Influence of the structural features of amino-based pyranoanthocyanins on their acid-base equilibria in aqueous solutions

The equilibrium forms of three different families of dimethylamino-based pyranoanthocyanins (1, 2 and 3) were studied in aqueous solutions at different pH values from 1 to 12 using UV–Visible spectroscopy. The forms present under those conditions are strongly correlated to the pyranoanthocyanin structural features. The increase of the electronic delocalization helps the protonation at the amino group. At very acidic pH condition (pH < 0) the protonation at the amino group is observed for the three pigments, but under less acidic conditions (pH∼1) it only occurs for pigment 3 (pK_a1 = 2.4 ± 0.1) and at a lesser extent for pigment 2 (pK_a1 = 1.1 ± 0.1). At the same time, the increase of the electronic delocalization on the amino-based pigments also favors the deprotonation at the hydroxyl group present at carbon C-7 yielding the neutral quinoidal base (pK_a2 = 2.7 ± 0.1, pK_a2 = 4.8 ± 0.1 and pK_a2 = 5.4 ± 0.1 for pigment 3, 2 and 1, respectively). For pigment 3, the maximum molar fraction obtained for the pyranoflavylium cation form is ∼0.4 due to the proximity of the two acid-base constants (pK_a1 and pK_a2) which indicates that at the pH range 1–5 three forms of the compound are present in equilibrium (pyranoflavylium dication, pyranoflavylium cation and neutral quinoidal base). The second deprotonation at the 4′-OH was less affected by the structural features of the pigment with the ionization constant situated at pK_a3∼9 (pK_a3 = 9.5 ± 0.1, pK_a3 = 8.9 ± 0.1 and pK_a3 = 9.8 ± 0.1 for pigment 1, 2 and 3, respectively).