Chromatic and structural features of blue anthocyanin-derived pigments present in Port wine

A new group of blue anthocyanin-derived pigments was recently found to occur in ageing Port wines and were named as portisins. The UV–vis spectrum of these pigments was found to have a λmax around 570 nm that is bathochromically shifted from that of original anthocyanins, exhibiting a more bluish hue in acidic solution. Three portisins were synthesised through the reaction between malvidin 3-O-glucoside, malvidin 3-O-coumaroylglucoside and cyanidin 3-O-glucoside-pyruvic acid adducts and catechin in presence of acetaldehyde. These portisins were structurally and chromatically characterized (pH, SO2 stability assays and determination of the molar extinction coefficient) and showed to have high resistance to the attack of water or bisulfite, specially when compared to malvidin-3-glucoside. This portisin resistance to discoloration may be explained by a higher protection of the chromophore group against the water or bisulfite nucleophilic attack that gives rise to the hemiacetal form, which is usually colourless. Portisins also showed to have a very high colorant capacity as they displayed a higher molar extinction coefficient than anthocyanins and the respective pyruvic acid adducts. These chromatic characteristics may help better understand the contribution of these anthocyanin-derived pigments to the overall wine colour.