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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 second.order 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.
Highlights
Anthocyanins are the natural plant pigments responsible for most of the red, blue and purple colors of flowers and fruit.[1,2,3,4] The thousands of different anthocyanins that have been described in the literature[4,5] differ basically in the chemical nature of the sugars and other residues attached to one of six basic anthocyanidin cation chromophores (Scheme 1)
Colors;[6,7] (ii) what are the relationships between structure and stability, not just chemical, and photochemical, in order to withstand large doses of solar radiation without fading;[8,9,10] and (iii) how are color, structure and stability related to their biological roles in plants?8 A more profound understanding of these questions is central to the rational design of new anthocyanin-inspired dyes and pigments with desirable colors and stability and for the color stabilization of natural anthocyanins in practical applications such as cosmetics or foods.[11,12,13]
CyCoum consists of a p-coumaric acid (Coum) residue covalently attached via an ester linkage to a hydroxyl group of one of the 3-O-sugar residues of a cyanidin3,5‐O‐diglycoside (Cy) chromophore (Scheme 2)
Summary
Anthocyanins are the natural plant pigments responsible for most of the red, blue and purple colors of flowers and fruit.[1,2,3,4] The thousands of different anthocyanins that have been described in the literature[4,5] differ basically in the chemical nature of the sugars and other residues attached to one of six basic anthocyanidin cation chromophores (Scheme 1). The 7-hydroxy group is almost always free, the 3-hydroxy group is always glycosylated and the 5-hydroxy group of the anthocyanidin cation chromophore may or may not be glycosylated. The maximum wavelength of the absorption, and the color of anthocyanidin cations shifts toward the red (longer wavelengths) with increasing number of hydroxy or methoxy substituents in the B-ring,[5,6] this by itself cannot account for the much larger color changes observed in nature.
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