A theoretical interpretation of the color of two classes of pyranoanthocyanins

Anthocyanins are natural pigments that present a characteristic red color at low pH (<2). Anthocyanin-pyruvic acid adducts, commonly referred to as vitisins, are anthocyanin derivatives usually found in red wines. Looking at the two-dimensional structures, vitisins apparently have a greater conjugation system than anthocyanins. Following the common conjugation rules, making the chromophore more conjugated increases the value of λmax, and vitisins should have a longer λmax than anthocyanins. However, they have a surprising lower λmax, presenting an orange color, under the same pH conditions. Another class of anthocyanin-derived pigments, portisins have the expected relative behaviour with a longer λmax than anthocyanins. In this work, a quantum study on molecular models of pigment models was carried out. The results obtained enabled to present a putative explanation of the color change in these series.