The Application of NMR Spectroscopy to the Study of Pyranoanthocyanins: Structural Elucidation, Solution Equilibria and Exhibited Color in Foods and Beverages

Anthocyanins are an important class of natural pigments widely distributed throughout the Plant Kingdom and foodstuffs. They are rather reactive compounds that are involved in several equilibria in aqueous solution and give rise to new classes of anthocyanin-derived pigments. This reactivity creates changes in the exhibited color that is especially interesting with regards to foods and beverages. Among the anthocyanin-derived pigments, pyranoanthocyanins have gained attention in the last two decades because of their higher stability, mainly the resistance to discoloration by changing pH or after the addition of bisulfite, in comparison to their precursor anthocyanins. One of the first challenges regarding pyranoanthocyanins was their structural elucidation, leading to an increasing difficulty due to the finding of several new subclasses of chemical structures, beginning with one the first discovered, relatively simple, vitisin-type pyrano-anthocyanins and continuing with other more complex structures like those of flavanyl-vinyl-pyranoanthocyanins or pyranoanthocyanin dimers. Multidimensional Mass Spectrometry (MSn) and Nuclear Magnetic Resonance (NMR) spectroscopy have been crucial tools for the structure elucidation of pyranoanthocyanins. In addition, NMR spectroscopy has contributed to the gaining of knowledge about the aqueous solution equilibria in which pyranoanthocyanins are involved, thus helping the interpretation of the molecular basis behind the higher stability of such anthocyanin-derived pigments. This review introduces the pyranoanthocyanins by means of a description of the anthocyanin reactivity and the formation of anthocyanin-derived pigments in foods and beverages. The review continues with an overview of the current knowledge about different structures of pyranoanthocyanins, their main properties related to exhibited color, especially their behavior against changing pH and bisulfite bleaching, their reactivity, and the occurrence of pyranoanthocyanins in foods and beverages. Finally, this review deals with the application of NMR spectroscopy to several interesting issues related to pyranoanthocyanin chemistry, namely the structure elucidation of the different classes of pyrano-anthocyanins, the behavior of these compounds in aqueous equilibria of hydration and proton transfer, and the formation of new pyranoanthocyanin-related compounds.

Keywords: Anthocyanin, Bleaching resistance, Color, Food, Fruit, Hydration, Juice, NMR, Pigment, Pinotin, Portisin, Pyranoanthocyanin, Vitisin, Wine.