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

Abstract

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.

The Chemical Reactivity of Anthocyanins and Its Consequences in Food Science and Nutrition

“…In this context, the most important C-centered nucleophiles are electron-rich C–C double bonds, such as 4-vinylphenols (4-hydroxystyrenes), formed upon microbial decarboxylation of 4-hydroxycinnamic acids ( Figure 6 ) and the enol forms of various aldehydes and ketones such as pyruvic acid and ethanal (acetaldehyde) [ 8 , 9 ]. In the process, new pigments, called pyranoanthocyanins, are formed, which are resistant to nucleophilic addition at C2 and C4 [ 10 , 11 , 12 ]. Their color (shifted to orange-red, compared to the corresponding flavylium ion) is thus more stable.…”

Mathematical modelling to enhance winemaking efficiency: a review of red wine colour and polyphenol extraction and evolution

“…Perceived hue and colour stability are determined by the unique chemical structure of a range of phenolic substances, consisting of at least one benzene ring with one or more hydroxyl groups, and a system of conjugated π electrons which may or may not absorb in the visible region (Aleixandre‐Tudo et al 2017). Conjugation lowers the energy required for electron transitions to take place between OH‐phenolic groups, causing an increase of absorption wavelengths in the UV–Vis spectra and subsequent displays of colour (Vallverdú‐Queralt et al 2016, Waterhouse et al 2016b). As such, the visible colour of a compound is influenced by its degree of conjugation, with less conjugation causing a material to absorb light at shorter wavelengths (perceived as yellow and red hues) and greater conjugation causing absorption at longer wavelengths (perceived as blue and green hues) (Sanna et al 2014).…”

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

“…Regarding the molecular structure modification, the change in the absorption spectra was analyzed in a theoretical study of pyranoanthocyanins [ 31 ] showing that the site of deprotonation could change the theoretical UV-Vis spectra. Thus, in DpS, the position where the hydrogen atom was substituted by the aliphatic chain also produces different spectra for the anthocyanin-esters.…”