Substituent Effect versus Aromaticity─A Curious Case of Fulvene Derivatives

A computational study on amino- and nitro-substituted penta- and heptafulvenes reveals the interplay between the aromaticity and the substituent effect (SE). Ring substitution alone has little influence on the aromaticity, but in combination with an exo substituent of opposite properties, it substantially enhances the cyclic π-electron delocalization. Despite the SE being stronger for β substitution, only γ substitution leads to higher aromaticity. An explanation is provided by the electron density of delocalized bonds (EDDB) method, which proves to be a valuable tool in analyzing both cyclic delocalization and the SE.

Theoretical Evaluation of Oleocanthal Reactive Centers

BACKGROUND

Decarboxymethyl ligstroside aglycone (Oleocanthal) is an essential component of olive oil. It is therefore interesting to study its metabolism in the human body. In order to find the best possible starting point for this metabolism, a theoretical study was carried out using DFT calculations and docking studies.

METHODS

The DFT, B3LYP/6-311++G** and the PCM solvation model calculations were used to study the initial process of Oleocanthal metabolism by the CYP1A2 enzyme. Structures of radicals formed by homolytic dissociation of hydrogen atoms from the Oleocanthal structure were obtained and their properties were studied. Several parameters such as HOMO and LUMO energy gaps, Bond Dissociation Energy (BDE), hardness, and spin density of possible Oleocanthal radicals were taken into account. Docking of Oleocanthal into an enzyme binding pocket was also performed to locate the most probably metabolic site. Detailed analysis of the theoretical results allows the determination of the most likely reaction sites in Oleocanthal. The mode of binding of Oleocanthal to the CYP1A2 enzyme was also predicted.

RESULTS

The results of the molecular docking studies are in agreement with the calculated quantum parameters. The theoretical predictions were compared with experimental data available in the scientific literature. A high correlation between theoretical calculations and experimental data was observed. The most likely site of Oleocanthal metabolism was identified.

CONCLUSION

The results of our research support the usefulness of theoretical calculations in predicting metabolic pathways.

Pyrrole and Pyridine in the Water Environment-Effect of Discrete and Continuum Solvation Models

Properties of pyrrole and pyridine molecules upon different hydrations were investigated through density functional theory. Complexes of studied molecules with a cluster of 50 water molecules were considered, and the polarizable continuum model of solvation (PCM) was also taken into account. For comparative purposes, all mentioned calculations were repeated for single pyrrole and pyridine molecules and their complexes with one water molecule. Aromaticities of solvated pyrrole and pyridine rings were studied using several geometric- and electronic-based aromaticity criteria. Special attention was paid to studying the properties of formed hydrogen bonds between pyrrole or pyridine and either a single water molecule or several water molecules of the cluster. Overall, a comprehensive description of two very important heterocyclic compounds, that is, pyrrole and pyridine, in both discrete and continuum water solutions, is extensively presented.

Solvent influence on intramolecular interactions and aromaticity in meta and para nitroanilines

Theoretical density functional theory (B3LYP/6-31G**) was used to study the intra- and intermolecular interactions of nitrobenzene, aniline, and meta and para nitroaniline in various solvation models. The studied molecules were solvated by one or two water molecules in the presence of continuum solvation (the PCM model) or without it. Finally, the studied molecules were surrounded by a cluster of water molecules. For comparison, calculations were also made for separated molecules. Geometries, energies, hydrogen bonding between solutes and solvent molecules, atomic charges, and aromaticity were examined. The analysis was based on the Atoms in Molecules methodology and the Harmonic Oscillator Model of Aromaticity (HOMA) index. As a result, an extensive description of the solvation of nitro and amino groups and the effect of solvation on mutual interactions between these groups in meta and para nitroanilines is provided. It was found that in general, the PCM description of the hydration effect on the electronic structure of the studied systems (substituents) is consistent with the approach taking into account all individual interactions (cluster model).

Synthesis, spectroscopy, and theoretical calculations of some 2-thiohydantoin derivatives as possible new fungicides

We present the synthesis and structure determination for two thiohydantoin compounds (5-benzylidene-2-sulfanylideneimidazolidin-4-one and 5-cinnamylidene-2-sulfanylideneimidazolidin-4-one), proposed as potential novel fungicides. The exact chemical structure of these molecules has not yet been determined since they can potentially exist in several tautomeric and geometric forms (Z-E isomerism). The geometries of all the theoretically possible structures of the studied compounds were optimised. The calculations were performed at the density functional theory level using the B3LYP functional and the 6-311++G** basis set. Based on our calculations, the most probable structures of the studied compounds were proposed. The theoretical predictions were verified by comparing the calculated IR as well as the ¹H and ¹³C NMR spectra with the experimental data. It was documented that both the studied compounds exist predominantly in the tautomeric structure, in which the movable hydrogen is connected to the nitrogen atom in the hydantoin ring. It has been experimentally proven that one of the studied compounds occurs only as a single structure, whereas the other one exists as a mixture of two geometric isomers. Schematic presentation of synthesis reaction and predicted percentage contents of studied compounds in products mixture.

Antioxidant properties of Aloe vera components: a DFT theoretical evaluation

Prediction of the antioxidant activity of three Aloe vera components (aloesone, aloe-emodin, and isoeleutheol) was performed based on density functional theory calculations using the B3LYP hybrid functional and the 6-311++ G** basis set. Calculation of highest occupied molecular orbital (HOMO), lowest occupied molecular orbital (LUMO), and E_gap revealed that aloe-emodin has the lowest E_gap value, indicating good antioxidant activity. Also in terms of electron affinity, softness, electrophilicity, and chemical potential, aloe-emodin is a potent structure with potential high radical scavenging activity. Calculation of the ionisation potential revealed that isoeleutherol likely also possesses a high degree of antiradical scavenging. To study the conjugating system of the radicals, density plots of HOMO, natural bond orbital analyses, and spin density plots were used. According to calculations, the isoeleutherol radical is more delocalised and the most stable radical. Calculated proton affinity values revealed that the most probable antioxidant mechanism is sequential proton loss-electron transfer. Our results were compared with available experimental data. Published experimental data were found to correlate well with our theoretical predictions. These results support the usefulness of theoretical calculations not only for identifying potentially useful structures of studied compounds but also for predicting their relative activity.