From Vine to Wine: The Chemistry of the Anthocyanin and Pyranoanthocyanin Pigments

Frank H. Quina

Instituto de Química, Universidade de São Paulo, São Paulo, Brazil

quina@usp.br

The basic chromophore of anthocyanins, the principal natural pigments responsible for the red, purple and blue colors of plants, is a flavylium cation [1]. During the maturation of red wine, the anthocyanins present in the grapes (predominantly oenin) are transformed into pyranoanthocyanins, which contain a pyranflavylium cation chromophore. Flavylium cations (FC) like that in the Scheme can be transformed into the corresponding pyranoflavylium cation (PFC) by acid-catalyzed reaction with a substituted benzaldehyde [2]. Like FCs, the PFCs are weak acids in the ground state, but have much simpler pH-dependent chemistry (only the acid-base equilibrium with pK_a ca. 4). In addition, the substituents of the benzaldehyde employed in the synthesis strongly influence the color of PFCs, which can be varied across the visible spectrum.  In the first excited singlet state, FCs have fluorescence lifetimes of 5-10 ps, are super photoacids and are potentially super photooxidants (2-2.5 eV available), though clear examples of net excited state electron transfer reactions are rare. By comparison, PCFs are less water soluble, have longer fluorescence lifetimes and less pronounced photoacidity. The relationship of the chemistry of these natural plant pigments to their roles in vivo (anthocyanins) and in vitro (pyranoanthocyanins) will be outlined.

References:

[1]. “Chemistry and photochemistry of natural plant pigments: the anthocyanins”, V. O. Silva, A. A. Freitas, A. L. Maçanita, F. H. Quina, J. Phys. Org. Chem., DOI: 10.1002/poc.3534 (2016).

[2]. “Aged red wine pigments as a source of inspiration for organic synthesis—the cases of the color-stable pyranoflavylium and flavylium-(4→8)-flavan chromophores”. S. Chassaing, G. Isorez-Mahler, M. Kueny-Stotz, R. Brouillard, Tetrahedron, 71, 3066 (2015).