Tuning color variation in grape anthocyanins at the molecular scale

Anthocyanins are the main grape pigments. Due to their aromatic cyclic arrangements, they are able to absorb the radiation in the low energy range of the visible spectrum. In the fruit of Vitis vinifera L., the five main anthocyanidins (cyanidin, peonidin, delphinidin, petunidin and malvidin) are present as 3-O-glucosides, as well as their acetyl, p-coumaroyl and caffeoyl ester forms. Despite the huge number of experimental studies dedicated to the anthocyanin profile analysis of grapes and wines, the complete theoretical elucidation of the optical properties of grape anthocyanins is missing. The present work carried out this task through quantum chemistry calculations based on time-dependent density functional theory (TD-DFT), compared to experimental spectra. The differences in visible absorption spectra between the most common grape anthocyanins were rationalized according to B-ring substitution, glucosylation and esterification. A particular attention was given to the intra-molecular copigmentation effect, demonstrating the existence of an intra-molecular charge transfer excited state for the p-coumaroyl and caffeoyl ester forms.