NMR Spectroscopy Experiments Research Articles

A cell-compatible phenolphthalein-aminophenol scaffold for Al3+sensing assisted by CHEF phenomenon

Monitoring of excess Al3+ and its selective detection in biological and environmental samples are important tasks for researcher due to its harmful effects. Herein, we present a phenolphthalein-aminophenol derived sensor (PFAP) for the selective fluorescent response of Al3+ in an "OFF-ON" mode over a pool of analytes including eighteen cations with striking greenish emission. It has a prominent limit of detection (LOD) value (1.5 µM), fast response time (10 s) for Al3+ detection. The complexation properties of PFAP with Al3+ ions were clarified by UV–vis, 1H & 13C NMR, HRMS, and FTIR spectroscopic experiments. The recognition mechanism of PFAP for Al3+working with chelation enhanced fluorescence (CHEF) and is verified with two model compounds PF and PFAN in the light of UV–vis, PL, TRPL, and NMR experiment. To check its applicability, easily prepared test paper and TLC strip of PFAP was produced for rapid and selective onsite detection of Al3+ ions. Bio-imaging application of PFAP in human lung cancer cell lines A549 demonstrated excellent results with negligible cytotoxicity and as an excellent marker to detect traces of Al3+ ion in a time-dependent as well as concentration-dependent manner. Actual sample analysis for Al3+ with the probe PFAP produces fruitful result.

The Preparation and Resolution of Novel Axially Chiral Pyrazine‐Containing P,N Ligands for Asymmetric Catalysis and Their Application in Palladium‐Catalysed Allylic Substitution

AbstractThe design and synthesis of two novel pyrazine‐containing ligands, 3,6‐diisopropyl‐Pyrazinap and Np−Cy‐Pyrazinap, has been described. Their synthesis was accomplished in six and nine steps, respectively, in which the key steps were the formation of the biaryl linkage through a Suzuki cross‐coupling reaction and the formation of the aryl to phosphorus bond employing nickel catalysis. Both ligands were resolved through the formation of diastereomeric palladacycles and in the case of the isolated diastereomer formed from 3,6‐diisopropyl‐Pyrazinap, an X‐ray crystal structure showed the axial chirality to possess (S)‐configuration. The two enantiopure ligands were applied to the palladium‐catalysed allylic alkylation of 1,3‐diphenylprop‐2‐enyl acetate with dimethyl malonate and afforded conversions of up to 100 % and 99 % with enantioselectivities of up to 92 % ee and 78 % ee, respectively. A series of 1,1,3‐triphenylallyl and 1,3‐diphenylallyl palladium complexes were prepared and their solution structures and dynamic behaviour elucidated by 2D‐COSY and NOESY NMR spectroscopic experiments.

Indium-Catalyzed CO2/Epoxide Copolymerization: Enhancing Reactivity with a Hemilabile Phosphine Donor.

Group 13 metal complexes have emerged as powerful catalysts for transforming CO2 into added-value products. However, direct comparisons of reactivity between Al, Ga, and In catalysts are rare. We report aluminum (1), gallium (2), and indium (3) complexes supported by a half-salen H[PNNO] ligand with a pendent phosphine donor and investigate their activity as catalysts for the copolymerization of CO2 and cyclohexene oxide. In solution, the P-donor is dissociated for the Al and Ga complexes while for the In complex it exhibits hemilabile behavior. The indium complex shows higher conversion and selectivity than the Al or Ga analogues. The mechanism of the reaction was studied by NMR and FTIR spectroscopy experiments as well as structural characterization of off-cycle catalytic intermediate indium trichloride complex [(PNNO)InCl3][TBA] (4). This study highlights the impact of a hemilabile phosphine group on group 13 metals and provides a detailed analysis of the initiation step in CO2/epoxide copolymerization reactions.

BN-Embedded Perylenes.

Transition metal catalyzed coupling reaction strategy has been utilized in the synthesis of two novel BN-perylenes starting from halogenated BN-naphthalene derivatives. The molecular structures and packing modes of BN-perylenes were confirmed by NMR spectroscopy and X-ray single-crystal diffraction experiments. Their photophysical properties were further investigated using UV-vis and fluorescence spectroscopy and DFT calculations. Interestingly, the isosteric BN-insertion in perylene system resulted in stronger π-π stacking interaction both in solid and solution phases. The synthesized BN-perylenes are proved to be highly stable and thus provide a new valuable platform for novel organic materials applications which is otherwise inaccessible to date.

Convenient Synthesis of Pyrazolo[4',3':5,6]pyrano[4,3-c][1,2]oxazoles via Intramolecular Nitrile Oxide Cycloaddition.

A simple and efficient synthetic route to the novel 3a,4-dihydro-3H,7H- and 4H,7H-pyrazolo[4′,3′:5,6]pyrano[4,3-c][1,2]oxazole ring systems from 3-(prop-2-en-1-yloxy)- or 3-(prop-2-yn-1-yloxy)-1H-pyrazole-4-carbaldehyde oximes has been developed by employing the intramolecular nitrile oxide cycloaddition (INOC) reaction as the key step. The configuration of intermediate aldoximes was unambiguously determined using NOESY experimental data and comparison of the magnitudes of 1JCH coupling constants of the iminyl moiety, which were greater by approximately 13 Hz for the predominant syn isomer. The structures of the obtained heterocyclic products were confirmed by detailed 1H, 13C and 15N NMR spectroscopic experiments and HRMS measurements.

Two seco-norabietane diterpenoids with unprecedented skeletons from the roots of Salvia abrotanoides (Kar.) Sytsma

Two seco-norabietane diterpenes with unique structures, namely abrotafuran and abrotacid, were isolated from the roots of Salvia abrotanoides (Kar.) Sytsma. The compounds were characterized by 1D and 2D NMR spectroscopic techniques, ECD, and HR-ESIMS experiments. Plausible biosynthetic pathways of abrotafuran and abrotacid were proposed. These compounds did not exhibit antimicrobial activity against Staphylococcus aureus and Pseudomonas aeruginosa. However, the rearranged seco-norabietane abrotafuran showed antiproliferative activity on HeLa (cervical carcinoma) and Jurkat (T-cell leukemia) cell lines.

A New Solid-State Proton Conductor: The Salt Hydrate Based on Imidazolium and 12-Tungstophosphate.

We report the structure and charge transport properties of a novel solid-state proton conductor obtained by acid–base chemistry via proton transfer from 12-tungstophosphoric acid to imidazole. The resulting material (henceforth named Imid3WP) is a solid salt hydrate that, at room temperature, includes four water molecules per structural unit. To our knowledge, this is the first attempt to tune the properties of a heteropolyacid-based solid-state proton conductor by means of a mixture of water and imidazole, interpolating between water-based and ionic liquid-based proton conductors of high thermal and electrochemical stability. The proton conductivity of Imid3WP·4H2O measured at truly anhydrous conditions reads 0.8 × 10–6 S cm–1 at 322 K, which is higher than the conductivity reported for any other related salt hydrate, despite the lower hydration. In the pseudoanhydrous state, that is, for Imid3WP·2H2O, the proton conductivity is still remarkable and, judging from the low activation energy (Ea = 0.26 eV), attributed to structural diffusion of protons. From complementary X-ray diffraction data, vibrational spectroscopy, and solid-state NMR experiments, the local structure of this salt hydrate was resolved, with imidazolium cations preferably orienting flat on the surface of the tungstophosphate anions, thus achieving a densely packed solid material, and water molecules of hydration that establish extremely strong hydrogen bonds. Computational results confirm these structural details and also evidence that the path of lowest energy for the proton transfer involves primarily imidazole and water molecules, while the proximate Keggin anion contributes with reducing the energy barrier for this particular pathway.

A Water Molecule Triggers Guest Exchange at a Mono-Zinc Centre Confined in a Biomimetic Calixarene Pocket: a Model for Understanding Ligand Stability in Zn Proteins.

In this study, the ligand exchange mechanism at a biomimetic ZnII centre, embedded in a pocket mimicking the possible constrains induced by a proteic structure, is explored. The residence time of different guest ligands (dimethylformamide, acetonitrile and ethanol) inside the cavity of a calix[6]arene-based tris(imidazole) tetrahedral zinc complex was probed using 1D EXchange SpectroscopY NMR experiments. A strong dependence of residence time on water content was observed with no exchange occurring under anhydrous conditions, even in the presence of a large excess of guest ligand. These results advocate for an associative exchange mechanism involving the transient exo-coordination of a water molecule, giving rise to 5-coordinate ZnII intermediates, and inversion of the pyramid at the ZnII centre. Theoretical modelling by DFT confirmed that the associative mechanism is at stake. These results are particularly relevant in the context of the understanding of kinetic stability/lability in Zn proteins and highlight the key role that a single water molecule can play in catalysing ligand exchange and controlling the lability of ZnII in proteins.

Chromone glycosides and phenolic glycoside from Scindapsus officinalis (Roxb.) Schott.

• We provided more information about the chemical constituents of Scindapsus officinalis . • We found one anti-inflammatory chromone glycoside reported firstly in literature. • Structures were elucidated by NMR, mass spectrometry and acid hydrolysis methods. Phytochemical and pharmacological investigations of the plants of Scindapsus officinalis (Roxb.) Schott, rendered the isolation of two new chromone glycosides and one phenolic glycoside, named officinalisides F, G (1, 2) and 4- O -[ α - l -rhamnopyranosyl-(1–3)- β - d -glucopyranosyl]-( E )-3-(4-hydroxy-3-methoxyphenyl) acrylic acid (3). Structures of the three undescribed compounds were elucidated by NMR spectroscopic and acid hydrolysis experiments. All isolates were evaluated for anti-inflammatory activity against nitric oxide (NO) production assay in vitro . As indicated in the results, compound 2 exhibited significant NO inhibitory effects with IC 50 value of 8.02 μM, while the other two ones showed weak NO inhibitory activities.

Co-factor-free aggregation of tau into seeding-competent RNA-sequestering amyloid fibrils

Pathological aggregation of the protein tau into insoluble aggregates is a hallmark of neurodegenerative diseases. The emergence of disease-specific tau aggregate structures termed tau strains, however, remains elusive. Here we show that full-length tau protein can be aggregated in the absence of co-factors into seeding-competent amyloid fibrils that sequester RNA. Using a combination of solid-state NMR spectroscopy and biochemical experiments we demonstrate that the co-factor-free amyloid fibrils of tau have a rigid core that is similar in size and location to the rigid core of tau fibrils purified from the brain of patients with corticobasal degeneration. In addition, we demonstrate that the N-terminal 30 residues of tau are immobilized during fibril formation, in agreement with the presence of an N-terminal epitope that is specifically detected by antibodies in pathological tau. Experiments in vitro and in biosensor cells further established that co-factor-free tau fibrils efficiently seed tau aggregation, while binding studies with different RNAs show that the co-factor-free tau fibrils strongly sequester RNA. Taken together the study provides a critical advance to reveal the molecular factors that guide aggregation towards disease-specific tau strains.

Enantioseparation of phenethylamines by using high-performance liquid chromatography column permanently coated with methylated β-cyclodextrin.

Cyclodextrins and their derivatives have been used for chiral high-performance liquid chromatography selectors, while they are costly to use as mobile phase additives in high-performance liquid chromatography. Here, we report application of phenyl column coated permanently with methylated β-cyclodextrin for chiral high-performance liquid chromatography. A 0.1% (v/v) phosphoric acid solution containing 1M NaCl and 0.5% (w/v) methylated β-cyclodextrin was subjected to a phenyl column at a flow rate of 0.5mL/min at 30°C for 2 h. Using the precoating phenyl column, all the enantiomers of the four phenethylamines (norepinephrine, epinephrine, octopamine, and synephrine) were successfully separated simultaneously by high-performance liquid chromatography with a mobile phase without methylated β-cyclodextrin at a flow rate of 0.2mL/min at 30°C. The enantioseparation ability was retained for successive analyses during 1 week. It is suggested that inclusion complex of methylated β-cyclodextrin with a phenyl group on the surface of the stationary phase could be formed and that the inclusion complex could form the ternary complex with the injected analytes. The longer retention time of (S)-enantiomers of analytes than corresponding (R)-enantiomers for high-performance liquid chromatography could be explained from the higher stability of the methylated β-cyclodextrin complexes with (S)-enantiomers, which were confirmed by capillary electrophoresis and 1 H NMR spectroscopy experiments.

Synthesis of Purine Analogues: Photocatalyst-Free Visible-Light-Enhanced Annulation Approach to Pyrazolo[1,5-a][1,3,5]triazine-2,4-diamines

A new photocatalyst-free visible-light-enhanced strategy for the synthesis of pyrazolo[1,5-a][1,3,5]triazine-2,4-diamines via the formation of electron donor-acceptor (EDA) complexes is reported. The in situ generated pyrazolthiourea intermediates from 1H-pyrazol-3-amines and isothiocyanates undergo formal [4 + 2] annulation with 1,1,3,3-tetramethylguanidines (TMG) to deliver the corresponding products involved in three C-N bond formations in a one-pot protocol. The formation of EDA complex from pyrazolthiourea and TMG is confirmed by UV-vis spectroscopy and 1H NMR experiments. Moreover, this mild reaction proceeds in the absence of any external transition metals, oxidants, bases, and ligands. This efficient methodology for the synthesis of purine analogues pyrazolo[1,5-a][1,3,5]triazine-2,4-diamines provides potential synthetic applications in the field of drug research and development.

Incorporation of TePhe into Expressed Proteins is Minimally Perturbing.

Tellurium is a versatile heavy chalcogen with numerous applications in chemical biology, providing valuable probes in mass cytometry, fluorescence imaging and structural biology. L-Tellurienylalanine (TePhe) is an analogue of the proteinogenic amino acid L-phenylalanine (Phe) in which the phenyl side chain has been replaced by a 5-membered tellurophene moiety. High incorporation level of TePhe in expressed proteins at defined sites is expected to facilitate studies in proteomics, protein NMR spectroscopy, and structure elucidation. As a model we chose immunoglobulin-binding Protein G, B1 domain (GB1) to validate TePhe as a suitable structural analogue for Phe. We demonstrate that approximately 1 in 2 of all Phe sites within GB1 can be substituted with TePhe through expression in standard non-Phe-auxotrophic E. coli in Phe-deficient media containing glyphosate, an inhibitor of aromatic amino acid biosynthesis. The TePhe content of the GB1 sample can be further increased to 85 % through HPLC. Using NMR and CD spectroscopy, we confirm that the Phe-to-TePhe substitution has negligible impact on the global structure and stability of GB1.

Phorbol Diesters and 12-Deoxy-16-hydroxyphorbol 13,16-Diesters Induce TGFα Release and Adult Mouse Neurogenesis.

A small library of phorbol 12,13-diesters bearing low lipophilicity ester chains was prepared as potential neurogenic agents in the adult brain. They were also used in a targeted UHPLC-HRMS screening of the latex of Euphorbia resinifera. Two new 12-deoxy-16-hydroxyphorbol 13,16-diesters were isolated, and their structures were deduced using two-dimensional NMR spectroscopy and NOE experiments. The ability of natural and synthetic compounds to stimulate transforming growth factor alpha (TFGα) release, to increase neural progenitor cell proliferation, and to stimulate neurogenesis was evaluated. All compounds that facilitated TGFα release promoted neural progenitor cell proliferation. The presence of two acyloxy moieties on the tigliane skeleton led to higher levels of activity, which decreased when a free hydroxyl group was at C-12. Remarkably, the compound bearing isobutyryloxy groups was the most potent on the TGFα assay and at inducing neural progenitor cell proliferation in vitro, also leading to enhanced neurogenesis in vivo when administered intranasally to mice.

Protein unfolded states populated at high and ambient pressure are similarly compact

The relationship between the dimensions of pressure-unfolded states of proteins compared with those at ambient pressure is controversial; resolving this issue is related directly to the mechanisms of pressure denaturation. Moreover, a significant pressure dependence of the compactness of unfolded states would complicate the interpretation of folding parameters from pressure perturbation and make comparison to those obtained using alternative perturbation approaches difficult. Here, we determined the compactness of the pressure-unfolded state of a small, cooperatively folding model protein, CTL9-I98A, as a function of temperature. This protein undergoes both thermal unfolding and cold denaturation, and the temperature dependence of the compactness at atmospheric pressure is known. High-pressure small angle x-ray scattering studies, yielding the radius of gyration and high-pressure diffusion ordered spectroscopy NMR experiments, yielding the hydrodynamic radius were carried out as a function of temperature at 250 MPa, a pressure at which the protein is unfolded. The radius of gyration values obtained at any given temperature at 250 MPa were similar to those reported previously at ambient pressure, and the trends with temperature are similar as well, although the pressure-unfolded state appears to undergo more pronounced expansion at high temperature than the unfolded state at atmospheric pressure. At 250 MPa, the compaction of the unfolded chain was maximal between 25 and 30°C, and the chain expanded upon both cooling and heating. These results reveal that the pressure-unfolded state of this protein is very similar to that observed at ambient pressure, demonstrating that pressure perturbation represents a powerful approach for observing the unfolded states of proteins under otherwise near-native conditions.

Methyl 2-Amino-4-[1-(tert-butoxycarbonyl)azetidin-3-yl]-1,3-selenazole-5-carboxylate

Methyl 2-amino-4-[1-(tert-butoxycarbonyl)azetidin-3-yl]-1,3-selenazole-5-carboxylate as a newly functionalized heterocyclic amino acid was obtained via [3+2] cycloaddition. The structure of the novel 1,3-selenazole was unequivocally confirmed by detailed 1H, 13C, 15N, and 77Se NMR spectroscopic experiments, HRMS and elemental analysis.

Dissertation Project for the Forensic Science Laboratory: Birch Reduction of Ephedrine and Analysis of Byproducts of Forensic Science Interest

This undergraduate dissertation project was designed to enhance organic and analytical chemistry skills of final year forensic science students. A modified Birch reaction method was employed to synthesize amphetamine from ephedrine and detect the most known byproduct of clandestine manufacture of amphetamines, e.g., 1-(1,4-cyclohexadienyl)-2-methylaminopropane (CMP). The use of this synthetic method led to simultaneous N-demethylation and dehydroxylation of the alkaloid. Harmful liquid ammonia and lithium metal, which are the typical reagents of the classic Birch reaction, were replaced by the safer sodium silica gel stage I (Na-SG-I) and an alcoholic proton source. Liquid chromatography–mass spectrometry (LC-MS) was used to monitor reaction progress and identify starting material, products, and byproducts, whereas proton and carbon nuclear magnetic resonance (1H and 13C NMR) and heteronuclear single-quantum correlation (HSQC) NMR spectroscopy experiments were employed to characterize amphetamine and the parent compound ephedrine. This multiweek project-based learning experience has been running for three years at De Montfort University (DMU), which offers a well-established Forensic Science BSc (Hons) undergraduate course. The students were guided with the help of an academic instructor through a research-like setting delving into synthesis, characterization, and analysis of a drug of abuse and its byproduct. As part of this learning experience, which culminated in a body of work that informed the content of final year dissertations, the students gained useful organic chemistry skills and consolidated previously acquired knowledge in trace analysis. A survey of the dissertations’ final grades and evaluation of end-of-project questionnaires revealed that students responded well to the challenges posed by the organic chemistry aspect of the lab and that the project topic and methods were engaging and very relevant to the forensic science course.

Charge‐Transfer‐Adaptable Double‐Strand Formation of a Porphyrin‐BODIPY‐Porphyrin Triad

AbstractA quadrupolar porphyrin–BODIPY–porphyrin array (1) displayed a moderate negative solvatochromism. The elastic 3,4,5‐tri((S)‐dihydrocitronellyloxy)phenyl groups of 1 facilitated the solubility of such a polarized chromophore array in a nonpolar solvent, and 1 was assembled into a double‐strand as elucidated by extensive NMR spectroscopic experiments in [D8]toluene and electronic absorption spectra in cyclohexane. Thermodynamic analyses revealed a small entropy change upon double‐strand formation, indicating that cyclohexane did not preferably solvate the porphyrin rings and BODIPY segment presumably because of their polarized nature. At the same time, the significant enthalpy change likely explained the intermolecular attractive force, such as the BF2⋅⋅⋅Zn cation‐dipole interaction. The solvent polarity effect on the electronic spectra together with thermodynamic analyses suggested that the charge‐transfer (CT) character was decreased by the double‐strand formation in a nonpolar solvent. It was rationalized that insufficient solvent polarity stabilized the CT character of 1 only in part, and the residual polarization character might be stabilized by supramolecular assembly. The charge transfer character of 1 was stabilized by double‐strand formation which partially decreased the CT character enough to be stabilized by solvation with cyclohexane.

Cytotoxic and Anti-Inflammatory Sesquiterpenes from the Whole Plants of Centipeda minima.

Sixteen new sesquiterpene lactones (1-16) along with 13 known analogues (17-29) were isolated from the whole plants of Centipeda minima. The structures of 1-16 were delineated by the combination of NMR spectroscopic experiments, HRESIMS, single-crystal X-ray diffraction analyses, and ECD spectra. Compounds 23-26 showed potent cytotoxicity against Hela, HCT-116, and HepG2 cells with IC50 values of 0.8-2.6, 0.4-3.3, and 1.1-2.6 μM, respectively. Compounds 8, 15, and 24 exhibited significant inhibitory activity on the production of nitric oxide in the lipopolysaccharide-activated RAW 264.7 mouse macrophage cell line, with IC50 values ranging from 0.1 to 0.2 μM.

O- and S-containing 1-azadiene derivatives of 3-aminopropylsilatrane

Silatranes bearing the aryl(hetaryl)-substituted 1-azadienyl moiety ((Het)ArCH=C(YR) C(H)=N- (Y = O, S)) were synthesized by the reaction of 1-(3-aminopropyl)silatrane with 2-alkoxy- and 2-alkylthio-3-(het)arylpropenals. The structures and stereoisomeric compositions of the synthesized compounds were proved by elemental analysis, IR spectroscopy, 1D 1H, 13C, 15N, 29Si, and 2D NMR experiments, mass spectrometry, and DFT calculations. O-Containing compounds exist exclusively as the 1E,2Z isomers, while S-containing derivatives are the mixtures of the 1E,2Z and 1E,2E stereoisomers. Newly synthesized chalcogen-containing poly-functional compounds represent the promising building blocks for design and synthesis of advanced materials and pharmaceuticals.