Optical properties of wine pigments: theoretical guidelines with new methodological perspectives
Wine pigmentation results from the complex chemistry of anthocyanins. Their flavylium cation form is stabilized either by chemical transformation occurring during wine aging (e.g., pyranoanthocyanin formation), or by the formation of non-covalent complexes with (phenolic) copigments. Molecular modelling (quantum mechanics and molecular dynamics) is more and more adapted to understand wine chemistry and pigmentation. The constant developments of theoretical methodologies might get
... easily lost. This manuscript is a review of the theoretical studies dedicated to the field of wine pigments, showing conformational analysis, energetics of the various forms, pigment/copigment (non-)covalent association and charge transfer excited states. QM/MM calculations are newly performed here, which improve solvent description. The conclusion is a comprehensive guideline for an accurate prediction of light absorption by wine pigments and all related supramolecular processes. Introduction Wine pigmentation has been the subject of intense research over the last twenty years, revealing a rather sophisticated wine chemistry regarding the complexity of dye composition and molecular electronic parameters that influence the color of the wine. 1-4 The most important group of phenolic wine pigments * are anthocyanin derivatives. Wine anthocyanins are mainly derived from five aglycones (anthocyanidins), namely cyanidin, peonidin, petunidin, delphinidin and malvidin (Figure 1 ). They are usually represented in their flavylium cation form (AH + ), which is mainly responsible for the red color of the wine. However, this form is only favored at pH < 2, whereas under ambient wine conditions, at a pH of about 3.6, an acid-base equilibrium exists (Figure 1 ). Under the latter conditions, the corresponding quinonoidal bases (i.e., neutral purple A, anionic blue Aand possibly dianionic A 2-) coexist with their hydrated forms, i.e., colorless hemiketals (AOH), in equilibrium with the corresponding tautomeric chalcones (yellowish hue). Accordingly, at wine pH, colorless anthocyanin derivatives should prevail (75-95%), 1 which does not sufficiently explain the actual red wine color. In fact, various natural processes stabilize the AH + form, and thus red (or close to red) colors. These processes can essentially be classed as chemical transformations and/or the formation of non-covalently stacked complexes. * We use the term 'pigments' as commonly used in life sciences to refer to both soluble and non-soluble natural organic colorants in contrast to chemistry, where in principle a distinction between 'dye' (soluble) and 'pigment' (non-soluble) is made. * ∆m is defined in the Hückel formalism as the difference in m k between both energy levels involved in the π → π* electronic transition. Within the Hückel formalism and for unsaturated hydrocarbon chains, m k appears in the energy of state k as E k = α + m k β, α and β being the Coulomb integral related to p-orbitals of carbons and the carbon-carbon bond integral, respectively. † HF/6-31G*//HF/3-21G also result from the underestimation of π-delocalization within these schemes, 21 which is (as we now know 22 ) incorrect. This work showed rather good agreement with X-ray diffraction crystal structure analysis. 23 However, torsional angles in conjugated organic compounds are known to easily adapt to the external constraints within the crystal. Thus, comparisons to gas phase and solvent calculations are not necessarily valid.