Towards understanding non-equivalence of α and β subunits within human hemoglobin in conformational relaxation and molecular oxygen rebinding

Picosecond to millisecond laser time-resolved transient absorption spectroscopy was used to study molecular oxygen (O₂) rebinding and conformational relaxation following O₂ photodissociation in the α and β subunits within human hemoglobin in the quaternary R-like structure. Oxy-cyanomet valency hybrids, α₂(Fe²⁺–O₂)β₂(Fe³⁺–CN) and α₂(Fe³⁺–CN)β₂(Fe²⁺–O₂), were used as models for oxygenated R-state hemoglobin. An extended kinetic model for geminate O₂ rebinding in the ferrous hemoglobin subunits, ligand migration between the primary and secondary docking site(s), and nonexponential tertiary relaxation within the R quaternary structure, was introduced and discussed. Significant functional non-equivalence of the α and β subunits in both the geminate O₂ rebinding and concomitant structural relaxation was revealed. For the β subunits, the rate constant for the geminate O₂ rebinding to the unrelaxed tertiary structure and the tertiary transition rate were found to be greater than the corresponding values for the α subunits. The conformational relaxation following the O₂ photodissociation in the α and β subunits was found to decrease the rate constant for the geminate O₂ rebinding, this effect being more than one order of magnitude greater for the β subunits than for the α subunits. Evidence was provided for the modulation of the O₂ rebinding to the individual α and β subunits within human hemoglobin in the R-state structure by the intrinsic heme reactivity through a change in proximal constraints upon the relaxation of the tertiary structure on a picosecond to microsecond time scale. Our results demonstrate that, for native R-state oxyhemoglobin, O₂ rebinding properties and spectral changes following the O₂ photodissociation can be adequately described as the sum of those for the α and β subunits within the valency hybrids. The isolated β chains (hemoglobin H) show similar behavior to the β subunits within the valency hybrids and can be used as a model for the β subunits within the R-state oxyhemoglobin. At the same time, the isolated α chains behave differently to the α subunits within the valency hybrids.

O₂ rebinding and conformational relaxation following O₂ photodissociation were studied on picosecond to millisecond time scale in the α and β subunits within human hemoglobin in the quaternary R-like structure.     

Thermodynamics and kinetics of the amyloid-β peptide revealed by Markov state models based on MD data in agreement with experiment

The amlyoid-β peptide (Aβ) is closely linked to the development of Alzheimer''s disease. Molecular dynamics (MD) simulations have become an indispensable tool for studying the behavior of this peptide at the atomistic level. General key aspects of MD simulations are the force field used for modeling the peptide and its environment, which is important for accurate modeling of the system of interest, and the length of the simulations, which determines whether or not equilibrium is reached. In this study we address these points by analyzing 30-μs MD simulations acquired for Aβ40 using seven different force fields. We assess the convergence of these simulations based on the convergence of various structural properties and of NMR and fluorescence spectroscopic observables. Moreover, we calculate Markov state models for the different MD simulations, which provide an unprecedented view of the thermodynamics and kinetics of the amyloid-β peptide. This further allows us to provide answers for pertinent questions, like: which force fields are suitable for modeling Aβ? (a99SB-UCB and a99SB-ILDN/TIP4P-D); what does Aβ peptide really look like? (mostly extended and disordered) and; how long does it take MD simulations of Aβ to attain equilibrium? (at least 20–30 μs). We believe the analyses presented in this study will provide a useful reference guide for important questions relating to the structure and dynamics of Aβ in particular, and by extension other similar disordered proteins.

The convergence of MD simulations is tested using varying measures for the intrinsically disordered amyloid-β peptide (Aβ). Markov state models show that 20–30 μs of MD is needed to reliably reproduce the thermodynamics and kinetics of Aβ.     

Bio-Guided Fractionation of Stem Bark Extracts from Phyllanthus muellarianus: Identification of Phytocomponents with Anti-Cholinesterase Activity

A combination of flash chromatography, solid phase extraction, high-performance liquid chromatography, and in vitro bioassays was used to isolate phytocomponents endowed with anticholinesterase activity in extract from Phyllanthus muellarianus. Phytocomponents responsible for the anti-cholinesterase activity of subfractions PMF1 and PMF4 were identified and re-assayed to confirm their activity. Magnoflorine was identified as an active phytocomponent from PMF1 while nitidine was isolated from PMF4. Magnoflorine was shown to be a selective inhibitor of human butyrylcholinesterase—hBChE (IC₅₀ = 131 ± 9 μM and IC₅₀ = 1120 ± 83 μM, for hBuChE and human acetylcholinesterase—hAChE, respectively), while nitidine showed comparable inhibitory potencies against both enzymes (IC₅₀ = 6.68 ± 0.13 μM and IC₅₀ = 5.31 ± 0.50 μM, for hBChE and hAChE, respectively). When compared with the commercial anti-Alzheimer drug galanthamine, nitidine was as potent as galanthamine against hAChE and one order of magnitude more potent against hBuChE. Furthermore, nitidine also showed significant, although weak, antiaggregating activity towards amyloid-β self-aggregation.     

Preferred Endocytosis of Amyloid Precursor Protein from Cholesterol-Enriched Lipid Raft Microdomains

Amyloid precursor protein (APP) at the plasma membrane is internalized via endocytosis and delivered to endo/lysosomes, where neurotoxic amyloid-β (Aβ) is produced via β-, γ-secretases. Hence, endocytosis plays a key role in the processing of APP and subsequent Aβ generation. β-, γ-secretases as well as APP are localized in cholesterol-enriched lipid raft microdomains. However, it is still unclear whether lipid rafts are the site where APP undergoes endocytosis and whether cholesterol levels affect this process. In this study, we found that localization of APP in lipid rafts was increased by elevated cholesterol level. We also showed that increasing or decreasing cholesterol levels increased or decreased APP endocytosis, respectively. When we labeled cell surface APP, APP localized in lipid rafts preferentially underwent endocytosis compared to nonraft-localized APP. In addition, APP endocytosis from lipid rafts was regulated by cholesterol levels. Our results demonstrate for the first time that cholesterol levels regulate the localization of APP in lipid rafts affecting raft-dependent APP endocytosis. Thus, regulating the microdomain localization of APP could offer a new therapeutic strategy for Alzheimer’s disease.     

Modulation of amyloid-β aggregation by metal complexes with a dual binding mode and their delivery across the blood–brain barrier using focused ultrasound

One of the key hallmarks of Alzheimer''s disease is the aggregation of the amyloid-β peptide to form fibrils. Consequently, there has been great interest in studying molecules that can disrupt amyloid-β aggregation. While a handful of molecules have been shown to inhibit amyloid-β aggregation in vitro, there remains a lack of in vivo data reported due to their inability to cross the blood–brain barrier. Here, we investigate a series of new metal complexes for their ability to inhibit amyloid-β aggregation in vitro. We demonstrate that octahedral cobalt complexes with polyaromatic ligands have high inhibitory activity thanks to their dual binding mode involving π–π stacking and metal coordination to amyloid-β (confirmed via a range of spectroscopic and biophysical techniques). In addition to their high activity, these complexes are not cytotoxic to human neuroblastoma cells. Finally, we report for the first time that these metal complexes can be safely delivered across the blood–brain barrier to specific locations in the brains of mice using focused ultrasound.

We report a series of non-toxic cobalt(iii) complexes which inhibit Aβ peptide aggregation in vitro; these complexes can be safely delivered across the blood–brain barrier in mice using focused ultrasound.     

Novel Copper(II) Complexes with Dipinodiazafluorene Ligands: Synthesis,Structure, Magnetic and Catalytic Properties

The reactions of CuX₂ (X = Cl, Br) with dipinodiazafluorenes yielded four new complexes [CuX₂L₁]₂ (X = Cl (1), Br (2), L₁ = (1R,3R,8R,10R)-2,2,9,9-Tetramethyl-3,4,7,8,9,10-hexahydro-1H-1,3:8,10-dimethanocyclopenta [1,2-b:5,4-b’]diquinolin-12(2H)-one) and [(CuX₂)₂L₂]n (X = Cl (3), Br (4), L₂ = (1R,3R,8R,10R,1’R,3’R,8’R,10’R)-2,2,2’,2’,9,9,9’,9’-Octamethyl-1,1’,2,2’,3,3’,4,4’,7,7’,8,8’,9,9’,10,10’-hexadecahydro-1,3:1’,3’:8,10:8’,10’-tetramethano-12,12’-bi(cyclopenta [1,2-b:5,4-b’]diquinolinylidene). The complexes were characterized by IR and EPR spectroscopy, HR-ESI-MS and elemental analysis. The crystal structures of compounds 1, 2 and 4 were determined by X-ray diffraction (XRD) analysis. Complexes 1–2 have a monomeric structure, while complex 4 has a polymeric structure due to additional coordinating N,N sites in L₂. All complexes contain a binuclear fragment {Cu₂(μ-X)_2×2} (X = Cl, Br) in their structures. Each copper atom has a distorted square-pyramidal coordination environment formed by two nitrogen atoms and three halogen atoms. The Cu-N_ax distance is elongated compared to Cu-N_eq. The EPR spectra of compounds 1–4 in CH₃CN confirm their paramagnetic nature due to the d⁹ electronic configuration of the copper(II) ion. The magnetic properties of all compounds were studied by the method of static magnetic susceptibility. For complexes 1 and 2, the effective magnetic moments are µ_eff ≈ 1.87 and 1.83 µ_B (per each Cu²⁺ ion), respectively, in the temperature range 50–300 K, which are close to the theoretical spin value (1.73 µ_B). Ferromagnetic exchange interactions between Cu(II) ions inside {Cu₂(μ-X)₂X₂} (X = Cl, Br) dimers (J/k_B ≈ 25 and 31 K for 1 and 2, respectively) or between dimers (θ′ ≈ 0.30 and 0.47 K for 1 and 2, respectively) were found at low temperatures. For compounds 3 and 4, the magnetic susceptibility is well described by the Curie–Weiss law in the temperature range 1.77–300 K with µ_eff ≈ 1.72 and 1.70 µ_B for 3 and 4, respectively, and weak antiferromagnetic interactions (θ ≈ −0.4 K for 3 and −0.65 K for 4). Complexes 1–4 exhibit high catalytic activity in the oxidation of alkanes and alcohols with peroxides. The maximum yield of cyclohexane oxidation products reached 50% (complex 3). Based on the data on the study of regio- and bond-selectivity, it was concluded that hydroxyl radicals play a decisive role in the oxidation reaction. The initial products in reactions with alkanes are alkyl hydroperoxides.     

An explicitly designed paratope of amyloid-β prevents neuronal apoptosis in vitro and hippocampal damage in rat brain

Synthetic antibodies hold great promise in combating diseases, diagnosis, and a wide range of biomedical applications. However, designing a therapeutically amenable, synthetic antibody that can arrest the aggregation of amyloid-β (Aβ) remains challenging. Here, we report a flexible, hairpin-like synthetic paratope (SP1, ∼2 kDa), which prevents the aggregation of Aβ monomers and reverses the preformed amyloid fibril to a non-toxic species. Structural and biophysical studies further allowed dissecting the mode and affinity of molecular recognition events between SP1 and Aβ. Subsequently, SP1 reduces Aβ-induced neurotoxicity, neuronal apoptosis, and ROS-mediated oxidative damage in human neuroblastoma cells (SH-SY5Y). The non-toxic nature of SP1 and its ability to ameliorate hippocampal neurodegeneration in a rat model of AD demonstrate its therapeutic potential. This paratope engineering module could readily implement discoveries of cost-effective molecular probes to nurture the basic principles of protein misfolding, thus combating related diseases.

Herein, the therapeutic potentials of an explicitly designed peptide probe are systematically illuminated in vitro and in vivo against Aβ aggregation. The probe demonstrates remarkable potency for attenuating neurotoxicity and hippocampal damage.     

Ti-catalyzed ring-opening oxidative amination of methylenecyclopropanes with diazenes

The ring-opening oxidative amination of methylenecyclopropanes (MCPs) with diazenes catalyzed by py₃TiCl₂(NR) complexes is reported. This reaction selectively generates branched α-methylene imines as opposed to linear α,β-unsaturated imines, which are difficult to access via other methods. Products can be isolated as the imine or hydrolyzed to the corresponding ketone in good yields. Mechanistic investigation via density functional theory suggests that the regioselectivity of these products results from a Curtin–Hammett kinetic scenario, where reversible β-carbon elimination of a spirocyclic [2 + 2] azatitanacyclobutene intermediate is followed by selectivity-determining β-hydrogen elimination of the resulting metallacycle. Further functionalizations of these branched α-methylene imine products are explored, demonstrating their utility as building blocks.

The ring-opening oxidative amination of methylenecyclopropanes (MCPs) with diazenes catalyzed by py₃TiCl₂(NR) complexes is reported.     

Impact of sphingosine and acetylsphingosines on the aggregation and toxicity of metal-free and metal-treated amyloid-β

Pathophysiological shifts in the cerebral levels of sphingolipids in Alzheimer''s disease (AD) patients suggest a link between sphingolipid metabolism and the disease pathology. Sphingosine (SP), a structural backbone of sphingolipids, is an amphiphilic molecule that is able to undergo aggregation into micelles and micellar aggregates. Considering its structural properties and cellular localization, we hypothesized that SP potentially interacts with amyloid-β (Aβ) and metal ions that are found as pathological components in AD-affected brains, with manifesting its reactivity towards metal-free Aβ and metal-bound Aβ (metal–Aβ). Herein, we report, for the first time, that SP is capable of interacting with both Aβ and metal ions and consequently affects the aggregation of metal-free Aβ and metal–Aβ. Moreover, incubation of SP with Aβ in the absence and presence of metal ions results in the aggravation of toxicity induced by metal-free Aβ and metal–Aβ in living cells. As the simplest acyl derivatives of SP, N-acetylsphingosine and 3-O-acetylsphingosine also influence metal-free Aβ and metal–Aβ aggregation to different degrees, compared to SP. Such slight structural modifications of SP neutralize its ability to exacerbate the cytotoxicity triggered by metal-free Aβ and metal–Aβ. Notably, the reactivity of SP and the acetylsphingosines towards metal-free Aβ and metal–Aβ is determined to be dependent on their formation of micelles and micellar aggregates. Our overall studies demonstrate that SP and its derivatives could directly interact with pathological factors in AD and modify their pathogenic properties at concentrations below and above critical aggregation concentrations.

The reactivity of sphingosine and acetylsphingosines towards both metal-free and metal-treated amyloid-β is demonstrated showing a correlation of their micellization properties.     

Dehydrogenation of iron amido-borane and resaturation of the imino-borane complex

We report on the first isolation and structural characterization of an iron phosphinoimino-borane complex Cp*Fe(η²-H₂B NC₆H₄PPh₂) by dehydrogenation of iron amido-borane precursor Cp*Fe(η¹-H₃B–NHC₆H₄PPh₂). Significantly, regeneration of the amido-borane complex has been realized by protonation of the iron(ii) imino-borane to the amino-borane intermediate [Cp*Fe(η²-H₂B–NHC₆H₄PPh₂)]⁺ followed by hydride transfer. These new iron species are efficient catalysts for 1,2-selective transfer hydrogenation of quinolines with ammonia borane.

Dehydrogenation of an amido-borane iron complex provides an imino-borane complex. Regeneration of the amido-borane precursor was achieved by protonation of the imino-borane followed by hydride transfer to the amino-borane intermediate.

Because of relevance to H₂ storage^1–10 and hydrogenation catalysis,^11–15 metal amine-borane complexes^16–18 and their dehydrogenated forms, such as amino-boranes^20–22 and imino-boranes⁴ are arising as a significant family in organometallic chemistry. In transition metal-catalyzed dehydrocoupling of amine-boranes and related transfer hydrogenations, the interactions between the metal and the borane fragment are essential to dehydrogenation and the consequent transformations.^16–20 Specifically, amino-borane complexes containing a M–H₂B NR₂ moiety are the primary dehydrogenated species and are often identified as a resting point in the catalysis (Scheme 1a).^20–22 Management of reversible dehydrogenation–regeneration reactions on a M–BH₂ NR₂ platform could provide a strategy with which to design efficient catalysts capable of operating sustainable syntheses.Open in a separate windowScheme 1Schematic representation of metal-based amine-borane dehydrogenation.Wider exploration of metal amino-borane chemistry is challenging since M–H₂B NH₂ species are very reactive toward H₂ release. In 2010, Aldridge et al. reported the isolation of [(IMes)₂Rh(H)₂(η²-H₂B NR₂)] and [(IMes)₂Ir(H)₂(η²-H₂B NR₂)] from the metal-catalyzed dehydrogenation of R₂HN·BH₃.^21a At the same time, Alcaraz and Sabo-Etienne reported the preparation of (PCy₃)₂Ru(H)₂(η²-H₂B NH_nMe_2−n) (n = 0–2) complexes^22a by the dehydrogenation of amine-boranes with the corresponding ruthenium precursors. Subsequently, a straightforward synthesis of Ru, Rh, and Ir amino-borane complexes by reaction of H₂B NR₂ (R = iPr or Cy) with the bis(hydrogen) complexes of M(H)₂(η²-H₂)₂(PCy₃)₂ or [CpRu(PR₃)₂]⁺ fragments was developed.^21b,22b Turculet et al. have shown that the ruthenium-alkoxide complex is able to activate H₃B·NHR₂ producing hydrido ruthenium complex.²³ Notably, Weller and Macgregor found that dehydrocoupling of ammonia-borane by [Ph₂P(CH₂)₃PPh₂Rh(η⁶-C₆H₅F)] affords a μ-amino-borane bimetallic Rh complex, in which the simplest H₂B NH₂ moiety is trapped on a rhodium dimer.^20aAlthough iron-catalyzed dehydrocoupling of amine-boranes has attracted great interest,^24–29 iron amine-borane complexes, their dehydrogenated derivatives, and especially the catalysis relevant to organic synthesis are largely unexplored. Recently, Kirchner et al. reported a pincer-type iron complex generated by protonation of the borohydride iron complex (PNP)Fe(H)(η²-BH₄) with ammonium salts.³⁰ Inspired by earlier research on M–H₂B NR₂ chemistry, we intended to establish the reversible conversions of amino-borane complexes and their dehydrogenated forms in a synthetic piano-stool iron system. Herein, we report dehydrogenation of iron amido-borane complex Cp*Fe(η¹-H₃B–NHC₆H₄PPh₂) (2) (Cp* = Me₅C₅⁻) to the imino-borane complex Cp*Fe(η²-H₂B NC₆H₄PPh₂) (3), and resaturation of the imino-borane by stepwise protonation and hydride transfer (Scheme 1b). This new class of iron species is capable of catalyzing 1,2-selective transfer hydrogenation of quinolines with H₃N·BH₃.To synthesize the iron amido-borane complex, a new monomer, the iron tetrahydridoborate precursor Cp*Fe(η²-BH₄)(NCMe) (1), was prepared in situ by the reaction of [Cp*Fe(NCMe)₃]PF₆ with Bu₄NBH₄ in acetonitrile at room temperature for 5 min. Such ferrous borohydrides have been documented only rarely,³¹ since they are prone to form polynuclear iron borate clusters.^32,33 The ¹¹B NMR spectrum of the reaction solution shows a quintet at δ 15.4 (J_BH = 88 Hz) for the BH₄⁻ ligand of 1, and this stands in contrast to the signal at δ −32.0 observed for Bu₄NBH₄. Upon storing the reaction mixture at −30 °C overnight, single crystals suitable for X-ray diffraction were obtained. Crystallographic analysis confirmed the structure of 1 as a piano-stool iron tetrahydridoborate compound (ESI, Fig. S1^†).Addition of phosphinoamine ligand 1,2-Ph₂PC₆H₄NH₂ to a solution of 1 in acetonitrile caused an instantaneous color change from deep blue to dark brown (Scheme 2). ESI-MS studies indicated the production of the iron amido-borane compound (2) with m/z = 481.1793 (calcd m/z = 481.1770), which was isolated in 87% yield. NMR spectra showed a boron resonance at δ −17.5, and a phosphorus resonance at δ 85.9. The ¹H NMR spectrum exhibits a characteristic hydride signal at δ −13.98, which is assigned to the bridging hydride Fe–H–B. Owing to exchange between the hydrogen atoms at the boron,³⁴ the terminal B–H resonances in the ¹H NMR spectrum are very broad and are obscured by the distinct Cp* signals. To assign the B–H hydride signals, the deuterated compound Cp*Fe(D₃B–NHC₆H₄PPh₂) (d-2) was synthesized from Cp*Fe(BD₄)(NCMe). In addition to the Fe–D–B signal at δ −13.98, the ²H NMR spectrum of d-2 displayed discrete peaks at δ 2.23 and 0.19 for the terminal B–D hydrides (Fig. 1).Open in a separate windowFig. 1 ²H NMR spectra for dehydrogenation of d-2 to d-3.Open in a separate windowScheme 2Synthetic route to imino-borane complex.When a C₆H₆ solution of 2 was held at 50 °C for 6 h the dehydrogenated imino-borane compound (3) was produced in 92% yield. The ESI-MS spectrum of 3 has a strong peak at m/z 479.1626 (calcd m/z = 479.1637) which can be compared to the peak at m/z = 481.1793 for 2. The isotopic distributions match well with the calculated values (see Fig. S3^†). GC analysis shows that the reaction produced H₂ nearly quantitatively (see Fig. S4^†). In solution, the ³¹P NMR spectrum of 3 displays a sharp signal at δ 71.9, in contrast to the peak at δ 85.9 for 2. The ¹¹B resonance shifts significantly, from δ −17.5 for 2 to δ 42.7 for 3 (Fig. S16^†), and is particularly diagnostic of a three-coordinate boron atom.^21,35 This result indicates the B N double bond character in the dehydrogenated form of the amido-borane complex. In the ¹H NMR spectrum, the Fe–H–B signal was observed at δ −17.91 with the integral of 2H, and no characteristic signal for a terminal B–H hydride was found. To confirm the formation of an imino-borane compound, the hydrogen decoupling was also carried out with compound d-2 and monitored by ²H NMR spectra. Only a deuterium signal was observed at δ −17.91 for Fe–D–B, indicating the formation of d-3 (Fig. 1). When the dehydrogenation was conducted in a J-Young tube in C₆D₆, a characteristic triplet corresponding to HD appeared at δ 4.43 (J_HD = 45 Hz) in the ¹H NMR spectrum (Fig. S18^†).³⁶The structures of 2 and 3 were verified by X-ray crystallographic analysis (Fig. 2). Consistent with NMR spectroscopic analysis, the BH₃ moiety in 2 is stabilized by one of the B–H bonds binding at the Fe–NH unit to form an Fe–H–B–N four-membered metallacycle. This metal–ligand cooperative binding mode increased the B–H bond length in the bridging B–H(1) bond to 1.362 Å vs. 1.129 Å and 1.121 Å for the two terminal B–H bonds. The B–N bond length of 1.545(3) Å in 2 is slightly shorter than that in H₃B·NH₃ (d_B–N = 1.58(2) Å).³⁷ Crystallographic analysis of 3 confirmed an imino-borane complex with a Cp*Fe(η²-H₂B NC₆H₄PPh₂) framework. After dehydrogenation of 2, striking structural changes were observed. The N atom has been become detached from Fe, while the BH₂ fragment acts as a bis(σ-borane) ligand coordinated to the metal center.^21–23 The B–N bond distance of 1.455(5) Å in 3 is shorter by 0.09 Å than that in 2, and is close to that reported for the cyclic trimer borazine (1.4355(21) Å).³⁸ Combined with the NMR results, the B–N bond length in 3 suggests some double bond character.^21,22 As the imino-borane fragment is tethered in the coordination sphere, the boron center adopts a quasi-tetrahedral geometry, and the B–N bond appears to be partially sp³ hybridized. Dehydrogenation of the amido-borane complex also caused the decrease of the Fe⋯B distances from 2.223(3) Å to 2.026(4) Å which is shorter than the sum of the covalent radii of Fe and B atom (2.16 Å), indicating that the borane and the metal are bonded.Open in a separate windowFig. 2Solid-sate structure (50% probability thermal ellipsoids) of (a) complex 2 and (b) 3. For clarity, hydrogen atoms of Cp* and phenyl rings are omitted.Notably, the amido-borane compound 2 can be regenerated by stepwise protonation of 3 and transfer of a hydride (Scheme 3). Complex 3 reacts readily with H(Et₂O)₂BAr^F₄ in C₆H₅F. The reaction solution was analyzed by ESI-MS spectroscopy, which showed an ionic peak at m/z = 480.1726 (calcd m/z = 480.1715), suggesting the formation of [3H]⁺. Alternatively, the reaction of complex 2 with H(Et₂O)₂BAr^F₄ unambiguously provides [3H]⁺ and produces H₂. X-ray crystallographic analysis reveals that the resulting cationic complex [3H]⁺ exhibits a similar framework to its imino-borane precursor (3). The BH₂ moiety retains a binding mode of the bis(σ-BH₂) fashion (Fig. 3). In contrast, the B–N distance in [3H]⁺ (1.586(6) Å) is extended by 0.13 Å and the [3H]⁺ framework becomes much less compact than that of 3. Probably due to the fluxional structure of the seven-membered Fe–P–C–C–N–B(H) ring, the solution of [3H][BAr^F₄] gives broad ¹H NMR resonances even at −60 °C. The phosphorus resonance arose at δ 72.0 as a singlet when the solution sample was cooled to −40 °C (Fig. S20 and S21^†).Open in a separate windowFig. 3Solid-state structures of (a) complex [3H]⁺ and (b) [3H(PPh₃)]⁺. For clarity, counterion [BAr^F₄]⁻, hydrogen atoms of Cp* and phenyl rings have been omitted.Open in a separate windowScheme 3Conversions of iron imino-borane, amino-borane and amido-borane complexes.In [3H]⁺, the boron is coordinatively unsaturated, as manifested by its interaction with a σ-donor. For instance, treatment of 2 with [HPPh₃][BAr^F₄] (pK^MeCN_a = 7.6)³⁹ provides a Ph₃P-stabilized borane complex, [3H(PPh₃)]⁺ (m/z = 742.2620, calcd m/z = 742.2626). The ¹H NMR spectrum of [3H(PPh₃)]⁺ exhibits an NH resonance at δ 4.68, suggesting that protonation occurred at the N site. The distinctive upfield hydride signal for Fe–H–B is observed at δ −15.58. In the ³¹P NMR spectrum, two phosphorus signals at δ 78.90 and −1.26 correspond to the Fe–P and the B–P resonances, respectively. The ¹¹B signal at δ −13.72 indicates a tetracoordinated boron, which is further confirmed by crystallographic analysis of [3H(PPh₃)]⁺ (Fig. 3). In the solid-sate structure, a Ph₃P molecule is bound to the B center (d_B–P = 1.982(4) Å), leading to the formation of a new Fe–H–B–N four-membered metallacycle. As a amido-borane complex, [3H(PPh₃)]⁺ has a B–N bond length of 1.527(5) Å, somewhat shorter than 1.545(3) Å in 2.After attaching a proton at the N atom, we subsequently explored restoration of the original borane moiety. Treatment of freshly prepared [3H][BAr^F₄] in fluorobenzene with catecholborane-NEt₃ adduct (δ_B = 10.56, J_HB = 142.4 Hz)⁴⁰ results in the regeneration of 2, as evidenced by the NMR spectra (Fig. S29 and S30^†). The ¹H NMR spectrum of the reaction mixture displays a characteristic hydride signal at −13.97 ppm, indicating the recovery of the iron amido-borane complex. On the other side, concomitant formation of the borenium ion (δ_B = 13.86) was also observed in the ¹¹B NMR spectrum, which agrees with the hydride transfer from the organohydride reagent to [3H]⁺. It was interesting that the ion [3H]⁺ is stable towards 5,6-dihydrophenanthridine and Hantszch ester. These results indicate that the hydride-donating ability (ΔG_H⁻) of 2 is in the range of 55–59 kcal mol⁻¹.⁴¹ The reactive nature of the hydride in 2 was demonstrated by the reaction with [HPPh₃][BAr^F₄], which produces [3H(PPh₃)]⁺ and releases H₂ (Scheme 3).1The metal amine-borane complexes and their dehydrogenated derivatives are implicated throughout the catalytic cycle of amine-borane dehydrogenation. We found both the iron complexes 2 and 3 are efficient catalysts for H₃N·BH₃ dehydrogenation at room temperature. In the presence of 1 mol% catalyst, a THF solution of H₃N·BH₃ (1.0 mmol) generates about 2.2 equivalent of H₂ within 6 h based on GC quantification (Fig. S33^†). More importantly, such catalytic dehydrocoupling systems allow for selective transfer hydrogenation of quinolines to dihydroquinolines, which are valuable synthons leading to many bio-active compounds.⁴² For instance, addition of methyl-6-quinolineacetate (4) to the catalytic system containing one equiv. of H₃N·BH₃ and 1 mol% of 3 gave 1,2-dihydro-methyl-6-quinolineacetate (5) in excellent yield within 6 h (eqn (1)). The outcome of this reaction was unaffected by switching the catalyst from 3 to 2, or by use of excess reducing agent or by an increase in the reaction temperature (Table S1^†).     

3D-visualization of amyloid-β oligomer interactions with lipid membranes by cryo-electron tomography

Amyloid-β (Aβ) assemblies have been shown to bind to lipid bilayers. This can disrupt membrane integrity and cause a loss of cellular homeostasis, that triggers a cascade of events leading to Alzheimer''s disease. However, molecular mechanisms of Aβ cytotoxicity and how the different assembly forms interact with the membrane remain enigmatic. Here we use cryo-electron tomography (cryoET) to obtain three-dimensional nano-scale images of various Aβ assembly types and their interaction with liposomes. Aβ oligomers and curvilinear protofibrils bind extensively to the lipid vesicles, inserting and carpeting the upper-leaflet of the bilayer. Aβ oligomers concentrate at the interface of vesicles and form a network of Aβ-linked liposomes, while crucially, monomeric and fibrillar Aβ have relatively little impact on the membrane. Changes to lipid membrane composition highlight a significant role for GM1-ganglioside in promoting Aβ-membrane interactions. The different effects of Aβ assembly forms observed align with the highlighted cytotoxicity reported for Aβ oligomers. The wide-scale incorporation of Aβ oligomers and curvilinear protofibrils into the lipid bilayer suggests a mechanism by which membrane integrity is lost.

Cryo-electron tomography 3D imaging of amyloid-β oligomers carpeting the surface of lipid bilayers in near native conditions.     

Transition-metal-like bonding behaviors of a boron atom in a boron-cluster boronyl complex [(η7-B7)-B-BO]−

Boron displays many unusual structural and bonding properties due to its electron deficiency. Here we show that a boron atom in a boron monoxide cluster (B₉O⁻) exhibits transition-metal-like properties. Temperature-dependent photoelectron spectroscopy provided evidence of the existence of two isomers for B₉O⁻: the main isomer has an adiabatic detachment energy (ADE) of 4.19 eV and a higher energy isomer with an ADE of 3.59 eV. The global minimum of B₉O⁻ is found surprisingly to be an umbrella-like structure (C_6v, ¹A₁) and its simulated spectrum agrees well with that of the main isomer observed. A low-lying isomer (C_s, ¹A′) consisting of a BO unit bonded to a disk-like B₈ cluster agrees well with the 3.59 eV ADE species. The unexpected umbrella-like global minimum of B₉O⁻ can be viewed as a central boron atom coordinated by a η⁷-B₇ ligand on one side and a BO ligand on the other side, [(η⁷-B₇)-B-BO]⁻. The central B atom is found to share its valence electrons with the B₇ unit to fulfill double aromaticity, similar to that in half-sandwich [(η⁷-B₇)-Zn-CO]⁻ or [(η⁷-B₇)-Fe(CO)₃]⁻ transition-metal complexes. The ability of boron to form a half-sandwich complex with an aromatic ligand, a prototypical property of transition metals, brings out new metallomimetic properties of boron.

The global minimum of the B₉O⁻ cluster is found to have an umbrella-like structure, where the central B atom exhibits transition-metal-like bonding properties, coordinated by a η⁷-B₇ ligand on one side and a BO ligand on the other.     

Synthesis,bonding properties and ether activation reactivity of cyclobutadienyl-ligated hybrid uranocenes

A series of hybrid uranocenes consisting of uranium(iv) sandwiched between cyclobutadienyl (Cb) and cyclo-octatetraenyl (COT) ligands has been synthesized, structurally characterized and studied computationally. The dimetallic species [(η⁴-Cb′′′′)(η⁸-COT)U(μ:η²:η⁸-COT)U(THF)(η⁴-Cb′′′′)] (1) forms concomitantly with, and can be separated from, monometallic [(η⁴-Cb′′′′)U(THF)(η⁸-COT)] (2) (Cb′′′′ = 1,2,3,4-tetrakis(trimethylsilyl)cyclobutadienyl, COT = cyclo-octatetraenyl). In toluene solution at room temperature, 1 dissociates into 2 and the unsolvated uranocene [(η⁴-Cb′′′′)U(η⁸-COT)] (3). By applying a high vacuum, both 1 and 2 can be converted directly into 3. Using bulky silyl substituents on the COT ligand allowed isolation of base-free [(η⁴-Cb′′′′)U{η⁸-1,4-(ⁱPr₃Si)₂C₈H₆}] (4), with compounds 3 and 4 being new members of the bis(annulene) family of actinocenes and the first to contain a cyclobutadienyl ligand. Computational studies show that the bonding in the hybrid uranocenes 3 and 4 has non-negligible covalency. New insight into actinocene bonding is provided by the complementary interactions of the different ligands with uranium, whereby the 6d orbitals interact most strongly with the cyclobutadienyl ligand and the 5f orbitals do so with the COT ligands. The redox-neutral activation of diethyl ether by [(η⁴-Cb′′′′)U(η⁸-C₈H₈)] is also described and represents a uranium-cyclobutadienyl cooperative process, potentially forming the basis of further small-molecule activation chemistry.

The synthesis, structure and bonding in a series of hybrid uranocenes consisting of cyclobutadienyl and cyclo-octatetraenyl ligands is described.     

Beta-Amyloid (Aβ1-42) Increases the Expression of NKCC1 in the Mouse Hippocampus

Alzheimer’s disease (AD) is a neurodegenerative disorder with an increasing need for developing disease-modifying treatments as current therapies only provide marginal symptomatic relief. Recent evidence suggests the γ-aminobutyric acid (GABA) neurotransmitter system undergoes remodeling in AD, disrupting the excitatory/inhibitory (E/I) balance in the brain. Altered expression levels of K-Cl-2 (KCC2) and N-K-Cl-1 (NKCC1), which are cation–chloride cotransporters (CCCs), have been implicated in disrupting GABAergic activity by regulating GABA_A receptor signaling polarity in several neurological disorders, but these have not yet been explored in AD. NKCC1 and KCC2 regulate intracellular chloride [Cl⁻]_i by accumulating and extruding Cl⁻, respectively. Increased NKCC1 expression in mature neurons has been reported in these disease conditions, and bumetanide, an NKCC1 inhibitor, is suggested to show potential therapeutic benefits. This study used primary mouse hippocampal neurons to explore if KCC2 and NKCC1 expression levels are altered following beta-amyloid (Aβ_1-42) treatment and the potential neuroprotective effects of bumetanide. KCC2 and NKCC1 expression levels were also examined in 18-months-old male C57BL/6 mice following bilateral hippocampal Aβ_1-42 stereotaxic injection. No change in KCC2 and NKCC1 expression levels were observed in mouse hippocampal neurons treated with 1 nM Aβ_1-42, but NKCC1 expression increased 30-days post-Aβ_1-42-injection in the CA1 region of the mouse hippocampus. Primary mouse hippocampal cultures were treated with 1 nM Aβ_1-42 alone or with various concentrations of bumetanide (1 µM, 10 µM, 100 µM, 1 mM) to investigate the effect of the drug on cell viability. Aβ_1-42 produced 53.1 ± 1.4% cell death after 5 days, and the addition of bumetanide did not reduce this. However, the drug at all concentrations significantly reduced cell viability, suggesting bumetanide is highly neurotoxic. In summary, these results suggest that chronic exposure to Aβ_1-42 alters the balance of KCC2 and NKCC1 expression in a region-and layer-specific manner in mouse hippocampal tissue; therefore, this process most likely contributes to altered hippocampal E/I balance in this model. Furthermore, bumetanide induces hippocampal neurotoxicity, thus questioning its suitability for AD therapy. Further investigations are required to examine the effects of Aβ_1-42 on KCC2 and NKCC1 expression and whether targeting CCCs might offer a therapeutic approach for AD.     

Hydrogenation of β-Keto Sulfones to β-Hydroxy Sulfones with Alkyl Aluminum Compounds: Structure of Intermediate Hydroalumination Products

β-Hydroxy sulfones are important in organic synthesis. The simplest method of β-hydroxy sulfones synthesis is the hydrogenation of β-keto sulfones. Herein, we report the reducing properties of alkyl aluminum compounds R₃Al (R = Et, i-Bu, n-Bu, t-Bu and n-Hex); i-Bu₂AlH; Et₂AlCl and EtAlCl₂ in the hydrogenation of β-keto sulfones. The compounds i-Bu₂AlH, i-Bu₃Al and Et₃Al are the at best reducing agents of β-keto sulfones to β-hydroxy sulfones. In reactions of β-keto sulfones with aluminum trialkyls, hydroalumination products with β-hydroxy sulfone ligands [R₂AlOC(C₆H₅)CH₂S(O)₂(p-R¹C₆H₄]_n [where n = 1,2; 2aa: R = i-Bu, R¹ = CH₃; 2ab: R = i-Bu, R¹ = Cl; 2ba: R = Et, R¹ = CH₃; 2bb: R = Et, R¹ = Cl] and {[Et₂AlOC(C₆H₅)CH₂S(O)₂(p-ClC₆H₄]∙Et₃Al}_n 3bb were obtained. These complexes in the solid state have a dimeric structure, while in solutions, they appear as equilibrium monomer–dimer mixtures. The hydrolysis of both the isolated 2aa, 2ab, 2ba, 2bb and 3bb and the postreaction mixtures quantitatively leads to pure racemic β-hydroxy sulfones. Hydroalumination reaction of β-keto sulfones with alkyl aluminum compounds and subsequent hydrolysis of the complexes is a simple and very efficient method of β-hydroxy sulfones synthesis.     

Mechanistic insights into rhodium-catalyzed enantioselective allylic alkylation for quaternary stereogenic centers

Installing quaternary stereogenic carbon is an arduous task of contemporary importance in the domain of asymmetric catalysis. To this end, an asymmetric allylic alkylation of α,α-disubstituted aldehydes by using allyl benzoate in the presence of Wilkinson''s catalyst [Rh(Cl)(PPh₃)₃], (R)-BINOL–P(OMe) as the external ligand, and LiHMDS as the base has been reported to offer high enantioselectivity. The mechanistic details of this important reaction remain vague, which prompted us to undertake a detailed density functional theory (SMD_(THF)/B3LYP-D3) investigation on the nature of the potential active catalyst, energetic features of the catalytic cycle, and the origin of high enantioselectivity. We note that a chloride displacement from the native Rh-phosphine [Rh(Cl)(PPh₃)₃] by BINOL–P(OMe) phosphite and an ensuing MeCl elimination can result in the in situ formation of a Rh-phosphonate [Rh(BINOL–P O)(PPh₃)₃]. A superior energetic span (δE) noted with such a Rh-phosphonate suggests that it is likely to serve as an active catalyst. The uptake of allyl benzoate by the active catalyst followed by the turnover determining C–O bond oxidative addition furnishes a Rh-π-allyl intermediate, which upon interception by (Z)-Li-enolate (derived from α,α-disubstituted aldehyde) in the enantiocontrolling C–C bond generates a quaternary stereogenic center. The addition of the re prochiral face of the (Z)-Li-enolate to the Rh-bound allyl moiety leading to the R enantiomer of the product is found to be 2.4 kcal mol⁻¹ more preferred over the addition through its si face. The origin of the stereochemical preference for the re face addition is traced to improved noncovalent interactions (NCIs) and less distortion in the enantiocontrolling C–C bond formation transition state than that in the si face addition. Computed enantioselectivity (96%) is in very good agreement with the experimental value (92%), so is the overall activation barrier (δE of 17.1 kcal mol⁻¹), which is in conformity with room temperature reaction conditions.

The origin of high enantioselectivity in the formation of quaternary stereogenic carbon.     

Tuning π-Acceptor/σ-Donor Ratio of the 2-Isocyanoazulene Ligand: Non-Fluorinated Rival of Pentafluorophenyl Isocyanide and Trifluorovinyl Isocyanide Discovered

Isocyanoazulenes (CNAz) constitute a relatively new class of isocyanoarenes that offers rich structural and electronic diversification of the organic isocyanide ligand platform. This article considers a series of 2-isocyano-1,3-X₂-azulene ligands (X = H, Me, CO₂Et, Br, and CN) and the corresponding zero-valent complexes thereof, [(OC)₅Cr(2-isocyano-1,3-X₂-azulene)]. Air- and thermally stable, X-ray structurally characterized 2-isocyano-1,3-dimethylazulene may be viewed as a non-benzenoid aromatic congener of 2,6-dimethyphenyl isocyanide (2,6-xylyl isocyanide), a longtime “workhorse” aryl isocyanide ligand in coordination chemistry. Single crystal X-ray crystallographic {Cr–CNAz bond distances}, cyclic voltametric {E_1/2(Cr^0/1+)}, ¹³C NMR {δ(¹³CN), δ(¹³CO)}, UV-vis {dπ(Cr) → pπ*(CNAz) Metal-to-Ligand Charge Transfer}, and FTIR {ν_N≡C, ν_C≡O, k_C≡O} analyses of the [(OC)₅Cr(2-isocyano-1,3-X₂-azulene)] complexes provided a multifaceted, quantitative assessment of the π-acceptor/σ-donor characteristics of the above five 2-isocyanoazulenes. In particular, the following inverse linear relationships were documented: δ(¹³CO_trans) vs. δ(¹³CN), δ(¹³CO_cis) vs. δ(¹³CN), and δ(¹³CO_trans) vs. k_C≡O,trans force constant. Remarkably, the net electron withdrawing capability of the 2-isocyano-1,3-dicyanoazulene ligand rivals those of perfluorinated isocyanides CNC₆F₅ and CNC₂F₃.     

Synthesis of unstrained Criegee intermediates: inverse α-effect and other protective stereoelectronic forces can stop Baeyer–Villiger rearrangement of γ-hydroperoxy-γ-peroxylactones

How far can we push the limits in removing stereoelectronic protection from an unstable intermediate? We address this question by exploring the interplay between the primary and secondary stereoelectronic effects in the Baeyer–Villiger (BV) rearrangement by experimental and computational studies of γ-OR-substituted γ-peroxylactones, the previously elusive non-strained Criegee intermediates (CI). These new cyclic peroxides were synthesized by the peroxidation of γ-ketoesters followed by in situ cyclization using a BF₃·Et₂O/H₂O₂ system. Although the primary effect (alignment of the migrating C–R_m bond with the breaking O–O bond) is active in the 6-membered ring, weakening of the secondary effect (donation from the OR lone pair to the breaking C–R_m bond) provides sufficient kinetic stabilization to allow the formation and isolation of stable γ-hydroperoxy-γ-peroxylactones with a methyl-substituent in the C6-position. Furthermore, supplementary protection is also provided by reactant stabilization originating from two new stereoelectronic factors, both identified and quantified for the first time in the present work. First, an unexpected boat preference in the γ-hydroperoxy-γ-peroxylactones weakens the primary stereoelectronic effects and introduces a ∼2 kcal mol⁻¹ Curtin–Hammett penalty for reacquiring the more reactive chair conformation. Second, activation of the secondary stereoelectronic effect in the TS comes with a ∼2–3 kcal mol⁻¹ penalty for giving up the exo-anomeric stabilization in the 6-membered Criegee intermediate. Together, the three new stereoelectronic factors (inverse α-effect, misalignment of reacting bonds in the boat conformation, and the exo-anomeric effect) illustrate the richness of stereoelectronic patterns in peroxide chemistry and provide experimentally significant kinetic stabilization to this new class of bisperoxides. Furthermore, mild reduction of γ-hydroperoxy-γ-peroxylactone with Ph₃P produced an isolable γ-hydroxy-γ-peroxylactone, the first example of a structurally unencumbered CI where neither the primary nor the secondary stereoelectronic effect are impeded. Although this compound is relatively unstable, it does not undergo the BV reaction and instead follows a new mode of reactivity for the CI – a ring-opening process.

Protecting stereoelectronic effects prevent Baeyer–Villiger rearrangement and stabilize γ-OX-γ-peroxylactones (X = H, OH), the previously elusive non-strained Criegee intermediates.     

Diverse ring-opening reactions of rhodium η4-azaborete complexes

Sequential treatment of [Rh(COE)₂Cl]₂ (COE = cyclooctene) with PⁱPr₃, alkyne derivatives and ^tBuN BMes (Mes = 2,4,6-trimethylphenyl) provided functionalized rhodium η⁴-1,2-azaborete complexes of the form (η⁴-azaborete)RhCl(PⁱPr₃). The scope of this reaction was expanded to encompass alkynes with hydrogen, alkyl, aryl, ferrocenyl, alkynyl, azaborinyl and boronate ester substituents. Treatment of these complexes with PMe₃ led to insertion of the rhodium atom into the B–C bond of the BNC₂ ring, forming 1-rhoda-3,2-azaboroles. Addition of N-heterocyclic carbenes to azaborete complexes led to highly unusual rearrangements to rhodium η²,κ¹-allenylborylamino complexes via deprotonation and C–N bond cleavage. Heating and photolysis of an azaborete complex also led to rupture of the C–N bond followed by subsequent rearrangements, yielding an η⁴-aminoborylallene complex and two isomeric η⁴-butadiene complexes.

Rhodium η⁴-azaborete complexes can be transformed into a variety of species with ring-opened, BN-containing ligands by treatment with Lewis bases.