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

• Type: Article in International Scientific Journal
• Date: 2017

Authors

Joana Oliveira (Author) FCUP; Araujo, P (Author) Other; Fernandes, A (Author) Other; Natercia F Bras (Author) FCUP; Nuno Mateus (Author) FCUP; Pina, F (Author) Other; Victor de Freitas (Author) FCUP

Journal

Title: Dyes and Pigments

Vol. 141

Pages: 479-486

ISSN: 0143-7208

Publisher: Elsevier

Indexing

ISI Web of Knowledge - 15 Citations

Scopus - 16 Citations

Other information

Authenticus ID: P-00M-HKW

DOI: 10.1016/j.dyepig.2017.03.005

Abstract (EN): 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 similar to 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 similar to 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)similar to 9 (PKa3 = 9.5 +/- 0.1, pK(a3) = 8.9 +/- 0.1 and pK(a3) = 9.8 +/- 0.1 for pigment 1, 2 and 3, respectively).

Language: English

Type (Professor's evaluation): Scientific

No. of pages: 8