Structure and electronic properties of (+)-catechin: aqueous solvent effects

Revealing the variances in color formation and bioactivities of seven catechin monomers throughout the enzymatic reaction by colorimetric and mass spectrometry

The capacity differences of seven catechin monomers to produce colors after treating with catechin-free extract were investigated. After 240-min reaction, only (-)-epicatechin (EC) and (+)-catechin (C) presented obvious luminous red color with L* values of 63.32-71.73, a* values of 37.13-46.44, and b* values of 65.64-69.99. Meanwhile, the decrease rate of EC and C was 43.52 %-50.35 %, which were significantly lower than those of other catechin monomers (85.91 %-100 %). The oxidized products of catechin monomers were analyzed by ultra-high performance liquid chromatography-quadrupole-time of flight-mass spectrometry coupled with diode array detector, wherein dehydro-dimers and -trimers (oxidative coupling products of catechins' A-B ring) were found to be the major chromogenic compounds of EC and C. Additionally, the antioxidant capacity of catechin monomers only decreased after 30-min reaction, while along with further enzymatic reaction, catechin monomers presented comparable oxyradical scavenging ability (e.g., the DPPH inhibitory rates of catechin monomers were in the range of 24.42 %-50.77 %) to vitamin C (positive control, DPPH inhibitory rate was 27.66 %). Meanwhile, the inhibitory effects of most catechin monomers on α-glucosidase were enhanced in different degrees. These results provided basis for the development of enzymatically-oxidized catechin monomers as functional food color additives.

Resonance structure contributions, flexibility, and frontier molecular orbitals (HOMO-LUMO) of pelargonidin, cyanidin, and delphinidin throughout the conformational space: application to antioxidant and antimutagenic activities

This research refers to the study and understanding of the conformational space of the positive-charged anthocyanidin structures in relation with the known chemical reactivities and bioactivities of these compounds. Therefore, the planar (P) and nonplanar (Z) conformers of the three hydroxylated anthocyanidins pelargonidin, cyanidin, and delphinidin were analyzed throughout the conformational space at the B3LYP/6-311 ++ G** level of theory. The outcome displayed eleven new conformers for pelargonidin, fifty-four for cyanidin, and thirty-one for delphinidin. Positive-charged quinoidal structures showed a significant statistical weight in the conformational space, thus coexisting simultaneously with other resonance structures, such that under certain reaction conditions, the anthocyanidins behave as positive-charged quinoidal structures instead of oxonium salts. The calculations of the permanent dipole moment and the polarizability showed relationships with the quantity and arrangement of hydroxyls in the structure. In addition, theoretical calculations were used to analyze the frontier molecular orbitals (HOMO-LUMO) of the three anthocyanidins. The novel conception of this work lies in the fact that dipole moment, polarizability, and HOMO-LUMO values were related to the reactivity/bioactivity of these three anthocyanidins. HOMO-LUMO energy gaps were useful to explain the antioxidant activity, while the percent atom contributions to HOMO were appropriate to demonstrate the antimutagenic activity as enzyme inhibitors, as well as the steric and electrostatic requirements to form the pharmacophore. Delphinidin was the strongest antioxidant anthocyanidin, and pelargonidin the best anthocyanidin with antimutagenic activity.

Study on the cytotoxic, antimetastatic and albumin binding properties of the oxidovanadium(IV) chrysin complex. Structural elucidation by computational methodologies

We have previously synthesized and characterized the chrysin coordination complex with the oxidovanadium(IV) cation (V^IVO(chrys)₂) and characterized in ethanolic solution and in solid state. Because suitable single crystals for X-ray diffraction determinations could not be obtained, in the present work, we elucidate the geometrical parameters of this complex by computational methodologies. The optimization and vibrational investigation were carried out both in ethanolic solution and in gas phase. The computational results support the experimentally proposed geometries of the V^IVO(chrys)₂ complex, thus leading to the conclusion that the complex exists as conformers with trans-octahedral geometry in ethanolic solution and as conformers with cis-octahedral geometry in the solid state. The complex also exists as conformers with trans-octahedral geometry in aqueous media. The active species formed after dissolution in DMSO showed anticancer and antimetastatic behavior in human lung cell line A549 with moderate binding (K_aca. 10⁵ M^-1) to bovine serum albumin (BSA). The interaction through hydrogen bonding and van der Waals forces resulted in a spontaneous process. Site marker competitive experiments showed binding sites for chrysin mainly located in site II (subdomain IIIA) and in site I (subdomain IIIA) for the complex. FT-IR spectral measurements showed evidences of the alterations of protein secondary structure in the presence of chrysin and V^IVO(chrys)₂.

Theoretical investigation of the conformational space of baicalin

Flavonoids are a large group of polyphenolic compounds ubiquitously present in plants. They are important components of human diet. They are recognized as potential drug candidates to be used in the treatment and prevention of a lot of pathological disorders, due to their protective effects. Baicalin (7-glucuronic acid 5, 6-dihydroxyflavone) is one of the main single active constituents isolated from the dried roots of Scutellaria baicalensis Georgi. The great interest on this flavonoid is due to its various pharmacological properties, such as antioxidant, antimicrobial, anti-inflammatory, anticancer and so on, and its high accumulation in the roots of S. baicalensis. The aim of our work was to analyze the geometric and electronic properties of baicalin conformers (BCL), thus performing a complete search on the conformational space of this flavonoid in gas phase and in aqueous solution. The results indicate that the conformational space of baicalin is formed by eight conformers in gas phase and five conformers in aqueous solution optimized at B3LYP/6-311++G** theory level. BCLa2_TT and BCLa1_TT conformers have low stability in gas phase and very high stability in aqueous solution. This variation is related to a modification in the τ₁ angle that represents the relative position of the glucuronide unit respect to the central rings of the flavan nucleus (A and C). This modification was successfully explained by examining the changes in the hydrogen bond (HB) interactions that occur in the region around the hydroxyl group located in position 6 of ring A. Besides, the molecular electrostatic potential (MEP) and frontier molecular orbital (FMO) analyses indicate that BCLa2_TT and BCLa1_TT conformers are the most favorable conformers for interacting with positively charged species (such as metal ions) in aqueous media (such as biological fluids).

Conformations of Flavan-3-ols in Water: Analysis Using Density Functional Theory

To elucidate the conformations and their relative stabilities of flavan-3-ols in water, the optimized structures and energies were calculated by density functional theory, in which M06-2X, ωB97X-D, B3LYP, and CAM-B3LYP were examined as exchange-correlation functionals. The results were evaluated by comparing calculated spin-spin coupling constants of vicinal protons with the corresponding values observed in experimental ¹H NMR spectra in D₂O. The M06-2X and ωB97X-D results showed good agreement with the experimental NMR data and revealed that the B-ring (pseudoequatorial)/3-O (pseudoaxial) conformers were more stable [5.4-8.7 kJ/mol (1.3-2.1 kcal/mol)] than the B-ring (pseudoaxial)/3-O (pseudoequatorial) conformers in the 2,3-cis-flavan-3-ols, while the B-ring (pseudoequatorial)/3-O (pseudoequatorial) conformers and the B-ring (pseudoaxial)/3-O (pseudoaxial) conformers had similar stabilities in the 2,3-trans-flavan-3-ols.

Z-Isomers of (4α→6″, 2α→O→1″)-phenylflavan substituted with R'=R=OH. Conformational properties, electronic structure and aqueous solvent effects

Procyanidins are highly hydroxylated polymers known as antioxidant compounds, thereby exhibiting beneficial effects. These compounds are protective agents against oxidative stress and the damage induced by free radicals in membranes and nucleic acids. This paper describes a study of the conformational space of (4α→6″, 2α→O→1″)-phenylflavan substituted with R'=R=OH as part of a larger study of similar structures with different substitutions. The relationships between aqueous solution-vacuum variations of some properties were studied, as well as the stabilization and reactivity of (4α→6″, 2α→O→1″)-phenylflavan substituted with R'=R=H, R'=H, R=OH, R'=R=OH, and (+)-catechin. The variations in geometric parameters and electronic properties due to conformational changes, as well as the effects of substituents and polar solvents, were evaluated and analyzed. Bader's theory of atoms in molecules was applied to characterize intramolecular interactions, along with a natural bond orbital analysis for each conformer described. The molecular electrostatic potential was rationalized by charge delocalization mechanisms and interatomic intramolecular interactions, relating them to the structural changes and topological properties of the electron charge density. Molecular polarizability and permanent electric dipole moment values were estimated. The results show the importance of a knowledge of the conformational space, and values for each conformer. Based on our previous results, we showed the existence of electron charge delocalization mechanisms acting cooperatively as "delocalization routes", showing interactions between different rings not even sharing the same plane. These "delocalization routes" were more effective for (4α→6″, 2α→O→1″)-phenylflavan substituted with R'=R=OH than for (+)-catechin, and are proposed as adding insight into the structure-antioxidant activity relationship of flavans.

Computational studies of free radical-scavenging properties of phenolic compounds

For more than half a century free radical-induced alterations at cellular and organ levels have been investigated as a probable underlying mechanism of a number of adverse health conditions. Consequently, significant research efforts have been spent for discovering more effective and potent antioxidants / free radical scavengers for treatment of these adverse conditions. Being by far the most used antioxidants among natural and synthetic compounds, mono- and polyphenols have been the focus of both experimental and computational research on mechanisms of free radical scavenging. Quantum chemical studies have provided a significant amount of data on mechanisms of reactions between phenolic compounds and free radicals outlining a number of properties with a key role for the radical scavenging activity and capacity of phenolics. The obtained quantum chemical parameters together with other molecular descriptors have been used in quantitative structure-activity relationship (QSAR) analyses for the design of new more effective phenolic antioxidants and for identification of the most useful natural antioxidant phenolics. This review aims at presenting the state of the art in quantum chemical and QSAR studies of phenolic antioxidants and at analysing the trends observed in the field in the last decade.