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Abstract
Anthocyanins are a broad family of natural dyes, increasingly finding application as substitutes for artificial colorants in the food industry. In spite of their importance and ubiquity, the molecular principles responsible for their extreme color variability are poorly known. We address these mechanisms by computer simulations and photoabsorption experiments of cyanidin-3-O-glucoside in water solution, as a proxy for more complex members of the family. Experimental results are presented in the range of pH 1-9, accompanied by a comprehensive systematic computational study across relevant charge states and tautomers. The computed spectra are in excellent agreement with the experiments, providing unprecedented insight into the complex behavior underlying color expression in these molecules. Besides confirming the importance of the molecule's charge state, we also unveil the hitherto unrecognized role of internal distortions in the chromophore, which affect its degree of conjugation, modulating the optical gap and in turn the color. This entanglement of structural and electronic traits is also shared by other members of the anthocyanin family (e.g. pelargonidin and delphinidin) highlighting a common mechanism for color expression across this important family of natural dyes.
Highlights
Anthocyanins are a broad family of natural dyes, increasingly finding application as substitutes for artificial colorants in the food industry
In spite of their importance and ubiquity, the molecular principles responsible for their extreme color variability are poorly known. We address these mechanisms by computer simulations and photoabsorption experiments of cyanidin-3-O-glucoside in water solution, as a proxy for more complex members of the family
Our Hamiltonian replica-exchange MD (HREMD) trajectory was analyzed in terms of the three dihedrals y, a6, and a7 (Fig. 2), in order to find representative geometries corresponding to conformers separated by free energy barrier of the order of a few kBT, impossible to overcome on the timescale accessible to ab initio MD (AIMD)
Summary
For most of the colors in the red-purple-blue gamut found in flowers, fruits, vegetables, leaves, tubers and food products (wines, jams, and syrups) and they account for a large amount of color expression in nature.[2,3,4,5]. Upon further pH increase, another hydroxyl will deprotonate, giving rise to the blue-colored anionic quinonoid forms.[3,6] This variability and complexity of color expression makes the industrial utilization of anthocyanins a challenging endeavor, compared to synthetic dyes which are more stable and predictable. Previous works have gone as far as taking into account thermal fluctuations in the minimum energy conformer,[9,10,20,21] realistically many conformers will coexist in solution, and each of their contributions to the final color should be taken into account Such a systematic study is lacking, and the present work intends to be a first step in this direction by applying a multi-scale simulation protocol to the simulation of anthocyanins. The agreement between our theoretical and experimental results is excellent, giving us confidence on the quality of the simulations and their ability to capture the fine details of microscopic mechanisms underlying color expression in this important family of natural dyes
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