identified. The first two reaction pathways start with H-atom abstraction from water by the O6 and N7 atoms in 9MG˙ + and are referred to as HA O6 and HA N7 , respectively. The primarymore »products of HA O6 and HA N7 reactions, including [9MG + H O6 ] + /[9MG + H N7 ] + and ˙OH, react further to either form [8OH-9MG + H O6 ]˙ + and [8OH-9MG + H N7 ]˙ + via C8-hydroxylation or form radical cations of 6- enol -guanine (6- enol -G˙ + ) and 7H-guanine (7HG˙ + ) via S N 2-type methanol elimination. The third reaction pathway corresponds to the formation of 8OH-9MG + by H elimination from the complex, referred to as HE. Among these product channels, [8OH-9MG + H N7 ]˙ + has the most favorable formation probability, especially in the presence of additional water molecules. This product may serve as a preceding structure to the 8-oxo-7,8-dihydroguanine lesion in DNA and has implications for health effects of radiation exposure and radiation therapy.« less
4. Oxo dicopper anchored on carbon nitride for selective oxidation of methane

   https://doi.org/10.1038/s41467-022-28987-1  

   Xie, Pengfei ; Ding, Jing ; Yao, Zihao ; Pu, Tiancheng ; Zhang, Peng ; Huang, Zhennan ; Wang, Canhui ; Zhang, Junlei ; Zecher-Freeman, Noah ; Zong, Han ; et al ( December 2022 , Nature Communications)

   Abstract Selective conversion of methane (CH 4 ) into value-added chemicals represents a grand challenge for the efficient utilization of rising hydrocarbon sources. We report here dimeric copper centers supported on graphitic carbon nitride (denoted as Cu 2 @C 3 N 4 ) as advanced catalysts for CH 4 partial oxidation. The copper-dimer catalysts demonstrate high selectivity for partial oxidation of methane under both thermo- and photocatalytic reaction conditions, with hydrogen peroxide (H 2 O 2 ) and oxygen (O 2 ) being used as the oxidizer, respectively. In particular, the photocatalytic oxidation of CH 4 with O 2 achieves >10% conversion, and >98% selectivity toward methyl oxygenates and a mass-specific activity of 1399.3 mmol g Cu −1 h −1 . Mechanistic studies reveal that the high reactivity of Cu 2 @C 3 N 4 can be ascribed to symphonic mechanisms among the bridging oxygen, the two copper sites and the semiconducting C 3 N 4 substrate, which do not only facilitate the heterolytic scission of C-H bond, but also promotes H 2 O 2 and O 2 activation in thermo- and photocatalysis, respectively.
5. Reaction of O ₂ with a diiron protein generates a mixed-valent Fe ²⁺ /Fe ³⁺ center and peroxide

   https://doi.org/10.1073/pnas.1809913116  

   Bradley, Justin M. ; Svistunenko, Dimitri A. ; Pullin, Jacob ; Hill, Natalie ; Stuart, Rhona K. ; Palenik, Brian ; Wilson, Michael T. ; Hemmings, Andrew M. ; Moore, Geoffrey R. ; Le Brun, Nick E. ( February 2019 , Proceedings of the National Academy of Sciences)

   The gene encoding the cyanobacterial ferritinSynFtn is up-regulated in response to copper stress. Here, we show that, whileSynFtn does not interact directly with copper, it is highly unusual in several ways. First, its catalytic diiron ferroxidase center is unlike those of all other characterized prokaryotic ferritins and instead resembles an animal H-chain ferritin center. Second, as demonstrated by kinetic, spectroscopic, and high-resolution X-ray crystallographic data, reaction of O₂with the di-Fe²⁺center results in a direct, one-electron oxidation to a mixed-valent Fe²⁺/Fe³⁺form. Iron–O₂chemistry of this type is currently unknown among the growing family of proteins that bind a diiron site within a four α-helical bundle in general and ferritins in particular. The mixed-valent form, which slowly oxidized to the more usual di-Fe³⁺form, is an intermediate that is continually generated during mineralization. Peroxide, rather than superoxide, is shown to be the product of O₂reduction, implying that ferroxidase centers function in pairs via long-range electron transfer through the protein resulting in reduction of O₂bound at only one of the centers. We show that electron transfer is mediated by the transient formation of a radical on Tyr40, which lies ∼4 Å from the diiron center. As well as demonstrating an expansion of the iron–O₂chemistry knownmore »to occur in nature, these data are also highly relevant to the question of whether all ferritins mineralize iron via a common mechanism, providing unequivocal proof that they do not.

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