Prominent Neuroprotective Potential of Indole-2-N-methylpropargylamine: High Affinity and Irreversible Inhibition Efficiency towards Monoamine Oxidase B Revealed by Computational Scaffold Analysis.

Background: Monoamine oxidases (MAO) are flavoenzymes that metabolize a range of brain neurotransmitters, whose dysregulation is closely associated with the development of various neurological disorders. This is why MAOs have been the central target in pharmacological interventions for neurodegeneration for more than 60 years. Still, existing drugs only address symptoms and not the cause of the disease, which underlines the need to develop more efficient inhibitors without adverse effects. Methods: Our drug design strategy relied on docking 25 organic scaffolds to MAO-B, which were extracted from the ChEMBL20 database with the highest cumulative counts of unique member compounds and bioactivity assays. The most promising candidates were substituted with the inactivating propargylamine group, while further affinity adjustment was made by its N-methylation. A total of 46 propargylamines were submitted to the docking and molecular dynamics simulations, while the best binders underwent mechanistic DFT analysis that confirmed the hydride abstraction mechanism of the covalent inhibition reaction. Results: We identified indole-2-propargylamine 4fH and indole-2-N-methylpropargylamine 4fMe as superior MAO-B binders over the clinical drugs rasagiline and selegiline. DFT calculations highlighted 4fMe as more potent over selegiline, evident in a reduced kinetic requirement (ΔΔG‡ = -2.5 kcal mol-1) and an improved reaction exergonicity (ΔΔGR = -4.3 kcal mol-1), together with its higher binding affinity, consistently determined by docking (ΔΔGBIND = -0.1 kcal mol-1) and MM-PBSA analysis (ΔΔGBIND = -1.5 kcal mol-1). Conclusions: Our findings strongly advocate 4fMe as an excellent drug candidate, whose synthesis and biological evaluation are highly recommended. Also, our results reveal the structural determinants that influenced the affinity and inhibition rates that should cooperate when designing further MAO inhibitors, which are of utmost significance and urgency with the increasing prevalence of brain diseases.

Interfacial Thermoconvection and Atomic Relay Catalysis Enable Equilibrium Shifting and Rapid Glucose-to-Fructose Isomerization.

The aqueous glucose-to-fructose isomerization is controlled by thermodynamics to an equilibrium limit of ~50% fructose yield. However, here we report an in-situ fructose removal strategy enabled by an interfacial local photothermal effect in combination with relay catalysis of geminal and isolated potassium single atoms (K SAs) on graphene-type carbon (Ksg/GT) to effectively bypass the equilibrium limit and markedly speed up glucose-to-fructose isomerization. At 25 ºC, an unprecedented fructose yield of 68.2% was obtained over Ksg/GT in an aqueous solution without any additives under 30-min solar-like irradiation. Mechanistic studies expounded that the interfacial thermoconvection caused by the local photothermal effect of the graphene-type carbon and preferable glucose adsorption on single-atom K could facilitate the release of in-situ formed fructose. The geminal K SAs were prone to form a stable metal-glucose complex via bidentate coordination, and could significantly reduce the C-H bond electron density by light-driven electron transfer toward K. This facilitated the hydride shift rate-determining step and expedited glucose isomerization. In addition, isolated K SAs favored the subsequent protonation and ring-closure process to furnish fructose. The integration of the interfacial thermoconvection-enhanced in-situ removal protocol and tailored atomic catalysis opens a prospective avenue for boosting equilibrium-limited reactions under mild conditions.

Computational Studies of Nucleophilic Substitution at Nitrogen Center: Reactions of NH2Cl with HO-, CH3O- and C2H5O.

The atomic-level mechanisms of the nucleophilic substitution reactions at the nitrogen center (SN2@N) were investigated for the reactions of chloramine (NH2Cl) with the alkoxide ions (RO-, where R=H, CH3, and C2H5) using DFT and MP2 methods. The computed potential energy profiles for the SN2@N pathways involving the back-side attack of the nucleophiles show the typical double-well potential with submerged barriers similar to the SN2 reactions at the carbon center (SN2@C). However, the pre-reaction and post-reaction complexes are, respectively, the N-H⋅⋅⋅O and N-H⋅⋅⋅Cl hydrogen-bonded intermediates, which are different from those generally seen in SN2@C reactions. The SN2@N pathways involving front-side attack of the nucleophiles have high-energy barriers. The potential energy surfaces (PESs) along the proton-transfer pathways were flat. In addition to the proton-transfer and SN2 pathways, we also observed a new path for the methoxide and ethoxide nucleophiles where a hydride-transfer from the nucleophile to chloramine resulted in the products Cl-+R'CHO+NH3, (R'=H, CH3), and was the most exoergic. A comparison of the energetics obtained used different DFT and MP2 methods with that of the benchmark coupled-cluster methods reveals that CAM-B3LYP best describes the PESs.

The role of the active site lysine residue on FAD reduction by NADPH in glutathione reductase.

Glutathione reductase (GR) is a two dinucleotide binding domain flavoprotein (tDBDF) that catalyzes the reduction of glutathione disulfide to glutathione coupled to the oxidation of NADPH to NADP+. An interesting feature of GR and other tDBDFs is the presence of a lysine residue (Lys-66 in human GR) at the active site, which interacts with the flavin group, but has an unknown function. To better understand the role of this residue, the dynamics of GR was studied using molecular dynamics simulations, and the reaction mechanism of FAD reduction by NADPH was studied using QM/MM molecular modeling. The two possible protonation states of Lys-66 were considered: neutral and protonated. Molecular dynamics results suggest that the active site is more structured for neutral Lys-66 than for protonated Lys-66. QM/MM modeling results suggest that Lys-66 should be in its neutral state for a thermodynamically favorable reduction of FAD by NADPH. Since the reaction is unfavorable with protonated Lys-66, the reverse reaction (the reduction of NADP+ by FADH-) is expected to take place. A phylogenetic analysis of various tDBDFs was performed, finding that an active site lysine is present in different the tDBDFs enzymes, suggesting that it has a conserved biological role. Overall, these results suggest that the protonation state of the active site lysine determines the energetics of the reaction, controlling its reversibility.

Chloride and Hydride Transfer as Keys to Catalytic Upcycling of Polyethylene into Liquid Alkanes.

Transforming polyolefin waste into liquid alkanes through tandem cracking-alkylation reactions catalyzed by Lewis-acid chlorides offers an efficient route for single-step plastic upcycling. Lewis acids in dichloromethane establish a polar environment that stabilizes carbenium ion intermediates and catalyzes hydride transfer, enabling breaking of polyethylene C-C bonds and forming C-C bonds in alkylation. Here, we show that efficient and selective deconstruction of low-density polyethylene (LDPE) to liquid alkanes is achieved with anhydrous aluminum chloride (AlCl3) and gallium chloride (GaCl3). Already at 60 °C, complete LDPE conversion was achieved, while maintaining the selectivity for gasoline-range liquid alkanes over 70 %. AlCl3 showed an exceptional conversion rate of 5000 g L D P E m o l c a t - 1 h - 1 ${{{\rm g}}_{{\rm L}{\rm D}{\rm P}{\rm E}}{{\rm \ }{\rm m}{\rm o}{\rm l}}_{{\rm c}{\rm a}{\rm t}}^{-1}{{\rm \ }{\rm h}}^{-1}}$ , surpassing other Lewis acid catalysts by two orders of magnitude. Through kinetic and mechanistic studies, we show that the rates of LDPE conversion do not correlate directly with the intrinsic strength of the Lewis acids or steric constraints that may limit the polymer to access the Lewis acid sites. Instead, the rates for the tandem processes of cracking and alkylation are primarily governed by the rates of initiation of carbenium ions and the subsequent intermolecular hydride transfer. Both jointly control the relative rates of cracking and alkylation, thereby determining the overall conversion and selectivity.

Activation of Coq6p, a FAD Monooxygenase Involved in Coenzyme Q Biosynthesis, by Adrenodoxin Reductase/Ferredoxin.

Adrenodoxin reductase (AdxR) plays a pivotal role in electron transfer, shuttling electrons between NADPH and iron/sulfur adrenodoxin proteins in mitochondria. This electron transport system is essential for P450 enzymes involved in various endogenous biomolecules biosynthesis. Here, we present an in-depth examination of the kinetics governing the reduction of human AdxR by NADH or NADPH. Our results highlight the efficiency of human AdxR when utilizing NADPH as a flavin reducing agent. Nevertheless, akin to related flavoenzymes such as cytochrome P450 reductase, we observe that low NADPH concentrations hinder flavin reduction due to intricate equilibrium reactions between the enzyme and its substrate/product. Remarkably, the presence of MgCl2 suppresses this complex kinetic behavior by decreasing NADPH binding to oxidized AdxR, effectively transforming AdxR into a classical Michaelis-Menten enzyme. We propose that the addition of MgCl2 may be adapted for studying the reductive half-reactions of other flavoenzymes with NADPH. Furthermore, in vitro experiments provide evidence that the reduction of the yeast flavin monooxygenase Coq6p relies on an electron transfer chain comprising NADPH-AdxR-Yah1p-Coq6p, where Yah1p shuttles electrons between AdxR and Coq6p. This discovery explains the previous in vivo observation that Yah1p and the AdxR homolog, Arh1p, are required for the biosynthesis of coenzyme Q in yeast.

Targeted Mutation of a Non-catalytic Gating Residue Increases the Rate of Pseudomonas aeruginosa d-Arginine Dehydrogenase Catalytic Turnover.

Commercial food and l-amino acid industries rely on bioengineered d-amino acid oxidizing enzymes to detect and remove d-amino acid contaminants. However, the bioengineering of enzymes to generate faster biological catalysts has proven difficult as a result of the failure to target specific kinetic steps that limit enzyme turnover, kcat, and the poor understanding of loop dynamics critical for catalysis. Pseudomonas aeruginosa d-arginine dehydrogenase (PaDADH) oxidizes most d-amino acids and is a good candidate for application in the l-amino acid and food industries. The side chain of the loop L2 E246 residue located at the entrance of the PaDADH active site pocket potentially favors the closed active site conformation and secures the substrate upon binding. This study used site-directed mutagenesis, steady-state, and rapid reaction kinetics to generate the glutamine, glycine, and leucine variants and investigate whether increasing the rate of product release could translate to an increased enzyme turnover rate. Upon E246 mutation to glycine, there was an increased rate of d-arginine turnover kcat from 122 to 500 s-1. Likewise, the kcat values increased 2-fold for the glutamine or leucine variants. Thus, we have engineered a faster biocatalyst for industrial applications by selectively increasing the rate of the PaDADH product release.

X-ray-driven chemistry and conformational heterogeneity in atomic resolution crystal structures of bacterial dihydrofolate reductases.

Dihydrofolate reductase (DHFR) catalyzes the NADPH-dependent reduction of dihydrofolate to tetrahydrofolate. Bacterial DHFRs are targets of several important antibiotics as well as model enzymes for the role of protein conformational dynamics in enzyme catalysis. We collected 0.93 Å resolution X-ray diffraction data from both Bacillus subtilis (Bs) and E. coli (Ec) DHFRs bound to folate and NADP+. These oxidized ternary complexes should not be able to perform chemistry, however electron density maps suggest hydride transfer is occurring in both enzymes. Comparison of low- and high-dose EcDHFR datasets show that X-rays drive partial production of tetrahydrofolate. Hydride transfer causes the nicotinamide moiety of NADP+ to move towards the folate as well as correlated shifts in nearby residues. Higher radiation dose also changes the conformational heterogeneity of Met20 in EcDHFR, supporting a solvent gating role during catalysis. BsDHFR has a different pattern of conformational heterogeneity and an unexpected disulfide bond, illustrating important differences between bacterial DHFRs. This work demonstrates that X-rays can drive hydride transfer similar to the native DHFR reaction and that X-ray photoreduction can be used to interrogate catalytically relevant enzyme dynamics in favorable cases.

Irreversible inactivation of lactate racemase by sodium borohydride reveals reactivity of the nickel-pincer nucleotide cofactor.

The nickel-pincer nucleotide (NPN) cofactor discovered in lactate racemase from Lactiplantibacillus plantarum (LarALp) is essential for the activities of racemases/epimerases in the highly diverse LarA superfamily. Prior mechanistic studies have established a proton-coupled hydride-transfer mechanism for LarALp, but direct evidence showing that hydride attacks the C4 atom in the pyridinium ring of NPN has been lacking. Here, we show that sodium borohydride (NaBH4) irreversibly inactivates LarALp accompanied by a rapid color change of the enzyme. The altered ultraviolet-visible spectra during NaBH4 titration supported hydride transfer to C4 of NPN, and the concomitant Ni loss unraveled by mass spectrometry experiments accounted for the irreversible inactivation. High resolution structures of LarALp revealed a substantially weakened C-Ni bond in the metastable sulfite-NPN adduct where the NPN cofactor is in the reduced state. These findings allowed us to propose a mechanism of LarALp inactivation by NaBH4 that provides key insights into the enzyme-catalyzed reaction and sheds light on the reactivity of small molecule NPN mimetics.

Computational Insights on the Hydride and Proton Transfer Mechanisms of D-Arginine Dehydrogenase.

D-Arginine dehydrogenase from Pseudomonas aeruginosa (PaDADH) is an amine oxidase which catalyzes the conversion of D-arginine into iminoarginine. It contains a non-covalent FAD cofactor that is involved in the oxidation mechanism. Based on substrate, solvent, and multiple kinetic isotope effects studies, a stepwise hydride transfer mechanism is proposed. It was shown that D-arginine binds to the active site of enzyme as α-amino group protonated, and it is deprotonated before a hydride ion is transferred from its α-C to FAD. Based on a mutagenesis study, it was concluded that a water molecule is the most likely catalytic base responsible from the deprotonation of α-amino group. In this study, we formulated computational models based on ONIOM method to elucidate the oxidation mechanism of D-arginine into iminoarginine using the crystal structure of enzyme complexed with iminoarginine. The calculations showed that Arg222, Arg305, Tyr249, Glu87, His 48, and two active site water molecules play key roles in binding and catalysis. Model systems showed that the deprotonation step occurs prior to hydride transfer step, and active site water molecule(s) may have participated in the deprotonation process.

Tris(pentafluorophenyl)borane-catalyzed Hydride Transfer Reactions in Polysiloxane Chemistry-Piers-Rubinsztajn Reaction and Related Processes.

Tris(pentafluorophenyl)borane (TPFPB) is a unique Lewis acid that catalyzes the condensation between hydrosilanes (Si-H) and alkoxysilanes (Si-OR), leading to the formation of siloxane bonds (Si-OSi) with the release of hydrocarbon (R-H) as a byproduct-the so-called Piers-Rubinsztajn reaction. The analogous reactions of hydrosilanes with silanols (Si-OH), alcohols (R-OH), ethers (R-OR') or water in the presence of TPFPB leads to the formation of a siloxane bond, alkoxysilane (Si-OR or Si-OR') or silanol (Si-OH), respectively. The above processes, often referred to as Piers-Rubinsztajn reactions, provide new synthetic tools for the controlled synthesis of siloxane materials under mild conditions with high yields. The common feature of these reactions is the TPFPB-mediated hydride transfer from silicon to carbon or hydrogen. This review presents a summary of 20 years of research efforts related to this field, with a focus on new synthetic methodologies leading to numerous previously difficult to synthesize well-defined siloxane oligomers, polymers and copolymers of a complex structure and potential applications of these new materials. In addition, the mechanistic aspects of the recently discovered reactions involving hydride transfer from silicon to silicon are discussed in more detail.

A Mechanism Study of Redox Reactions of the Ruthenium-oxo-polypyridyl Complex.

Over the years, RuIV(bpy)2(py)(O)2+([RuIVO]2+) has garnered considerable interest owing to its extensive use as a polypyridine mono-oxygen complex. However, as the active-site Ru=O bond changes during the oxidation process, [RuIVO]2+ can be used to simulate the reactions of various high-priced metallic oxides. In order to elucidate the hydrogen element transfer process between the Ruthenium-oxo-polypyridyl complex and organic hydride donor, the current study reports on the synthesis of [RuIVO]2+, a polypyridine mono-oxygen complex, in addition to 1H and 3H (organic hydride compounds) and 1H derivative: 2. Through 1H-NMR analysis and thermodynamics- and kinetics-based assessments, we collected data on [RuIVO]2+ and two organic hydride donors and their corresponding intermediates and established a thermodynamic platform. It was confirmed that a one-step hydride transfer reaction between [RuIVO]2+ and these organic hydride donors occurs, and here, the advantages and nature of the new mechanism approach are revealed. Accordingly, these findings can considerably contribute to the better application of the compound in theoretical research and organic synthesis.

The unusual chemical sequences of mammalian dihydropyrimidine dehydrogenase revealed by transient-state analysis.

Dihydropyrimidine dehydrogenase (DPD) catalyzes the reduction of the 5,6-vinylic bond of uracil and thymine with electrons from NADPH. The complexity of the enzyme belies the simplicity of the reaction catalyzed. To accomplish this chemistry DPD has two active sites that are ∼60Šapart, both of which house flavin cofactors, FAD and FMN. The FAD site interacts with NADPH, while the FMN site with pyrimidines. The distance between the flavins is spanned by four Fe4S4 centers. Though DPD has been studied for nearly 50years, it is only recently that the novel apects of its mechanism have been described. The primary reason for this is that the chemistry of DPD is not portrayed adequately by known descriptive steady-state mechanism categories. The highly chromophoric nature of the enzyme has recently been exploited in transient-state to document unexpected reaction sequences. Specifically, DPD undergoes reductive activation prior to catalytic turnover. Two electrons are taken up from NADPH and transmitted via the FAD and Fe4S4 centers to form the FAD•4(Fe4S4)•FMNH2 form of the enzyme. This form of the enzyme will only reduce pyrimidine substrates in the presence NADPH, establishing that hydride transfer to the pyrimidine precedes reductive reactivation that reinstates the active form of the enzyme. DPD is therefore the first flavoprotein dehydrogenase known to complete the oxidative half-reaction prior to the reductive half-reaction. Here we describe the methods and deduction that led to this mechanistic assignment.

Kinetics and mechanism for enzyme-catalyzed reactions of substrate pieces.

The most important difference between enzyme and small molecule catalysts is that only enzymes utilize the large intrinsic binding energies of nonreacting portions of the substrate in stabilization of the transition state for the catalyzed reaction. A general protocol is described to determine the intrinsic phosphodianion binding energy for enzymatic catalysis of reactions of phosphate monoester substrates, and the intrinsic phosphite dianion binding energy in activation of enzymes for catalysis of phosphodianion truncated substrates, from the kinetic parameters for enzyme-catalyzed reactions of whole and truncated substrates. The enzyme-catalyzed reactions so-far documented that utilize dianion binding interactions for enzyme activation; and, their phosphodianion truncated substrates are summarized. A model for the utilization of dianion binding interactions for enzyme activation is described. The methods for the determination of the kinetic parameters for enzyme-catalyzed reactions of whole and truncated substrates, from initial velocity data, are described and illustrated by graphical plots of kinetic data. The results of studies on the effect of site-directed amino acid substitutions at orotidine 5'-monophosphate decarboxylase, triosephosphate isomerase, and glycerol-3-phosphate dehydrogenase provide strong support for the proposal that these enzymes utilize binding interactions with the substrate phosphodianion to hold the protein catalysts in reactive closed conformations.

Reactivity of non-organometallic ruthenium(II) polypyridyl complexes and their application as catalysts for hydride transfer reactions.

Recently, we investigated the substitution behavior of a series of ruthenium(II) complexes of the general formula [RuII(terpy)(N∧N)Cl]Cl, where terpy = 2,2':6',2″-terpyridine, N∧N = bidentate ligand, in aqueous solutions. We have shown that the most and least reactive complexes of the series are [RuII(terpy)(en)Cl]Cl (en = ethylenediamine) and [RuII(terpy)(phen)Cl]Cl (phen = 1, 10-phenantroline), respectively, as a result of different electronic effects provided by the bidentate spectator chelates. Polypyridyl amine Ru(II) complex, viz. [Ru(terpy)(en)Cl]Cl and [Ru(terpy)(ampy)Cl]Cl (where ampy = 2-(aminomethyl)pyridine), in which the terpy chelate labilizes the metal center, are able to catalyze the conversion of NAD+ to 1,4-NADH using sodium formate as a source of hydride. We showed that this complex can control the [NAD+]/[NADH] ratio and potentially induce reductive stress in living cells, which is accepted as an effective method to kill cancer cells. Polypyridyl Ru(II) complexes, characterized in terms of the behavior in aqueous solutions, can be used as model systems to monitor heterogeneous multiphase ligand substitution reactions at the solid-liquid interface. Colloidal coordination compounds in the submicron range were synthesized from Ru(II)-aqua derivatives of starting chlorido complexes via the anti-solvent procedure and stabilized by a surfactant shell layer.

The Dichotomy of Mn-H Bond Cleavage and Kinetic Hydricity of Tricarbonyl Manganese Hydride Complexes.

Acid-base characteristics (acidity, pKa, and hydricity, ΔG°H- or kH-) of metal hydride complexes could be a helpful value for forecasting their activity in various catalytic reactions. Polarity of the M-H bond may change radically at the stage of formation of a non-covalent adduct with an acidic/basic partner. This stage is responsible for subsequent hydrogen ion (hydride or proton) transfer. Here, the reaction of tricarbonyl manganese hydrides mer,trans-[L2Mn(CO)3H] (1; L = P(OPh)3, 2; L = PPh3) and fac-[(L-L')Mn(CO)3H] (3, L-L' = Ph2PCH2PPh2 (dppm); 4, L-L' = Ph2PCH2-NHC) with organic bases and Lewis acid (B(C6F5)3) was explored by spectroscopic (IR, NMR) methods to find the conditions for the Mn-H bond repolarization. Complex 1, bearing phosphite ligands, features acidic properties (pKa 21.3) but can serve also as a hydride donor (ΔG≠298K = 19.8 kcal/mol). Complex 3 with pronounced hydride character can be deprotonated with KHMDS at the CH2-bridge position in THF and at the Mn-H position in MeCN. The kinetic hydricity of manganese complexes 1-4 increases in the order mer,trans-[(P(OPh)3)2Mn(CO)3H] (1) < mer,trans-[(PPh3)2Mn(CO)3H] (2) ≈ fac-[(dppm)Mn(CO)3H] (3) < fac-[(Ph2PCH2NHC)Mn(CO)3H] (4), corresponding to the gain of the phosphorus ligand electron-donor properties.

Mutation of a distal gating residue modulates NADH binding in NADH:Quinone oxidoreductase from Pseudomonas aeruginosa PAO1.

Enzymes require flexible regions to adopt multiple conformations during catalysis. The mobile regions of enzymes include gates that modulate the passage of molecules in and out of the enzyme's active site. The enzyme PA1024 from Pseudomonas aeruginosa PA01 is a recently discovered flavin-dependent NADH:quinone oxidoreductase (NQO, EC 1.6.5.9). Q80 in loop 3 (residues 75-86) of NQO is ∼15 Å away from the flavin and creates a gate that seals the active site through a hydrogen bond with Y261 upon NADH binding. In this study, we mutated Q80 to glycine, leucine, or glutamate to investigate the mechanistic significance of distal residue Q80 in NADH binding in the active site of NQO. The UV-visible absorption spectrum reveals that the mutation of Q80 minimally affects the protein microenvironment surrounding the flavin. The anaerobic reductive half-reaction of the NQO-mutants yields a ≥25-fold increase in the Kd value for NADH compared to the WT enzyme. However, we determined that the kred value was similar in the Q80G, Q80L, and wildtype enzymes and only ∼25% smaller in the Q80E enzyme. Steady-state kinetics with NQO-mutants and NQO-WT at varying concentrations of NADH and 1,4-benzoquinone establish a ≤5-fold decrease in the kcat/KNADH value. Moreover, there is no significant difference in the kcat/KBQ (∼1 × 106 M-1s-1) and kcat (∼24 s-1) values in NQO-mutants and NQO-WT. These results are consistent with the distal residue Q80 being mechanistically essential for NADH binding to NQO with minimal effect on the quinone binding to the enzyme and hydride transfer from NADH to flavin.

Photoinduced Catalytic Organic-Hydride Transfer to CO2 Mediated with Ruthenium Complexes as NAD+ /NADH Redox Couple Models.

The catalytic organic-hydride transfer to CO2 was first achieved through the photoinduced two-electron reduction of the [Ru(bpy)2 (pbn)]2+ /[Ru(bpy)2 (pbnHH)]2+ (bpy=2,2'-bipyridine, pbn=2-(pyridin-2-yl)benzo[b]-1,5-naphthyridine, and pbnHH=2-(pyridin-2-yl)-5,10-dihydrobenzo[b]-1,5-naphthyridine) redox couple in the presence of 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH). The active species for the catalytic hydride transfer to carbon dioxide giving formate is [Ru(bpy)(bpy⋅- )(pbnHH)]+ formed by one-electron reduction of [Ru(bpy)2 (pbnHH)]2+ with BI⋅.

Conformational transitions induced by NADH binding promote reduction half-reaction in 2-hydroxybiphenyl-3-monooxygenase catalytic cycle.

2-Hydroxybiphenyl-3-monoxygenase from Pseudomonas azelaica is an effective catalyst of the regiospecific conversions of various aromatic compounds. A comprehensive understanding of the complete catalytic cycle, including the as yet unclear details of NADH binding, NADH/FAD interaction as well as related conformational changes could facilitate the rational design of improved enzyme variants for practical applications. Induced fit formation of a specific pocket for the nicotinamide ring at NADH binding has been revealed using advanced molecular simulation methods including metadynamics and QM/MM modeling. The resulting triple stacking interaction of the nicotinamide as well as isoalloxazine rings and evolutionarily correlated amino acid residues of the active site greatly contributes to the stabilization of the charge-transfer complex and determines the Pro-S stereospecificity of the hydride transfer and the low energy barrier 11 kcal/mol. Then the resulting FADH- anion undergoes the consequent conformational transition of the FAD isoalloxazine ring from the open out to the closed in position which is followed by the binding of an oxygen molecule what is crucial for the next step of substrate oxidation and the completion of the catalytic cycle.

Homogeneous Dearomative Hydrogenation with a Co/P4 N2 Catalyst: A Nucleophilic Approach.

Arene hydrogenation is the most straightforward method to prepare carbo- and heterocycles. However, the high resonance energy prevents aromatic substrates from hydrogenation. Herein the homogeneous, nucleophilic hydrogenation of less electron-rich arenes and heteroarenes is reported. The Co(P4 N2 )H species that has been demonstrated to be a strong hydride donor could deliver a hydride ion to the cyano (hetero)arene substrates. Deuterium labeling experiments supported a Michael-type reaction pathway. Theoretical analyses have been conducted to investigate the hydricity of the catalytically active CoH species and the electrophilicity of the arene substrates. An outlook for the synthesis of more challenging substituted benzenes was proposed based on the in silico modification of the CoH species.