ABSTRACT Natural products have historically been a rich and diverse source of biologically active compounds. Some of the most interesting members of this category are cyclic and acyclic peptides. Although this structural class has a high potential for development into medicines and application to numerous therapeutic indications, the elucidation of their structures is not always trivial. The occurrence of nonstandard amino acids and extensive overlap of aliphatic 1 H α and amide 1 H N resonances can make the assignment of individual amino acid spin systems and connectivity between them difficult. Experiments like 1 H– 1 H TOCSY, 1 H– 13 C HSQC‐TOCSY, and 1 H– 13 C HMBC can be implemented to help disentangle these congested resonances, but the limited spectral dispersion of 1 H and restricted chemical shift differences in the 13 C dimension can limit the effectiveness of these experiments. To circumvent this issue, the larger spectral dispersion of 15 N (~900 ppm) can be leveraged with the use of 1 H– 15 N HSQC‐TOCSY and 1 H– 15 N HMBC experiments. Although these NMR experiments have been shown to be effective structure elucidation tools at natural isotopic abundance (0.4%), we demonstrate here that the quality of those data can be significantly enhanced, and their acquisition accelerated, through the production of 15 N‐labeled analyte.
ABSTRACT This work presents examples of the effect of the nitroxide ring structure, the bridge composition, and the solvent nature on EPR spectra of short biradicals with similar structures, on the value of the electron spin exchange integral | J |. The attenuation coefficients γ NH ≈ 4.1 and γ O ≈ 3.3 which characterize the decrease of the spin density on −NH− and −O− groups were determined for the first time. They do not depend on other groups forming the linker. The biradical O=C (NHR 6 ) 2 was found to be structurally rigid in all polar and nonpolar solvents studied, over the temperature range of 200–410 K. The suggested approach allows the prediction of the exchange integral value, | J/a | or | J |, without electronic structure computation based on the bridge composition only.
ABSTRACT This review summarizes how DQCC values in deuterated TMHC can be experimentally determined in solutions and the solid state applying 2 H T 1 relaxation measurements and a line shape analysis of 2 H NMR spectra. Advantages and limitations of these methods are discussed including errors in DQCC calculations. The considered concepts in interpretations of quadrupolar interactions demonstrate that DQCCs in TMHC characterize and distinguish different metal–hydrogen bonding modes providing determinations of metal–deuterium bond lengths and the metal–deuterium bond ionicity. The aim of this review is also to show perspectives in further studies of TMHC including more complex molecular hydride systems such as metal nanoparticles and immobilized metal hydrides interacting with D 2 .
ABSTRACT The identification of bioactive compounds in Ugni molinae Turcz. extracts has gained interest due to the well‐known biological properties of the plant. Previous research has identified two main types of bioactive compounds: polyphenols and pentacyclic triterpenes. In this study, an ethyl acetate extract from the leaves of the 14‐4 genotype of U. molinae was separated by column chromatography (CC), and one fraction exhibited an antioxidant capacity 11 times higher than the crude extract, evaluated through DPPH assay. The presence of the pentacyclic triterpene asiatic acid (AA) was identified through NMR experiments. The AA concentration, estimated by standard addition NMR experiments was 2.18 mg/mL, which corresponds to 14.5% of the active fraction (AF). Two polyphenolic compounds were confirmed by HPLC‐MS and NMR experiments: gallic acid and a stable conformer of quercetin. The structure was elucidated by coordinated analysis of HPLC‐MS, along with several NMR experiments. These included 1 H NMR, 13 C NMR, HSQC, HMBC, SELCOSY, SELTOCSY, variable temperature, and standard addition experiments. Variable‐temperature NMR experiments provide evidence of no isomer exchange over a broad temperature range. Consequently, the isomer previously described as a conformer was reclassified as E conformer. These findings expand current knowledge of the chemical composition of U. molinae extracts and polyphenols.
ABSTRACT Chemical investigation of the aerial parts of Salvia polystachya Ortega afforded a new neo ‐clerodane‐type diterpenoid named polystachyne I ( 1 ) and the previously known diterpenoid microphyllandiolide ( 2 ). Their structures were elucidated by 1D and 2D NMR experiments. The absolute configuration of 1 was established by single‐crystal X‐ray diffraction. Compound 1 was tested for cytotoxicity against three human cancer cell lines, showing moderate activity.
