TY - JOUR
T1 - Influence of the structural features of amino-based pyranoanthocyanins on their acid-base equilibria in aqueous solutions
AU - Oliveira, Joana
AU - Araújo, Paula
AU - Fernandes, Ana
AU - Brás, Natércia F.
AU - Mateus, Nuno
AU - Pina, Fernando
AU - de Freitas, Victor
N1 - sem pdf conforme despacho.
FCT/MEC (UID/QUI/50006/2013 - POCI/01/0145/FEDER/007265) ;
FEDER fund through FCT - SFRH/BPD/112465/2015 ;
IF/00225/2015 ;
IF/01355/2014;
PTDC/AGR-TEC/2789/2014 ;
REDE/1517/RMN/2005
PY - 2017/6/1
Y1 - 2017/6/1
N2 - 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 (pKa1 = 2.4 ± 0.1) and at a lesser extent for pigment 2 (pKa1 = 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 (pKa2 = 2.7 ± 0.1, pKa2 = 4.8 ± 0.1 and pKa2 = 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 (pKa1 and pKa2) 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 pKa3∼9 (pKa3 = 9.5 ± 0.1, pKa3 = 8.9 ± 0.1 and pKa3 = 9.8 ± 0.1 for pigment 1, 2 and 3, respectively).
AB - 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 (pKa1 = 2.4 ± 0.1) and at a lesser extent for pigment 2 (pKa1 = 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 (pKa2 = 2.7 ± 0.1, pKa2 = 4.8 ± 0.1 and pKa2 = 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 (pKa1 and pKa2) 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 pKa3∼9 (pKa3 = 9.5 ± 0.1, pKa3 = 8.9 ± 0.1 and pKa3 = 9.8 ± 0.1 for pigment 1, 2 and 3, respectively).
KW - Amino-derived pyranoanthocyanins
KW - Butadienylidene linkage
KW - Density functional theory (DFT)
KW - Deprotonation
KW - Protonation
KW - UV–Visible
KW - Vinylene linkage
UR - http://www.scopus.com/inward/record.url?scp=85014722594&partnerID=8YFLogxK
U2 - 10.1016/j.dyepig.2017.03.005
DO - 10.1016/j.dyepig.2017.03.005
M3 - Article
AN - SCOPUS:85014722594
VL - 141
SP - 479
EP - 486
JO - Dyes and Pigments
JF - Dyes and Pigments
SN - 0143-7208
ER -