Dearomatization of [2.2]Paracyclophane-Derived N-Sulfonylimines through Cyclopropanation with Sulfur Ylides.

An unprecedented dearomatization of [2.2]paracyclophane-derived cyclic N-sulfonylimines was conducted through cyclopropanation with sulfur ylides, giving a series of dearomative cyclopropanes with good yields. DFT calculations suggested that the dearomatization was attributed to the relatively weak aromaticity of [2.2]paracyclophane derivatives that resulted from the effect of the unique [2.2]paracyclophane skeleton and the electron-withdrawing N-sulfonyl group. Some downstream elaborations of the products were demonstrated.

Cysteine Radical and Glutamate Collaboratively Enable C-H Bond Activation and C-N Bond Cleavage in a Glycyl Radical Enzyme HplG.

Hydroxyprolines are abundant in nature and widely utilized by many living organisms. Isomerization of trans-4-hydroxy-d-proline (t4D-HP) to generate 2-amino-4-ketopentanoate has been found to need a glycyl radical enzyme HplG, which catalyzes the cleavage of the C-N bond, while dehydration of trans-4-hydroxy-l-proline involves a homologous enzyme of HplG. Herein, molecular dynamics simulations and quantum mechanics/molecular mechanics (QM/MM) calculations are employed to understand the reaction mechanism of HplG. Two possible reaction pathways of HplG have been explored to decipher the origin of its chemoselectivity. The QM/MM calculations reveal that the isomerization proceeds via an initial hydrogen shift from the Cγ site of t4D-HP to a catalytic cysteine radical, followed by cleavage of the Cδ-N bond in t4D-HP to form a radical intermediate that captures a hydrogen atom from the cysteine. Activation of the Cδ-H bond in t4D-HP to bring about dehydration of t4D-HP possesses an extremely high energy barrier, thus rendering the dehydration pathway implausible in HplG. On the basis of the current calculations, conserved residue Glu429 plays a pivotal role in the isomerization pathway: the hydrogen bonding between it and t4D-HP weakens the hydroxyalkyl Cγ-Hγ bond, and it acts as a proton acceptor to trigger the cleavage of the C-N bond in t4D-HP. Our current QM/MM calculations rationalize the origin of the experimentally observed chemoselectivity of HplG and propose an H-bond-assisted bond activation strategy in radical-containing enzymes. These findings have general implications on radical-mediated enzymatic catalysis and expand our understanding of how nature wisely and selectively activates the C-H bond to modulate catalytic selectivity.

Sequential C-H Methylation Catalyzed by the B12 -Dependent SAM Enzyme TokK: Comprehensive Theoretical Study of Selectivities.

TokK is a B12 -dependent radical SAM enzyme involved in the biosynthesis of the ß-lactam antibiotic asparenomycin A. It can catalyze three methylations on different sp3 -hybridized carbon positions to introduce an isopropyl side chain at the ß-lactam ring of pantetheinylated carbapenem. Herein, we report a quantum chemical study of the reaction mechanism of TokK. A stepwise ''push-pull'' radical relay mechanism is proposed for each methylation: a 5'-deoxyadenosine radical first abstracts a hydrogen atom from the substrate in the active site, then methylcobalamin directionally donates a methyl group to the substrate. More importantly, calculations were able to uncover the origin of observed chemoselectivity and stereoselectivity for the first methylation and regioselectivity for the following two methylations. Further detailed distortion/interaction analysis can help to unravel the main factors controlling the selectivities. Our findings of sequential methylations by TokK could have profound implications for studying other B12 -dependent radical SAM enzymes.

Transition-Metal-Free, Site-Selective C-F Arylation of Polyfluoroarenes via Electrophotocatalysis.

Direct CAr-F arylation is effective and sustainable for synthesis of polyfluorobiaryls with different degrees of fluorination, which are important motifs in medical and material chemistry. However, with no aid of transition metals, the engagement of CAr-F bond activation has proved difficult. Herein, an unprecedented transition-metal-free strategy is reported for site-selective CAr-F arylation of polyfluoroarenes with simple (het)arenes. By merging N,N-bis(2,6-diisopropylphenyl)perylene-3,4,9,10-bis(dicarboximide)-catalyzed electrophotocatalytic reduction and anodic nitroxyl radical oxidation in an electrophotocatalytic cell, various polyfluoroaromatics (2F-6F and 8F), especially inactive partially fluorinated aromatics, undergo sacrificial-reagents-free C-F bond arylation with high regioselectivity, and the yields are comparable to those for reported transition-metal catalysis. This atom- and step-economic protocol features a paired electrocatalysis with organic mediators in both cathodic and anodic processes. The broad substrate scope and good functional-group compatibility highlight the merits of this operationally simple strategy. Moreover, the easy gram-scale synthesis and late-stage functionalization collectively advocate for the practical value, which would promote the vigorous development of fluorine chemistry.

Plant begomoviruses subvert ubiquitination to suppress plant defenses against insect vectors.

Most plant viruses are vectored by insects and the interactions of virus-plant-vector have important ecological and evolutionary implications. Insect vectors often perform better on virus-infected plants. This indirect mutualism between plant viruses and insect vectors promotes the spread of virus and has significant agronomical effects. However, few studies have investigated how plant viruses manipulate plant defenses and promote vector performance. Begomoviruses are a prominent group of plant viruses in tropical and sub-tropical agro-ecosystems and are transmitted by whiteflies. Working with the whitefly Bemisia tabaci, begomoviruses and tobacco, we revealed that C2 protein of begomoviruses lacking DNA satellites was responsible for the suppression of plant defenses against whitefly vectors. We found that infection of plants by tomato yellow leaf curl virus (TYLCV), one of the most devastating begomoviruses worldwide, promoted the survival and reproduction of whitefly vectors. TYLCV C2 protein suppressed plant defenses by interacting with plant ubiquitin. This interaction compromised the degradation of JAZ1 protein, thus inhibiting jasmonic acid defense and the expression of MYC2-regulated terpene synthase genes. We further demonstrated that function of C2 protein among begomoviruses not associated with satellites is well conserved and ubiquitination is an evolutionarily conserved target of begomoviruses for the suppression of plant resistance to whitefly vectors. Taken together, these results demonstrate that ubiquitination inhibition by begomovirus C2 protein might be a general mechanism in begomovirus, whitefly and plant interactions.

Revealing the Mechanism of Isethionate Sulfite-Lyase by QM/MM Calculations.

Isethionate sulfite-lyase (IseG) is a recently characterized glycyl radical enzyme (GRE) that catalyzes radical-mediated C-S bond cleavage of isethionate to produce acetaldehyde and sulfite. Herein, we use quantum mechanical/molecular mechanical (QM/MM) calculations to investigate the detailed catalytic reaction mechanism of IseG. Our calculations indicate that a previously proposed direct 1,2-elimination mechanism is disfavored. Instead, we suggest a new 1,2-migration mechanism for this enzymatic reaction: a key stepwise 1,2-SO3- radical migration occurs after the catalytically active cysteinyl radical grabs a hydrogen atom from isethionate, followed by hydrogen atom transfer from cysteine to a 1-hydroxylethane-1-sulfonate radical intermediate. Finally, the elimination of sulfite from 1-hydroxylethane-1-sulfonate to result in the final product is likely to occur outside the enzyme. Glu468 in the active site is found to help orient the substrate rather than grabbing a proton from the hydroxyl group of the substrate. Our findings help reveal the mechanisms of radical-mediated C-S bond cleavage of organosulfonates catalyzed by GREs and expand the understanding of radical-based enzymatic catalysis.

Computational study revealed a "pull-push" radical transfer mechanism of Mmp10-catalyzed Cδ-methylation of arginine.

Mmp10 is a B12-dependent SAM radical enzyme that catalyzes Cδ-methylation of arginine. The quantum chemical cluster calculations of Mmp10 revealed a "pull-push" radical transfer mechanism in which a 5'-deoxyadenosine radical first abstracts a hydrogen atom from the arginine residue and then methylcobalamin donates a methyl group to the arginine residue. The stereoselectivity and regioselectivity of Mmp10 originated in a specific substrate binding mode enabled by the active site.

QM/MM Calculations Suggest Concerted O-O Bond Cleavage and Substrate Oxidation by Nonheme Diiron Toluene/o-Xylene Monooxygenase.

The nonheme diiron toluene/o-xylene monooxygenase (ToMO) is the most studied toluene monooxygenase that mediates an aromatic hydroxylation reaction. In this work, QM/MM calculations were performed to understand the reaction mechanism. It is revealed that the µ-η2 :η2 peroxodiferric species is the reactive intermediate after the binding of the O2 molecule to the reduced diferrous center. Subsequently, both a stepwise and a concerted mechanism involving the critical O-O bond cleavage and C-O bond formation were considered. The latter was calculated to be more favorable, suggesting that the formation of a high-valent diferryl Q intermediate is not needed. The isomeric formation of the phenol product was found very facile. The first step was calculated to be rate-limiting, with a barrier of 17.6 kcal/mol for the ortho-hydroxylation.

Thermal sensitivity of bacteriocytes constrains the persistence of intracellular bacteria in whitefly symbiosis under heat stress.

Temperature affects the persistence of diverse symbionts of insects. Our previous study indicates that the whitefly symbionts confined within bacteriocytes or scattered throughout the body cavity outside bacteriocytes may have differential thermal sensitivity. However, the underlying mechanisms remain largely unknown. Here, we report that following continuous heat stress, Portiera and Hamiltonella were almost completely depleted in two species of Middle East-Asia Minor 1 (MEAM1) and Mediterranean (MED) of the Bemisia tabaci whitefly cryptic species complex. Meanwhile, proliferation of bacteriocytes was severely inhibited and approximately 50% of the nymphs had lost one of the two bacteriomes. While cell size of bacteriocytes was increased, cell number was severely decreased leading to reduction of total volume of bacteriocytes. Moreover, bacteriocyte organelles and associated symbionts were lysed, and huge amount of electron-dense inclusions accumulated. Eventually, Portiera and Hamiltonella failed to be transmitted to the next generation. In contrast, Rickettsia could be detected although at a reduced level, and successfully transmitted to eggs. The results suggest that the thermal sensitivity of bacteriocytes may limit thermal tolerance and vertical transmission of the associated symbionts, and consequently different patterns of distribution of symbionts may affect their capacity to tolerate unfavourable temperatures and persistence in the host.