13C 1H Research Articles

Isolation and characterization of polyketide drug molecule from Streptomyces species with antimicrobial activity against clinical pathogens

BackgroundNatural products derived from marine microbes have more potential toward to treatment of various diseases. Among the microbes, the secondary metabolites recovered from the marine actinomycetes were more added values. ObjectiveA promising antimicrobial metabolite producing filamentous actinomycete SCA-7 recovered from Alkhobar marine region was investigated for its potential to inhibit Gram positive Enterococcus sp. In addition to the chemical characterization, the polyketide gene cluster of the actinomycete SCA-7 was sequenced. ResultsAmong the 8 actinomycetes isolated from the marine sample, the isolate SCA-7 produced significant antimicrobial activity against Enterococcus sp. The biochemical, physiological and morphological characteristics and fermentation assimilation pattern confirmed that the isolate belonged to the genus Streptomyces. The 16S rRNA gene amplification and sequencing results showed 99% sequence similarity to Streptomyces felleus. The antimicrobial activity of the crude ethyl acetate extract was performed by disc diffusion method. The spectral characterization was done by 13C NMR and 1H NMR. The compound was polyketide in nature. The Minimum Inhibitory Concentration (MIC) of the polyketide compound against Enterococcus sp. was 25μg/mL. Among the agro-industrial waste materials, wheat bran showed increased secondary metabolite production. Antibacterial activity was found to be high when the isolate SCA-7 was grown in wheat bran substrate and maximum zone of inhibition (22mm) was recorded in it. Among the carbon and nitrogen sources, lactose enhanced the production of secondary metabolites and the zone of inhibition against Enterococcus sp. was 25mm. The amplification and sequencing of the ketoacyl synthase gene clearly indicated that it was type I polyketide synthase (PKS) gene of Streptomyces species. ConclusionOverall, the therapeutic drug molecule isolated from the marine Streptomyces species might be used for the treatment of disease causing microbial clinical pathogen.

Novel 2-aminopyridine liganded Pd(II) N-heterocyclic carbene complexes: Synthesis, characterization, crystal structure and bioactivity properties

In this work, the synthesis, crystal structure, characterization, and enzyme inhibition effects of the novel a series of 2-aminopyridine liganded Pd(II) N-heterocyclic carbene (NHC) complexes were examined. These complexes of the Pd-based were synthesized from PEPPSI complexes and 2-aminopyridine. The novel complexes were characterized by using 13C NMR, 1H NMR, elemental analysis, and FTIR spectroscopy techniques. Also, crystal structures of the two compounds were recorded by using single-crystal X-ray diffraction assay. Also, these complexes were tested toward some metabolic enzymes like α-glycosidase, aldose reductase, butyrylcholinesterase, acetylcholinesterase enzymes, and carbonic anhydrase I, and II isoforms. The novel 2-aminopyridine liganded (NHC)PdI2(2-aminopyridine) complexes (1a-i) showed Ki values of in range of 5.78 ± 0.33–22.51 ± 8.59 nM against hCA I, 13.77 ± 2.21–30.81 ± 4.87 nM against hCA II, 0.44 ± 0.08–1.87 ± 0.11 nM against AChE and 3.25 ± 0.34–12.89 ± 4.77 nM against BChE. Additionally, we studied the inhibition effect of these derivatives on aldose reductase and α-glycosidase enzymes. For these compounds, compound 1d showed maximum inhibition effect against AR with a Ki value of 360.37 ± 55.82 nM. Finally, all compounds were tested for the inhibition of α-glycosidase enzyme, which recorded efficient inhibition profiles with Ki values in the range of 4.44 ± 0.65–12.67 ± 2.50 nM against α-glycosidase.

Asymmetric Ir-catalyzed hydrogenation of 4-R-1,3-dihydro-2H-1,5-benzodiazepin-2-ones using a novel phosphoramidite ligand

A novel chiral phosphoramidite ligand, (Sa)-2-[N-ethyl-N-(1-naphthylmethyl)amino]-dinaphtho[2,1-d:1´,2´-f][1,3,2]dioxaphosphepane, was obtained and tested in Ir-catalyzed asymmetric hydrogenation of a series of 4-R-1,3-dihydro-2H-1,5-benzodiazepin-2-ones, wherein up to 74% ee was achieved. The structure of the resulting compounds was determined based on one- and two-dimensional NMR (1H, 13C, 31P, 1H—1H COSY, 1H—1H ROESY, 1H—13C HSQC and 1H—13C HMBC) spectroscopy data.

Preserving Both Lignin and Cellulose Chemical Structures: Flow-Through Acid Hydrotropic Fractionation at Atmospheric Pressure for Complete Wood Valorization

Poplar wood was rapidly fractionated via a flow-through reaction using aqueous solutions of an acid hydrotrope (AH), p-toluenesulfonic acid (p-TsOH), at temperatures below 98 °C. 13C–1H two-dimensional nuclear magnetic resonance (NMR) spectroscopic analyses demonstrated that the AH-solubilized lignins (AHLs) from a range of fractionation conditions with yields up to approximately 80% had a very high content of β-aryl-ether linkages compared to milled wood lignin (MWL) with a low enough condensation to facilitate subsequent reductive catalytic depolymerization resulting in a lignin monomer yield of over 30%. Gel-permeation chromatographic (GPC) and differential scanning calorimetric (DSC) analyses showed that the AHLs have high molecular weights and low glass transition temperatures Tg. These AHLs also have a pinkish color suitable for applications such as cosmetics and dye dispersants. AH fractionation (AHF) preserved the cellulose fraction as solid fibers also with a light pinkish color for the materials...

Effect of Drug-Polymer Interactions through Hypromellose Acetate Succinate Substituents on the Physical Stability on Solid Dispersions Studied by Fourier-Transform Infrared and Solid-State Nuclear Magnetic Resonance.

The present study evaluated the specific intermolecular interactions between carbamazepine (CBZ) and substituents of hypromellose acetate succinate (HPMC-AS), as well as the mechanism of inhibition of recrystallization of solid dispersions (SDs) using Fourier-transform infrared (FTIR) and solid-state nuclear magnetic resonance (NMR) spectroscopy. CBZ and HPMC derivatives, including HPMC, hypromellose acetate (HPMC-A), and hypromellose succinate (HPMC-S), were spray-dried to prepare CBZ/polymer spray-dried samples (SPDs). CBZ/HPMC SPD and CBZ/HPMC-A SPD recrystallized within 10 days at 60 °C and 0% relative humidity, whereas CBZ/HPMC-S SPD maintained its amorphous state for a longer period. FTIR and solid-state NMR measurements using 13C cross polarization (CP), 1H single-pulse, and 1H-15N CP-based heteronuclear single quantum correlation filter experiment with very fast magic angle spinning (MAS) at 70 kHz identified molecular interactions in CBZ/polymer SPDs. Although the HPMC backbone and substituents did not interact notably with CBZ and disrupt CBZ-CBZ intermolecular interactions (formed in the amorphous CBZ), acetate and succinate substituents on HPMC-A and HPMC-S disrupted CBZ-CBZ intermolecular interactions through formation of CBZ/polymer interactions. The acetate substituent formed a hydrogen bond with the NH2 group of CBZ, whereas the succinate substituent formed molecular interactions with both the C═O and NH2 groups of CBZ. Formation of relatively strong molecular interactions between CBZ and the succinate substituent followed by disruption of CBZ-CBZ intermolecular interactions effectively stabilized the amorphous state of CBZ in CBZ/HPMC-S SPD. The correlation between CBZ-polymer interactions and ability of polymers to effectively inhibit CBZ recrystallization is reflected in various commercial HPMC-AS. For example, HPMC-AS LF grade, containing higher amounts of the succinate group, was found to effectively inhibit the recrystallization of CBZ through strong molecular interactions as compared with the HPMC-AS HF grade. The present study demonstrated that a detailed investigation of molecular interactions between the drug and the polymer using FTIR and solid-state NMR spectroscopy could contribute to a suitable selection of the SD carrier.

Polymer Infiltration into Metal-Organic Frameworks in Mixed-Matrix Membranes Detected in Situ by NMR.

Solid-state NMR has been used to study mixed-matrix membranes (MMMs) prepared with a metal-organic framework (MOF, UiO-66) and two different high molecular weight polymers (PEO and PVDF). 13C and 1H NMR data provide overwhelming evidence that most UiO-66 organic linkers are within 1 nm of PEO, which indicates that PEO is homogeneously distributed throughout the MOF. Systematic changes in MOF 13C NMR peak positions and 1H NMR line widths, as well as dramatic reductions in the MOF 1H T1ρ relaxation times, are observed as the PEO content increases, and when the pores have been filled, a further increase in PEO results in the formation of semicrystalline PEO outside the UiO-66 particles. In contrast, similar studies on PVDF MMMs show that the polymer contacts only a small fraction (<20%) of the MOF linkers. Simulations confirm that PEO penetrates into UiO-66 more easily than does PVDF. These studies are among the first to provide experimental insights into MOF-polymer interactions in an MMM.

NMR 1H,13C, 15N resonance assignment of the G12C mutant of human K-Ras bound to GppNHp.

K-Ras exists in two distinct structural conformations specific to binding of GDP and GTP nucleotides. The cycling between an inactive, GDP-bound state and an active, GTP-bound state is regulated by guanine nucleotide exchange factors and GTPase activating proteins, respectively. The activated form of K-Ras regulates cell proliferation, differentiation and survival by controlling several downstream signaling pathways. Oncogenic mutations that attenuate the GTPase activity of K-Ras result in accumulation of this key signaling protein in its hyperactivated state, leading to uncontrolled cellular proliferation and tumorogenesis. Mutations at position 12 are the most prevalent in K-Ras associated cancers, hence K-RasG12C has become a recent focus of research for therapeutic intervention. Here we report 1HN, 15N, and 13C backbone and 1H, 13C side-chain resonance assignments for the 19.3kDa (aa 1-169) human K-Ras protein harboring an oncogenic G12C mutation in the active GppNHp-bound form (K-RasG12C-GppNHp), using heteronuclear, multidimensional NMR spectroscopy at 298K. Triple-resonance data assisted the assignments of the backbone 1H, 15N, and 13C resonances of 126 out of 165 non-proline residues. The vast majority of unassigned residues are exchange-broadened beyond detection on the NMR time scale and belong to the P-loop and two flexible Switch regions.

Multidimensional high-resolution NMR structural characterization of a carborane cluster derivative: The case of 2-amino-3-(1,7-dicarba-closo-dodecaboranyl-1-thio)propanoic acid

Carbon-bearing boron hydrides, also known as carboranes, are polyhedral clusters composed of hydrogen, boron, and carbon and are closely related to boranes. Because of their esthetically pleasing symmetry as well as unusual chemical properties, carboranes have attracted immense interest. Studies over the past 40 years have revealed them to have chemical features that make them useful in a number of specialized applications such as conducting organic polymers, nuclear waste remediation, and as liquid crystalline materials. It has also been shown that carboranes can be used as boron delivery agents for boron neutron capture therapy against cancer. While a number of strides have been made in synthesizing these molecules, their characterization by NMR spectroscopy is challenging due to boron–boron and boron–hydrogen coupling. This gives rise to broad and unresolved peaks and makes peak assignment difficult and often based on best guesses. NMR structural characterization studies of carborane compounds are sparse, dated, and often performed at low resolution. This report provides a detailed structural characterization of an m-carborane derivative, featuring high-resolution multi-dimensional NMR spectroscopy (2-D 1H–1H, 2-D 11B–11B, 2-D 11B–1H, and 2-D 13C–1H). The asymmetry of the carborane cluster (a consequence of substitution at one of the two cluster carbons) adds a dimension of complexity to the spectra, whose peaks could be assigned nevertheless due to their high resolution.

NMR Crystallography: Evaluation of Hydrogen Positions in Hydromagnesite by 13C{1H} REDOR Solid‐State NMR and Density Functional Theory Calculation of Chemical Shielding Tensors

AbstractSolid‐state NMR measurements coupled with density functional theory (DFT) calculations demonstrate how hydrogen positions can be refined in a crystalline system. The precision afforded by rotational‐echo double‐resonance (REDOR) NMR to interrogate 13C–1H distances is exploited along with DFT determinations of the 13C tensor of carbonates (CO32−). Nearby 1H nuclei perturb the axial symmetry of the carbonate sites in the hydrated carbonate mineral, hydromagnesite [4 MgCO3⋅Mg(OH)2⋅4 H2O]. A match between the calculated structure and solid‐state NMR was found by testing multiple semi‐local and dispersion‐corrected DFT functionals and applying them to optimize atom positions, starting from X‐ray diffraction (XRD)‐determined atomic coordinates. This was validated by comparing calculated to experimental 13C{1H} REDOR and 13C chemical shift anisotropy (CSA) tensor values. The results show that the combination of solid‐state NMR, XRD, and DFT can improve structure refinement for hydrated materials.

15N–1H Transfer of Light-Induced Nuclear Hyperpolarization in Frozen Photosynthetic Reaction Centers

Recently, we have demonstrated the possibility to transfer the light-induced hyperpolarization occurring on select 13C nuclei of photosynthetic cofactors due to the solid-state photochemically induced dynamic nuclear polarization (photo-CIDNP) effect into the proton pool via inverse 13C–1H cross-polarization Bielytskyi et al. (J Magn Reson 293: 82–91, 2018). Such approach allowed us to observe the selective response from the protons belonging to photochemically active cofactors in their native protein environment. In the present study, we demonstrate that also 15N nuclei can be used as a source of polarization in a similar type of experiments. We present the 2D photo-CIDNP 15N–1H heteronuclear correlation (HETCOR) experiment acquired on uniformly 15N-labeled quinone-blocked reaction center (RC) from Rhodobacter sphaeroides R26. Obtained 1H chemical shifts match with previously observed ones on selectively 13C-labeled RC from Rhodobacter sphaeroides WT. We expect that using 15N as a source of polarization in potential heteronuclear spin-torch experiments could be advantageous for systems where the procedure for selective 13C labeling is not yet established.

Crystallization in PEG networks: The importance of network topology and chain tilt in crystals

We synthesized poly(ethylene glycol) (PEG) networks by Cu(I) catalyzed azide-alkyne cycloaddition (CuAAC) using three-arm star-PEG with azide end groups and linear PEG oligomers of different molar masses with alkyne end groups. After extensive characterization of the networks by 13C MAS and 1H multiple-quantum (MQ) NMR spectroscopy, their crystallization behavior is studied by differential scanning calorimetry (DSC), temperature dependent small and wide angle X-ray scattering (SAXS, WAXS) as well as 1H FID NMR. Since CuAAC introduces 1,4-disubstituted 1,2,3-triazole (TR) rings as chain defects into the network, we compare our results with those obtained from linear PEG which contained a purposely introduced TR ring in the middle of the polymer chain. PEG network chains are able to form the characteristic 72 helices upon cooling to −20 °C and crystallize into the monoclinic unit cell in complete analogy to the homopolymer. The crystallinity of the networks, though it is lower as compared to the linear samples, reached values of about 30–50%. Given the fact that the network chains are highly constrained in their configurational freedom, this is very surprising. However, cross-links and the TR defects have only a minor effect on the molecular dynamics of the PEG chains in the crystals since the characteristic αc-relaxation process (helix jumps) is also detected in the PEG networks by analysis of the 1H FIDs. The SAXS data reveal that randomly arranged stacks of lamellae are formed upon crystallization and the chains are tilted in the crystals at large angles of about 40°–55°. Supported by all experimental data, we propose a model to explain the formation of polymer crystals in the networks and concluded that primary and secondary loops which extent the network chains beyond the length of the linear precursors play an essential role for the crystallization.

Multivariate analysis applied to complex biological medicines.

A biological medicine (or biologicals) is a term for a medicinal compound that is derived from a living organism. By their very nature, they are complex and often heterogeneous in structure, composition and biological activity. Some of the oldest pharmaceutical products are biologicals, for example insulin and heparin. The former is now produced recombinantly, with technology being at a point where this can be considered a defined chemical entity. This is not the case for the latter, however. Heparin is a heterogeneous polysaccharide that is extracted from the intestinal mucosa of animals, primarily porcine, although there is also a significant market for non-porcine heparin due to social and economical reasons. In 2008 heparin was adulterated with another sulfated polysaccharide. Unfortunately this event was disastrous and resulted in a global public health emergency. This was the impetuous to apply modern analytical techniques, principally NMR spectroscopy, and multivariate analyses to monitor heparin. Initially, traditional unsupervised multivariate analysis (principal component analysis (PCA)) was applied to the problem. This was able to distinguish animal heparins from each other, and could also separate adulterated heparin from what was considered bona fide heparin. Taught multivariate analysis functions by training the analysis to look for specific patterns within the dataset of interest. If this approach was to be applied to heparin, or any other biological medicine, it would have to be taught to find every possible alien signal. The opposite approach would be more efficient; defining the complex heterogeneous material by a library of bona fide spectra and then filtering test samples with these spectra to reveal alien features that are not consistent with the reference library. This is the basis of an approach termed spectral filtering, which has been applied to 1D and 2D-NMR spectra, and has been very successful in extracting the spectral features of adulterants in heparin, as well as being able to differentiate supposedly biosimilar products. In essence, the filtered spectrum is determined by subtracting the covariance matrix of the library spectra from the covariance matrix of the library spectra plus the test spectrum. These approaches are universal and could be applied to biological medicines such as vaccine polysaccharides and monoclonal antibodies.

Assignment of NMR resonances of protons covalently bound to photochemically active cofactors in photosynthetic reaction centers by 13C-1H photo-CIDNP MAS-J-HMQC experiment.

Modified versions of through-bond heteronuclear correlation (HETCOR) experiments are presented to take advantage of the light-induced hyperpolarization that occurs on 13C nuclei due to the solid-state photochemically induced dynamic nuclear polarization (photo-CIDNP) effect. Such 13C-1H photo-CIDNP MAS-J-HMQC and photo-CIDNP MAS-J-HSQC experiments are applied to acquire the 2D 13C-1H correlation spectra of selectively 13C-labeled photochemically active cofactors in the frozen quinone-blocked photosynthetic reaction center (RC) of the purple bacterium Rhodobacter (R.) sphaeroides wild-type (WT). Resulting spectra contain no correlation peaks arising from the protein backbone, which greatly simplifies the assignment of aliphatic region. Based on the photo-CIDNP MAS-J-HMQC NMR experiment, we obtained assignment of selective 1H NMR resonances of the cofactors involved in the electron transfer process in the RC and compared them with values theoretically predicted by density functional theory (DFT) calculation as well as with the chemical shifts obtained from monomeric cofactors in the solution. We also compared proton chemical shifts obtained by photo-CIDNP MAS-J-HMQC experiment under continuous illumination with the ones obtained in dark by classical cross-polarization (CP) HETCOR. We expect that the proposed approach will become a method of choice for obtaining 1H chemical shift maps of the active cofactors in photosynthetic RCs and will aid the interpretation of heteronuclear spin-torch experiments.

A Novel Complex of Chitosan⁻Sodium Carbonate and Its Properties.

Chitosan has excellent properties, as it is nontoxic, mucoadhesive, biocompatible, and biodegradable. However, the poor water solubility of chitosan is a major disadvantage. Here, a novel chitosan-sodium carbonate complex was formed by adding a large amount of sodium carbonate to a chitosan/acetic acid solution, which is water-soluble. Fourier transform infrared spectroscopy, energy dispersive spectrometry, scanning electron microscopy, and solid-state nuclear magnetic resonance techniques were used to detect and characterize the aforementioned complex, which appeared to be a neat flake crystal. Solid-state nuclear magnetic resonance (SSNMR) was used to verify the connections between carbonate, sodium ions, and the protonated amino group in chitosan on the basis of 13C signals at the chemical shift of 167.745 ppm and 164.743 ppm. Further confirmation was provided by the strong cross-polarization signals identified by the SSNMR 2D 13C–1H frequency-switched Lee–Goldberg heteronuclear correlation spectrum. The cytotoxicity of a film prepared using this complex was tested using rat fibroblasts. The results show that the film promoted cell proliferation, which provides evidence to support its nontoxicity. The ease of film-forming and the results of cytocompatibility testing suggest that the chitosan-sodium carbonate complex has the potential for use in tissue engineering.

Exploration of Antifungal Potential of Carbohydrate‐Tethered Triazoles as CYP450 Inhibitors

AbstractIn search of potent and novel antifungal agents, herein, the authors report the synthesis, chemical characterization and biological evaluation of triazole tethered glycoconjugates. The structural verification of the molecules was carried out by 13C NMR, 1H NMR and mass spectra analysis. The in vitro antifungal activity was investigated against three filamentous fungi, Trichoderma viride 4329, Aspergillus fumigatus 3007 and Sporothrix schenckii. Among the synthesized compounds, it was observed that compound 10f completely inhibited the fungal growth giving a Minimum Inhibitory Concentration (MIC) value of 800 μg/ml against Aspergillus fumigatus 3007. Moreover through microscopic analysis, using Scanning Electron Microscopy and confocal imaging, it was detected that the compound had induced pronounced irreversible membrane damage and disruption of fungal mycelium.

3-Cyano thiophene-based π-conjugated mesogens: XRD and 13C NMR investigations

ABSTRACT3-Cyano thiophene-centred π-conjugated mesogens with alkoxy phenyl/biphenyl rings are synthesised by palladium acetate-catalysed direct arylation to investigate the mesophase properties and the molecular order. The synthesised mesogens mostly exhibit smectic A mesophase as confirmed by X-ray diffraction studies. Further, due to the manifestation of π-conjugated core, the mesogens exhibit photo luminescence in solution with emission maxima in the range 425–460 nm. Density functional theory and time-dependent density functional theory calculations of a representative mesogen indicate possible intramolecular charge transfer transition between the biphenyl to 3-cyanothiophene units. Despite structural simplicity, the observation of smectic A phase is attributed to location of strong dipole moment component perpendicular to the long axis of the molecules due to the presence of polar cyano group at 3-position of central thiophene ring. The 13C NMR studies are performed in mesophase and using the 13C–1H dipolar couplings determined from 2D separated local field experiment; the order parameters of central thiophene ring and side arm phenyl rings are calculated. Moreover, the order parameter ratios of thiophene and side arm phenyl rings indicate the bent–core nature of the mesogen.

Synthesis and application of sugarcane bagasse cellulose mixed esters. Part I: Removal of Co2+ and Ni2+ from single spiked aqueous solutions in batch mode using sugarcane bagasse cellulose succinate phthalate

Sugarcane bagasse cellulose mixed ester succinate phthalate (SBSPh) was synthesized by a novel one-pot reaction method. The effects of temperature, time and mole fraction of succinic anhydride (χSA) on the responses weight gain (wg), number of carboxylic acid groups (nT,COOH), and adsorption capacity (q) of Co2+ and Ni2+ were evaluated by a 23 experimental design. The chemical structure of the material was elucidated by Fourier transform infrared, 13C Multiple Cross-Polarization solid-state NMR spectroscopy and 1H NMR relaxometry. The best SBSPh synthesis condition (100 °C, 11 h, χSA of 0.2) yielded a wg of 59.1%, nT,COOH of 3.41 mmol g−1, and values of qCo2+ and qNi2+ of 0.348 and 0.346 mmol g−1, respectively. The Sips model fitted better the equilibrium data, and the maximum adsorption capacities (pH 5.75 and 25 °C) estimated by this model were 0.62 and 0.53 mmol g−1 for Co2+ and Ni2+, respectively. The ΔadsH° values estimated by isothermal titration calorimetry were 8.43 and 7.79 kJ mol−1 for Co2+ and Ni2+, respectively. Desorption and re-adsorption efficiencies were evaluated by a 22 experimental design, which showed that SBSPh adsorbent can be recovered and reused without significant loss of adsorption capacity.

Efficient 15N-13C Polarization Transfer by Third-Spin-Assisted Pulsed Cross-Polarization Magic-Angle-Spinning NMR for Protein Structure Determination.

We introduce a pulsed third-spin-assisted recoupling experiment that produces high-intensity long-range 15N-13C cross peaks using low radiofrequency (rf) energy. This Proton-Enhanced Rotor-echo Short-Pulse IRradiATION Cross-Polarization (PERSPIRATIONCP) pulse sequence operates with the same principle as the Proton-Assisted Insensitive-Nuclei Cross-Polarization (PAINCP) experiment but uses only a fraction of the rf energy by replacing continuous-wave 13C and 15N irradiation with rotor-echo 90° pulses. Using formyl-Met-Leu-Phe (f-MLF) and β1 immunoglobulin binding domain of protein G (GB1) as model proteins, we demonstrate experimentally how PERSPIRATIONCP polarization transfer depends on the CP contact time, rf power, pulse flip angle, and 13C carrier frequency and compare the PERSPIRATIONCP performance with the performances of PAINCP, RESPIRATIONCP, and SPECIFICCP for measuring 15N-13C cross peaks. PERSPIRATIONCP achieves long-range 15N-13C transfer and yields higher cross peak-intensities than that of the other techniques. Numerical simulations reproduce the experimental trends and moreover indicate that PERSPIRATIONCP relies on 15N-1H and 13C-1H dipolar couplings rather than 15N-13C dipolar coupling for polarization transfer. Therefore, PERSPIRATIONCP is an rf-efficient and higher-sensitivity alternative to PAINCP for measuring long-range 15N-13C correlations, which are essential for protein resonance assignment and structure determination. Using cross peaks from two PERSPIRATIONCP 15N-13C correlation spectra as the sole distance restraints, supplemented with (φ, ψ) torsion angles obtained from chemical shifts, we calculated the GB1 structure and obtained a backbone root-mean-square deviation of 2.0 Å from the high-resolution structure of the protein. Therefore, this rf-efficient PERSPIRATIONCP method is useful for obtaining many long-range distance restraints for protein structure determination.

Capturing Elusive Polymorphs of Curcumin: A Structural Characterization and Computational Study

Curcumin, a compound derived from the herb turmeric, has gained considerable attention because of its purported pharmacological activity, but its poor water solubility and low bioavailability impedes its therapeutic potential. In addition to the frequently studied curcumin polymorph, referred to as form I, another polymorph with enhanced water solubility, referred to as form II, or red curcumin, has also been reported. We discuss experimental challenges in isolating the red curcumin polymorph. In the course of our studies, we were unable to obtain a third reported polymorph, form III. We redetermined crystal structures of forms I and II of curcumin and present 13C and 1H solid-state nuclear magnetic resonance (NMR) spectra for these two forms, as well as 13C–1H two-dimensional heteronuclear correlation (HETCOR) data. The experimental 1H and 13C nuclear magnetic resonance (NMR) chemical shifts are compared with GIPAW density functional theory values computed using CASTEP. Our research illustrates the utili...

Persulfate initiated free-radical polymerization of itaconic acid: Kinetics, end-groups and side products

Persulfate initiated free radical homopolymerization of itaconic acid in aqueous media has been investigated at 65 °C. The research was focused on the elucidation of the chemical structure of low-molecular-weight side products and itaconic oligomers separated from polymeric fraction by dialysis (cut-off of 1000 Da). Nuclear magnetic resonance (NMR) 1H, 13C, 1H–13C me-HSQC and 1H–13C HMBC experiments and direct infusion electrospray ionization mass spectrometry (ESI-MS) operated in the negative ion mode have been combined to investigate the low-molecular fraction. It was found that 2-hydroxyparaconic and itatartaric acid are the primary oxidation products formed during free radical polymerization of itaconic acid. Besides, ESI-MS analysis revealed sulfate, hydroxyl and lactone terminal groups resulted from disproportionation, combination and transfer processes. Kinetics studies using Raman spectrometry in initiator range of 0.07–0.32 mol/L and for monomer concentration between 1.32 and 2.63 mol/L indicated non-classical dependency both polymerization rate and initiator decomposition rate on monomer and initiator concentration.