Quantum Chemical Investigation of the Intramolecular Copigmentation Complex of an Acylated Anthocyanin

Abstract

Anthocyanins are the natural plant pigments responsible for most of the red, blue and purple colors of flowers and fruit. One method of stabilization of the color of anthocyanins in nature is intramolecular copigmentation, in which a copigment molecule covalently attached to one of the sugar residues complexes with the anthocyanin cation chromophore. In the present work, two quantum chemical methodologies, time-dependent density functional theory (TD-DFT) and secondorder algebraic diagrammatic construction (ADC(2)), were employed to predict the absorption spectra in vacuum and conductor-like screening model (COSMO) water of a natural anthocyanin containing an ester of coumaric acid (copigment) bound to the sugar residue of a cyanidin chromophore. ADC(2) in water adequately reproduces the experimental spectra with and without intramolecular copigmentation, pointing to this theoretical technique as a promising approach for predicting the spectroscopic properties of natural (and nature-inspired) dyes and pigments.

Photoacidity of the 7‐Hydroxyflavylium Cation

“…Taken as a whole, the present results provide a coherent theoretical description of the photodissociation of the 7‐hydroxyflavylium cation, promoted by a significant shift of charge away from the OH group in the first singlet excited state, leading smoothly to the excited conjugate base and a protonated water cluster. The consistency of the results of the present study can be attributed to a combination of several factors: (1) our theoretical approach (TD‐DFT with the B3‐LYP functional and def2‐TZVP basis set utilizing Grimme's D3 dispersion correction), which was found to give results similar to high‐level ab initio methods for anthocyanins ; (2) the modeling of the solvent effect by immersing the photoacid, hydrogen‐bonded to a cluster of discrete water molecules , into a water‐like continuum solvent (COSMO); (3) the fact that the S 1 state of the flavylium cation (and of anthocyanins in general) is dominated by HOMO‐LUMO contributions and is well separated in energy from S 2 and higher excited states; and (4) unlike a neutral or anionic photoacid, the proton is transferred from the excited cationic photoacid to water to form a neutral excited conjugate base, avoiding Coulombic interactions between the proton and the conjugate base. As such, the present approach should serve as a useful framework for the theoretical treatment of intermolecular ESPT in other molecular systems.…”

“…A second photoprotective mechanism for anthocyanin color is copigmentation, involving the formation of bimolecular complexes with colorless electron-rich organic molecules such as hydroxybenzoic or hydroxycinnamic acids or their esters (28)(29)(30). Some *Corresponding authors' emails: quina@usp.br (Frank Quina) and adelia.aquino@univie.ac.at (Adelia Aquino) © 2019 American Society for Photobiology natural anthocyanins exhibit intramolecular copigmentation due to the presence of one or more copigment chromophores covalently attached to the sugar residues of the anthocyanin (31)(32)(33). These copigmentation complexes have charge-transfer character (28)(29)(30), are nonfluorescent and decay back to the ground state in less than a ps (29).…”

“…Highly efficient ultrafast energy dissipation pathways such as these, which rapidly deactivate a dye back to its ground state with minimal fluorescence or intersystem crossing to the excited triplet manifold, are particularly desirable in coloring agents. In order to obtain a deeper understanding of the electronic properties of the excited states of anthocyanins and their 7‐hydroxyflavylium cation synthetic analogs and the relationship to their photophysical behavior, we recently employed ab initio quantum chemical methodology to examine the ground and excited singlet state of an intramolecular copigmentation complex of a natural anthocyanin .…”

Natural Blues: Structure Meets Function in Anthocyanins

“…Anthocyanins are large, planar compounds which lend themselves to the formation of non-covalent interactions with molecules termed co-pigments ( Figure 2 ). Generally, these interactions are favourable towards the stabilisation of the coloured species through blocking hydration of the flavylium cation to the colourless hemiketal by the addition of water at C2 [ 19 ]. These interactions occur with the highest affinity between the coloured forms of the anthocyanin species and the co-pigment, and less so with the hemiketal and chalcone forms due to decreased conjugation of the tricyclic core upon ring opening [ 15 ] ( Figure 2 ).…”

Oenin/Syringic Acid Copigmentation: Insights From a Theoretical Study

“…Quantum mechanics (QM) screening is able to provide microscopic interactive conformation, spectrum and binding free energy of any copigmentation system (Quartarolo and Russo, 2011; Di Meo et al, 2012; Kalisz et al, 2013; Rustioni et al, 2013; Trouillas et al, 2016; Khalifa et al, 2018; Bayach et al, 2019; He et al, 2019), and is generally less money- and time-consuming compared with experimental approaches. The QM screening calls for a robust theoretical model composed of an efficient conformer-scanning strategy in search of the copigmentation conformers with the lowest energy in conformational space, appropriate algorithms for structural optimization, energetic and spectral evaluation, and solvent effect description (Li et al, 2011a,b; Nave et al, 2012; Rustioni et al, 2013; Trouillas et al, 2016; Marpaung et al, 2017).…”