Theoretical investigation of the structural and electronic properties of luteolin, apigenin and their deprotonated species

Density functional theory studies of the antioxidants-a review

The following review article attempts to compare the antioxidant activity of the compounds. For this purpose, density functional theory/Becke three-parameter Lee-Yang-Parr (DFT/B3LYP) methodology was carried out instead of using pharmacological methodologies because of economic benefits and high accuracy. This methodology filtrates the compounds with the lowest antioxidant activity. At first, the Koopmans' theorem was carried out to calculate some descriptors to compare antioxidants. The energy of the highest occupied molecular orbitals (HOMO) was accepted as the best indicator, and then some studies confirmed that the highest occupied molecular orbital/lowest unoccupied molecular orbital (HOMO-LUMO) energy gap is the more precise descriptor. Although it would be better to compare spin density distribution (SDD) on the oxygen of the corresponding radical in the polarizable continuum model (PCM) to evaluate their capability to chain reaction inhibition. Next, it was mentioned that in the multi-target directed ligands (MTDLs), the antioxidant is connected to other moieties in para positions to create better antioxidants or novel hybrid compounds. Indeed, SDD was introduced as a descriptor for MTDL antioxidant effectiveness. Then, the relation between antioxidants and aromaticity was investigated. The more the aromaticity of an antioxidant, the more stable the corresponding radical is. Subsequently, in preferred antioxidant activity, it was defined that the hydrogen atom transfer (HAT) mechanism is more favored in metabolism phase I. It has been seen that the solvent model can change the antioxidant mechanism. Therefore, the solvent model is more important than the chemical structure of antioxidants, and an ideal antioxidant should be evaluated in PCM for pharmacological evaluations.

Mechanism of Copigmentation of Monoglucosylrutin with Caffeine

4^G-α-Glucopyranosylrutin (monoglucosylrutin, MGR) is a flavonol glycoside with quercetin as an aglycone, is pale yellow in color, and engages in both copigmentation and anticopigmentation. In this study, we elucidated the mechanism underlying the copigmentation of MGR upon complexation with caffeine. Three approaches were used: binding analyses based on changes in the absorbance spectrum, NOESY experiments, and DFT and TDDFT calculations using an explicit solvation model. Our findings show that copigmentation mainly results from a bathochromic shift in the absorbance spectrum and not a from hyperchromic effect. MGR and caffeine form a complex in both 1:1 and 1:2 stoichiometric ratios. The calculated optimized 1:1 and 1:2 complex structures were supported by the NOESY spectrum and form a cluster with 13 and 11 water molecules, respectively, through hydrogen bonds. Although HOMO and LUMO contribute most to the excitation of both the MGR monomer and the complexes, these frontier molecular orbitals in the complexes are distributed more widely than those in the MGR monomer. In particular, LUMO in the complexes spreads into the copigment caffeine and the solvent water molecules. This increase in electron delocalization reduces the energy gap between the frontier molecular orbitals, resulting in copigmentation with a bathochromic shift.

Complex Structures of Monoglucosylrutin with ent-Gallocatechin-3- O-gallate and Epigallocatechin-3- O-gallate in Aqueous Solutions and the Mechanism of Color Change Induced by Complexation

The addition of ent-gallocatechin-3- O-gallate ( ent-GCg) or epigallocatechin-3- O-gallate (EGCg) to an aqueous solution of 4^G-α-glucopyranosylrutin (monoglucosylrutin, MGR) causes the color of the solution to weaken due to complexation between MGR and these flavan-3-ols. Copigmentation is a well-known color change phenomenon resulting from the complexation of flavonoids that deepens and strengthens the color of the solution, whereas MGR/catechin complexation results in the opposite change in color (i.e., weakening). In order to gain insight into the mechanism underlying the rare changes in the color of solutions of complexes between flavonoids, the structures of the MGR monomer and the complexes in aqueous solutions and their photochemical properties were investigated by computational methods. Molecular dynamics simulations and subsequent density functional theory (DFT) calculations revealed that the complex structures are stabilized through aromatic/aromatic, CH/π, and OH/O interactions as direct intermolecular forces and that many solvent water networks would contribute to the complexations. Time-dependent DFT calculations showed that the change in the color of an MGR/ ent-GCg solution is due only to a decrease in absorbance, whereas that of an MGR/EGCg solution is due to both a decrease in absorbance and a hypsochromic shift.

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).

Structural and electronic properties of the PbCrO4 chrome yellow pigment and of its light sensitive sulfate-substituted compounds

Chrome Yellows (CY) are a family of synthetic pigments of formula (PbCr_(1−x)S_xO₄) used by van Gogh. We investigate structure/property relations in CY by first-principles methods, providing insight into their possible degradation mechanisms.

Predicting the preferred conformations of luteolin-4'-O-β-D-glucoside in gas phase: a comparison of two computational approaches

A tree-step computational approach has been applied to determine the lowest-energy conformers of luteolin-4'-O-β-D-glucoside (L4'G). Fifty-seven starting structures of the L4'G have been built, and then by performing with density functional theory (DFT) optimizations and second-order Møller-Plesset (MP2) calculations, the preferred conformations of L4'G are predicted. In order to test the accuracy of the computational approach, a hybrid Monte-Carlo multiple minimum (MCMM)/quantum mechanical (QM) approach is applied to determine the favorable conformers of L4'G. The alternative classification is employed to put similar conformations into the same catalogue according to the dihedral angles among the luteolin rings, glycosidic dihedral angles, and the orientations of hydroxyl and hydroxymethyl groups. The low-energy conformations are located after the optimizations at the HF/6-31G(d) and B3LYP/6-311+G(d) levels. Compared with the hybrid MCMM/QM approach, the tree-step computational approach not only remains accurate but also saves a lot of computing resources.

Computational chemistry meets cultural heritage: challenges and perspectives

Chemistry is central to addressing topics of interest in the cultural heritage field, offering particular insight into the nature and composition of the original materials, the degradation processes that have occurred over the years, and the attendant physical and chemical changes. On the one hand, the chemical characterization of the constituting materials allows researchers to unravel the rich information enclosed in a work of art, providing insight into the manufacturing techniques and revealing aspects of artistic, chronological, historical, and sociocultural significance. On the other hand, despite the recognized contribution of computational chemistry in many branches of materials science, this tool has only recently been applied to cultural heritage, largely because of the inherent complexity of art materials. In this Account, we present a brief overview of the available computational methods, classified on the basis of accuracy level and dimension of the system to be simulated. Among the discussed methodologies, density functional theory (DFT) and time-dependent DFT represent a good compromise between accuracy and computational cost, allowing researchers to model the structural, electronic, and spectroscopic properties of complex extended systems in condensed phase. We then discuss the results of recent research devoted to the computer simulation of prototypical systems in cultural heritage, namely, indigo and Maya Blue, weld and weld lake, and the pigment minium (red lead). These studies provide insight into the basic interactions underlying the materials properties and, in some cases, permit the assignment of the material composition. We discuss properties of interest in the cultural heritage field, ranging from structural geometries and acid-base properties to IR-Raman vibrational spectra and UV-vis absorption-emission spectra (including excited-state deactivation pathways). We particularly highlight how computational chemistry applications in cultural heritage can complement experimental investigations by establishing or rationalizing structure-property relations of the fundamental artwork components. These insights allow researchers to understand the interdependence of such components and eventually the composition of the artwork materials. As a perspective, we aim to extend the simulations to systems of increasing complexity that are similar to the realistic materials encountered in works of art. A challenge is the computational investigation of materials degradation and their associated reactive pathways; here the possible initial components, intermediates, final materials, and various deterioration mechanisms must all be simulated.

Complexation of apigenin and luteolin in weld lake: a DFT/TDDFT investigation

A DFT-TDDFT investigation on the aluminium complexation of apigenin and luteolin has been carried out. We have focused our attention on these hydroxyflavonoids, which are the main components of weld, one of the earliest natural dyestuff used in art. In particular, weld, upon complexation with Al(iii) forms a highly prized lake which has been widely used in medieval manuscripts and easel paintings for its rich yellow colour and transparency. The experimental spectra of apigenin and luteolin upon addition of increasing [Al(3+)] show a general red-shift of the lowest absorption bands of both flavonoids spectra, associated with the presence of two and three isosbestic points for apigenin and luteolin, respectively. The molecular geometries of all the Al-apigenin and -luteolin complexes have been optimized, followed by calculation of the formation Gibbs free energies and UV-vis absorption spectra. The comparison between the computed absorption spectra of the Al-flavonoid complexes and the experimental ones corresponding to various limit [Al(3+)] concentrations has been used to discriminate between the possible complexation modes as well as the stoichiometry ratio. We have thus been able to associate specific Al-apigenin (-luteolin) complexes with the experimental absorption spectra as a function of the [Al(3+)] concentration, thus providing insights into the aluminium complexation of these hydroxyflavonoids and most importantly into the weld lake composition.

Absorption and emission of the apigenin and luteolin flavonoids: a TDDFT investigation

The absorption and emission properties of the two components of the yellow color extracted from weld (Reseda luteola L.), apigenin and luteolin, have been extensively investigated by means of DFT and TDDFT calculations. Our calculations reproduce the absorption spectra of both flavonoids in good agreement with the experimental data and allow us to assign the transitions giving rise to the main spectral features. For apigenin, we have also computed the electronic spectrum of the monodeprotonated species, providing a rationale for the red-shift of the experimental spectrum with increasing pH. The fluorescence emission of both apigenin and luteolin has then been investigated. Excited-state TDDFT geometry optimizations have highlighted an excited-state intramolecular proton transfer (ESIPT) from the 5-hydroxyl to the 4-carbonyl oxygen of the substituted benzopyrone moiety. By computing the potential energy curves at the ground and excited states as a function of an approximate proton transfer coordinate for apigenin, we have been able to trace an ESIPT pathway and thus explain the double emission observed experimentally.