TY - JOUR
T1 - Effect of water content on the acid-base equilibrium of cyanidin-3-glucoside
AU - Dias, Isabel Borges Coutinho Medeiros
AU - Freitas, Adilson A.
AU - Maçanita, Antonio L.
AU - Lima, João Carlos dos Santos Silva E Pereira de
N1 - Sem PDF.
FCT/MCTES (PTDC/QUI/65728/2006 )
PY - 2015/4/1
Y1 - 2015/4/1
N2 - Laser Flash Photolysis was employed to measure the deprotonation and reprotonation rate constants of cyanidin 3-monoglucoside (kuromanin) in water/methanol mixtures. It was found that the deprotonation rate constant k(d) decreases with decreasing water content, reflecting the lack of free water molecules around kuromanin, which may accommodate and stabilize the outgoing protons. On the other hand, the reprotonation rate constant, k(p), increases with the decrease in water concentration from a value of k(p) = 2 x 10(10) l mol (1) s (1) in water up to k(p) = 6 x 10(10) l mol (1) s (1) at 5.6 M water concentration in the mixture. The higher value of k(p) at lower water concentrations reflects the fact that the proton is not freely escaping the solvation shell of the molecule. The deprotonation rate constant decreases with decreasing water content, reflecting the lack of free water molecules around kuromanin that can accommodate the outgoing protons. Overall, the acidity constant of the flavylium cation decreases with the decrease in water concentration from pK(a) values of 3.8 in water to approximately 4.8 in water-depleted media, thus shifting the equilibrium towards the red-coloured form, AH(+), at low water contents.
AB - Laser Flash Photolysis was employed to measure the deprotonation and reprotonation rate constants of cyanidin 3-monoglucoside (kuromanin) in water/methanol mixtures. It was found that the deprotonation rate constant k(d) decreases with decreasing water content, reflecting the lack of free water molecules around kuromanin, which may accommodate and stabilize the outgoing protons. On the other hand, the reprotonation rate constant, k(p), increases with the decrease in water concentration from a value of k(p) = 2 x 10(10) l mol (1) s (1) in water up to k(p) = 6 x 10(10) l mol (1) s (1) at 5.6 M water concentration in the mixture. The higher value of k(p) at lower water concentrations reflects the fact that the proton is not freely escaping the solvation shell of the molecule. The deprotonation rate constant decreases with decreasing water content, reflecting the lack of free water molecules around kuromanin that can accommodate the outgoing protons. Overall, the acidity constant of the flavylium cation decreases with the decrease in water concentration from pK(a) values of 3.8 in water to approximately 4.8 in water-depleted media, thus shifting the equilibrium towards the red-coloured form, AH(+), at low water contents.
KW - Acid-base equilibrium
KW - Anthocyanin
KW - Laser Flash Photolysis
KW - Water/methanol
U2 - 10.1016/j.foodchem.2014.09.060
DO - 10.1016/j.foodchem.2014.09.060
M3 - Article
C2 - 25442581
VL - 172
SP - 476
EP - 480
JO - Food Chemistry
JF - Food Chemistry
ER -