Optical properties of wine pigments: theoretical guidelines with new methodological perspectives

Intermolecular interactions between malvidin-3-O-glucoside and caffeic acid: Structural and thermodynamic characterization and its effect on real wine color quality

The copigmentation effect between malvidin-3-O-glucoside and caffeic acid was comprehensive inquiry on the model wine solution, theoretical simulation and real wine. Thermodynamic parameters were determined by UV/Visible spectroscopy and Isothermal titration calorimetry (ITC). Theoretical data were obtained employing a dispersion-corrected density functional approach. The effects in real wines were investigated by adding the caffeic acid during different fermentation periods. Results shown that the copigmentation reaction between caffeic acid and malvidin-3-O-glucoside is a spontaneous exothermic reaction driven by hydrogen bonding and dispersions forces. Computations show that the polyhydroxyl sugar moiety and phenolic hydroxyl groups are the key active sites. The addition of caffeic acid in post-alcohol fermentation samples evidences an improving color characteristics in the wine.

Modeling UV/Vis Absorption Spectra of Food Colorants in Solution: Anthocyanins and Curcumin as Case Studies

We present a comprehensive computational study of UV/Vis absorption spectra of significant food colorants, specifically anthocyanins and curcumin tautomers, dissolved in polar protic solvents, namely water and ethanol. The absorption spectra are simulated using two fully polarizable quantum mechanical (QM)/molecular mechanics (MM) models based on the fluctuating charge (FQ) and fluctuating charge and dipoles (FQFμ) force fields. To accurately capture the dynamical aspects of the solvation phenomenon, atomistic approaches are combined with configurational sampling obtained through classical molecular dynamics (MD) simulations. The calculated QM/FQ and QM/FQFμ spectra are then compared with experiments. Our findings demonstrate that a precise reproduction of the UV/Vis spectra of the studied pigments can be achieved by adequately accounting for configurational sampling, polarization effects, and hydrogen bonding interactions.

Investigating the Role of Odorant-Polymer Interactions in the Aroma Perception of Red Wine: A Density Functional Theory-Based Approach

The aroma of red wine results from the intricate interplay between aroma compounds (odorants) and complex polymers generated during fermentation. This study combines density functional theory (DFT), human sensory experiments, and nuclear magnetic resonance to investigate the impact of odorant-polymer interactions on wine aroma. Molecular aggregation patterns of odorants with polymer segments are identified, indicating the crucial role of intermolecular noncovalent interactions, such as hydrogen bonds and van der Waals interactions, in stabilizing odorant-polymer conformations. Certain odorants, including 3-isobutyl-2-methoxypyrazine and cis-whisky lactone, exhibit high binding affinity to specific polymer segments, such as (+)-catechin and p-coumaric acid, resulting in substantial changes in the perceived aroma. Their strong binding affinities correlate with changes in sensory experiments for binary mixtures. The results provide insights into the molecular mechanisms of odorant-polymer interactions in red wine with the potential of DFT calculations as a tool for predicting and tailoring red wine aroma.

Acetylation of Malvidin-3-O-glucoside Impedes Intermolecular Copigmentation: Experimental and Theoretical Investigations

Malvidin-3-O-(6-O-acetyl)-glucoside is usually the most abundant acylated anthocyanin in red wines. In this work, intermolecular copigmentation of malvidin-3-O-(6-O-acetyl)-glucoside and malvidin-3-O-glucoside with three phenolic copigments was carried out in model wine solutions, and the influence of the acetylation group was evaluated emphatically using experimental and theoretical approaches. This study found that there was no distinct coloring difference in the two anthocyanins themselves; however, the hyperchromic effects were smaller in malvidin-3-O-(6-O-acetyl)-glucoside solutions with the existence of insufficient copigments. Thermodynamic analysis confirmed that malvidin-3-O-(6-O-acetyl)-glucoside showed weaker affinity (smaller K values) toward the three copigments compared with its non-acylated form. Theoretical analysis also indicated that the existence of the acetylation group changed the spatial conformations and non-covalent interactions (hydrogen bonds and van der Waals forces) of the copigmentation complexes, which might be due to the potential steric hindrance effect. In conclusion, the results revealed that the acetylation group on anthocyanin glycosyl could impede intermolecular copigmentation.

A 1000-year-old mystery solved: Unlocking the molecular structure for the medieval blue from Chrozophora tinctoria, also known as folium

The molecular structure of the medieval watercolor known as folium has finally been solved in the 21st century. The interdisciplinary approach taken was the key to producing extracts that had been prepared following medieval instructions, and shows the blue/purple chromophore as the major dye in Chrozophora tinctoria fruits (shell). A multi-analytical characterization of its structure was made using HPLC-DAD-MS, GC-MS, NMR (¹H, ¹³C, COSY, HSQC, HMBC, INADEQUATE), and computational studies. The results demonstrate that the blue compound corresponds to 6'-hydroxy-4,4'-dimethoxy-1,1'-dimethyl-5'-{[3,4,5-trihydroxy-6-(hydroxymethyl)tetrahydro-2H-pyran-2-yl]oxy}-[3,3'-bipyridine]-2,2',5,6(1H,1'H)-tetraone, a hermidin derivative, which we named chrozophoridin. Experimental data and computational modeling studies show that this mono-glycosylated dimer is represented by two stable isomers (atropisomers). This is an indispensable piece of knowledge for the characterization of this medieval dye in works of art such as medieval manuscript illuminations and for testing its stability and contributes to the preservation of our cultural heritage.

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.

Oenin/Syringic Acid Copigmentation: Insights From a Theoretical Study

On the basis of the dispersion-corrected density functional theory, a computational model is proposed to describe the oenin/syringic acid copigmentation and to explore the non-covalent interaction between the anthocyanin and the copigment in the framework of implicit solvent approach. The predicted binding free energy and visible spectrum shift of this copigmentation complex are in accordance with the experimental observations. The used model provides a good structural description of oenin/syringic acid complex and suggests that the intermolecular hydrogen bonding, in which the hydroxyl-rich sugar moiety in oenin plays a key role, may be the determinant for the formation and nature of the copigmentation complex.

Tuning Optical Properties of Chalcone Derivatives: A Computational Study

The conformational feature of noncovalent complexes of two borondifluoride chalcone derivatives was assessed using DFT-D2. The corresponding optical properties were analyzed based on time-dependent density functional theory calculations. As already described in such complexes, the π-stacking interaction existing between both fragments allowed formation of a new absorption band corresponding to the S0 → S1 transition. However, this band appears very close to the most intense band corresponding the S0 → S2 transition.

The electronic transitions of analogs of red wine pyranoanthocyanin pigments

High level theoretical methods have been used in order to understand structural effects on the absorption spectra of pyranoanthocyanins.

Theoretical Characterization by Density Functional Theory (DFT) of Delphinidin 3-O-Sambubioside and Its Esters Obtained by Chemical Lipophilization

Anthocyanins are water-soluble phenolic pigments. However, their poor solubility in lipidic media limits their use. This hurdle can be overcome with the lipophilization of anthocyanins, which consists of adding an aliphatic chain to a hydrophilic compound, in order to increase its solubility in lipids. Still, the unspecific chemical lipophilization of anthocyanin-esters produces molecules with different properties from their precursors. In this work, experimental changes of anthocyanin-esters obtained by chemical lipophilization are investigated in silico aiming specifically at observing their molecular behavior and comparing it with their anthocyanin precursor. Thus, the analysis of delphinidin 3-O-sambubioside and its esters employing Density Functional Theory (DFT) methods, such as the hybrid functional B3LYP in combination with the 6-31++G(d,p) Pople basis set, provides the ground state properties, the local reactivity and the molecular orbitals (MOs) of these compounds. Excited states properties were analyzed by TD-DFT with the B3LYP functional, and the M06 and M06-2X meta-GGA functionals. Local reactivity calculations showed that the electrophilic site for all the anthocyanin-esters was the same as the one for the anthocyanin precursor, however the nucleophilic site changed depending localization of the esterification. TD-DFT results indicate that the place of esterification could change the electronic transitions and the MOs spatial distribution.

Oenin and Quercetin Copigmentation: Highlights From Density Functional Theory

Making use of anthocyanin copigmentation, it is possible to effectively improve color quality and stability of red wines and other foods. This can be done by selecting strong copigments, but a 1-fold experimental screening usually entails a high cost and a low efficiency. The aim of this work is to show how a theoretical model based on density functional theory can be useful for an accurate and rapid prediction of copigmentation ability of a copigment. The present study, concerning the copigmentation between oenin and quercetin under the framework of implicit solvent, indicates that, in these conditions, the intermolecular hydrogen bonds play an important role in the system stabilization. The dispersion interaction slightly affects the structure, energies and UV-Vis spectral properties of the copigmentation complex.

Quercetin and its analogues: optical and acido–basic properties

This study is focused on eight structurally analogous natural flavonoids that exhibit a wide range of biological activities, which are of interest in pharmacy, cosmetics and the food industry.

p-Hydroxyphenyl-pyranoanthocyanins: An Experimental and Theoretical Investigation of Their Acid-Base Properties and Molecular Interactions

The physicochemical properties of the wine pigments catechyl-pyranomalvidin-3-O-glucoside (PA1) and guaiacyl-pyranomalvidin-3-O-glucoside (PA2) are extensively revisited using ultraviolet (UV)-visible spectroscopy, dynamic light scattering (DLS) and quantum chemistry density functional theory (DFT) calculations. In mildly acidic aqueous solution, each cationic pigment undergoes regioselective deprotonation to form a single neutral quinonoid base and water addition appears negligible. Above pH = 4, both PA1 and PA2 become prone to aggregation, which is manifested by the slow build-up of broad absorption bands at longer wavelengths (λ ≥ 600 nm), followed in the case of PA2 by precipitation. Some phenolic copigments are able to inhibit aggregation of pyranoanthocyanins (PAs), although at large copigment/PA molar ratios. Thus, chlorogenic acid can dissociate PA1 aggregates while catechin is inactive. With PA2, both chlorogenic acid and catechin are able to prevent precipitation but not self-association. Calculations confirmed that the noncovalent dimerization of PAs is stronger with the neutral base than with the cation and also stronger than π–π stacking of PAs to chlorogenic acid (copigmentation). For each type of complex, the most stable conformation could be obtained. Finally, PA1 can also bind hard metal ions such as Al³⁺ and Fe³⁺ and the corresponding chelates are less prone to self-association.

Stabilizing and Modulating Color by Copigmentation: Insights from Theory and Experiment

Natural anthocyanin pigments/dyes and phenolic copigments/co-dyes form noncovalent complexes, which stabilize and modulate (in particular blue, violet, and red) colors in flowers, berries, and food products derived from them (including wines, jams, purees, and syrups). This noncovalent association and their electronic and optical implications constitute the copigmentation phenomenon. Over the past decade, experimental and theoretical studies have enabled a molecular understanding of copigmentation. This review revisits this phenomenon to provide a comprehensive description of the nature of binding (the dispersion and electrostatic components of π-π stacking, the hydrophobic effect, and possible hydrogen-bonding between pigment and copigment) and of spectral modifications occurring in copigmentation complexes, in which charge transfer plays an important role. Particular attention is paid to applications of copigmentation in food chemistry.