Comparison Of 13C Chemical Shifts Research Articles

Cytotoxic, antimicrobial and antiviral secondary metabolites produced by the plant pathogenic fungus Cytospora sp. CCTU A309

Chemical analysis of extracts from cultures of the plant pathogenic fungus Cytospora sp. strain CCTU A309 collected in Iran led to the isolation of two previously unreported heptanedioic acid derivatives namely (2R,3S) 2-hydroxy-3-phenyl-4-oxoheptanedioic acid (1) and (2S,3S) 2-hydroxy-3-phenyl-4-oxoheptanedioic acid (2) as diastereomers, four previously undescribed prenylated p-terphenyl quinones 3–6 in addition to five known metabolites. Their structures were elucidated on the basis of extensive spectroscopic analysis and high-resolution mass spectrometry. For metabolites 1 and 2, the absolute configurations at C-2 were deduced from comparison of the 1H NMR difference of their (S)- and (R)-phenylglycine methyl ester derivatives while the relative configurations were tentatively assigned by a J-based analysis and confirmed by comparison of 13C chemical shifts to literature data. The isolated compounds were tested for their cytotoxic, antimicrobial (including biofilm inhibition), antiviral, and nematicidal activities. While only moderate antimicrobial effects were observed, the terphenyl quinone derivatives 3–6 and leucomelone (10) exhibited significant cytotoxicity against the mouse fibroblast L929 and cervix carcinoma KB-3-1 cell lines with IC50 values ranging from 2.4 to 26 μg/mL. Furthermore, metabolites 4–6 showed interesting antiviral activity against hepatitis C virus (HCV).

Activation of the NLRP3 Inflammasome by Hyaboron, a New Asymmetric Boron-Containing Macrodiolide from the Myxobacterium Hyalangium minutum

A Natural Compound Library containing myxobacterial secondary metabolites was screened in murine macrophages for novel activators of IL-1β maturation and secretion. The most potent of three hits in total was a so far undescribed metabolite, which was identified from the myxobacterium Hyalangium minutum strain Hym3. While the planar structure of 1 was elucidated by high resolution mass spectrometry and NMR data yielding an asymmetric boron containing a macrodiolide core structure, its relative stereochemistry of all 20 stereocenters of the 42-membered ring was assigned by rotating frame Overhause effect spectroscopy correlations, 1H,1H, and 1H,13C coupling constants, and by comparison of 13C chemical shifts to those of the structurally related metabolites tartrolon B-D. The absolute stereochemistry was subsequently assigned by Mosher's and Marfey's methods. Further functional studies revealed that hyaboron and other boronated natural compounds resulted in NLRP3 inflammasome dependent IL-1β maturation, which is most likely due to their ability to act as potassium ionophores. Moreover, besides its inflammasome-stimulatory activity in human and mouse cells, hyaboron (1) showed additional diverse biological activities, including antibacterial and antiparasitic effects.

Hydrogen bonded networks of methoxy-substituted α-phenylcinnamic acids studied by spectroscopic and computational methods

Measurements by infrared (FT-IR) spectroscopy revealed that in solutions of low concentration the major structure forming force (short-range ordering) between methoxy-substituted α-phenylcinnamic acid stereoisomers was the CO⋯H–O intermolecular hydrogen bonding between the carboxylic groups. Under 10−3mol/dm3 concentration no other secondary intermolecular interaction was found. In solid state, using IR spectroscopy, FT-IR and 13C NMR spectroscopies, it was possible to detect (aromatic)C–H⋯O hydrogen bonds, which were thought to be responsible for long-range ordering. Through the comparison of 13C chemical shifts in the liquid phase and the solid state (olefinic)C–H⋯O interactions were sought for. Such interactions could not be found for the methoxy derivatives. Molecular modeling provided structural semiquantitative information for the hydrogen bonds detected by IR spectroscopy and gave additional proof for the existence of (aromatic)C–H⋯O and the non-existence of (olefinic)C–H⋯O hydrogen bonds.

13C NMR, X-ray, and differential scanning calorimetry investigations of truncated BPTI (aprotinin) analogues.

Truncated BPTI missing residues 1 and 2 is investigated together with variants thereof (Lys-15, Arg-17, and Arg-42 are replaced by other residues in various combinations). A comparison of the X-ray structure of BPTI with that of 3-58BPTI(K15R,R17A,R42S) shows only minor variations for the backbone, but the lack of salt bridge between the terminals and the lack of two N-terminal residues provide a structure open at one end. Comparisons of amide exchange rates show a dramatic increase for the most slowly exchanging NH protons of 3-58BPTI and the analogues thereof, as compared to those of the wild-type despite only small differences in the structures. The amide exchange rates for truncated analogues increase with decreasing TTEP (temperature top endothermic peak) values. On the basis of the known structural changes comparisons to 13C chemical shifts are made. 13C chemical shifts are assigned using the D-isotope and HMBC techniques. Excellent resolution is obtained in these 1D natural abundance spectra. 13C NMR chemical shifts are shown to be able to gauge structural changes. A comparison of 13C chemical shifts of WT BPTI (aprotinin) and 3-58BPTI reveals effects caused by (i) the removal of the salt bridge of the terminii, (ii) the charge of the N-terminus, and (iii) the increased mobility of the side chain of Tyr-23. Small effects are also seen due to a conformational change of the aromatic ring of Phe-4. Ring current shifts at 13C chemical shifts are calculated. The difference in the calculated ring current effects are small comparing the wild-type with 3-58BPTI(K15R,R17A,R42S) provided the structures are relaxed. Protein unfolding as a function of pH and temperature is studied by DSC. Unfolding occurs at lower temperature with N-terminally truncated analogues, and the maximum is shifted toward higher pH.

Carbon-13 N.M.R. evidence for an unsymmetrical hydrogen bond in the CLS-enols of β-diketones

Comparison of 13C chemical shifts for enols of β-diketones with those for aluminium complexes and enol ethers shows that the enols are unsymmetrical.

Urea‐formaldehyde resins. III. Constitutional characterization by 13C fourier transform NMR spectroscopy

Abstract13C Fourier transform NMR has been used to characterize a random chemical structure of ureaformaldehyde resins. By comparison of 13C chemical shifts with synthesized standard derivatives from urea and formaldehyde the analysis of reacted formaldehyde was completed. In a 13C spectrum of resin each signal due to reacted formaldehyde (e.g., methylol group, methylene group, and dimethylene ether group) was isolated.Measurement of a 13C spectrum of resin by the gated decoupling of proton without nuclear Overhauser effect made a quantitative analysis of reacted formaldehyde possible. In this quantitative analysis a 13C quantity of carbonyl groups in urea residue can be directly compared with that of each combined formaldehyde.

Carbon magnetic resonance of organometallic Compounds: Hyperconjugative effects in benzylsilanes

A number of benzylsilanes have been synthesised and characterised, and the structural features of certain of these compounds are such that the CSi bond is constrained to varying degrees to the nodal plane of the aromatic system. The proton-decoupled, natural-abundance 13C NMR spectra of these compounds have been obtained and essentially completely assigned on the basis of signal intensities, chemical shifts, selective deuteration and (in some cases) the magnitudes of 13C-19F coupling constants. Comparison of 13C chemical shifts of carbons formally para to the silylmethyl group (CH2Si) in the bicyclic silatetralins and -indanes with the corresponding carbon shifts in the carbon analogues, indicates suppression of the electron-donating effect of the CH2Si group, when the optimum alignment for interaction of the CSi σ-bond and the π-system is prevented. Effects related to strain and special influences operating on the chemical shifts of carbon bearing fluorine, render interpretation for some compounds less straight-forward.