ss-NMR and single-crystal X-ray diffraction in the elucidation of a new polymorph of bischalcone (1E,4E)-1,5-bis(4-fluorophenyl)penta-1,4-dien-3-one

We report a new polymorph of (1E,4E)-1,5-bis(4-fluorophenyl)penta-1,4-dien-3-one, C₁₇H₁₂F₂O. Contrary to the precedent literature polymorph with Z' = 3, our polymorph has one half molecule in the asymmetric unit disordered over two 50% occupancy sites. Each site corresponds to one conformation around the single bond vicinal to the carbonyl group (so-called anti or syn). The other half of the bischalcone is generated by twofold rotation symmetry, giving rise to two half-occupied and overlapping molecules presenting both anti and syn conformations in their open chain. Such a disorder allows for distinct patterns of intermolecular C-H...O contacts involving the carbonyl and anti-oriented β-C-H groups, which is reflected in three ¹³C NMR chemical shifts for the carbonyl C atom. Here, we have also assessed the cytotoxicity of three symmetric bischalcones through their in vitro antitumour potential against three cancer cell lines. Cytotoxicity assays revealed that this biological property increases as halogen electronegativity increases.

Important issues in plant tissues analyses by HR-MAS NMR

INTRODUCTION

High-resolution magic-angle spinning nuclear magnetic resonance (HR-MAS NMR) spectroscopy enables the analysis of the metabolic profile of plant and animal tissues under close to natural conditions, as well as of other heterogeneous natural or synthetic materials. Neither sample pretreatment is required after fragmentation nor powdering of the sample before insertion into the rotor. However, the efficiency of the method depends strongly on the sample preparation, rotor insertion procedure, and analysis conditions.

OBJECTIVE

To identify some of the variables that affect the spectral data and to propose solutions that minimise their impact on the quality of the analyses and results.

METHODS

Dried plant tissues were powdered, weighed, and homogenised in a 50 μL rotor with an optimised volume of deuterated solvent and sample in order to prevent material from escaping during spacer insertion, avoiding variations in magnetic susceptibility. Factors affecting the quality of HR-MAS NMR analysis such as particle size, sample and solvent amounts, solvent polarity, swelling time, rotor manipulation and pulse sequence setting were evaluated.

RESULTS

A strong correlation was observed between the signal area and the particle size of the powdered sample. The spectral profile varied depending on the deuterated solvent used. An incubation period was necessary to achieve adequate swelling of the sample and to ensure good data reproducibility. Proper sealing of the rotor, number of cycles and τ time on cpmgpr1d pulse sequence were found to affect the signal areas.

CONCLUSION

The study highlights the need for standardised sample preparation and instrumental setup protocols in order to achieve high reproducibility and obtain reliable data from HR-MAS NMR analyses.

β-Carboline alkaloids from Galianthe ramosa inhibit malate synthase from Paracoccidioides spp

As part of our continuing chemical and biological analyses of Rubiaceae species from Cerrado, we isolated novel alkaloids 1 and 2, along with known compounds epicatechin, ursolic acid, and oleanolic acid, from Galianthe ramosa. Alkaloid 2 inhibited malate synthase from the pathogenic fungus Paracoccidioides spp. This enzyme is considered an important molecular target because it is not found in humans. Molecular docking simulations were used to describe the interactions between the alkaloids and malate synthase.

Real-time mechanistic monitoring of an acetal hydrolysis using ultrafast 2D NMR

Ultrafast (UF) 2D NMR makes it possible to obtain a 2D NMR spectrum in less than a second. Here, UF-HSQC experiments are used for the real-time mechanistic study of an acetal hydrolysis at ¹³C natural abundance, and it is possible to characterize the presence of the hemiacetal, an intermediate with a well-known short lifetime. The assignments are confirmed and rationalized by quantum calculations of ¹H and ¹³C NMR chemical shifts and natural bonding orbital analysis.

NMR property calculations and experimental study of the 1,6-epoxycarvone and α-epoxypinene: a comparison of models

This work aims at using theoretical calculations of shielding tensors (σ) through different methods [gauge-independent atomic orbital (GIAO), continuous set of gauge transformations (CSGT) and individual gauges for atoms in molecules (IGAIM)] and spin-spin coupling constants J using GIAO method to compare these methods and to corroborate the data obtained with the assignment of all of (1)H and (13)C NMR signals and the relative stereochemistry of the 1,6-epoxycarvone and the α-epoxypinene. All the (1)H and (13)C NMR signals were assigned unequivocally. The stereochemistry for the epoxides is trans and the B3LYP theory level with CSGT and IGAIM methods is the best choice to evaluate theoretical chemical shifts for compounds studied.