Establishment of the chemical equilibria of different types of pyranoanthocyanins in aqueous solutions: evidence for the formation of aggregation in pyranomalvidin-3-O-coumaroylglucoside-(+)-catechin.
The chemical equilibria of the pyranomalvidin-3-glucosides linked to (+)-catechin, (-)-epicatechin, and catechol moieties (and the respective coumaroylglucoside compounds) were established by means of UV-vis spectroscopy. The conjugated double bonds among pyranic rings C and D provide a higher electronic delocalization that prevents the nucleophilic attack of water at position 2. Consequently, besides flavylium cation (AH(+)), the bases A, A(-), and A(2-) have been identified by increasing pH, and the respective acidity constants were determined by spectrophotometry. The formation of dimers at higher concentration was observed for pyranomalvidin-3-O-coumaroylglucoside-(+)-catechin, and the respective data treated by the exciton model suggests the formation of a dimer where the monomers form J-type aggregates with the dipolar moments in opposite directions and rotated by 174° at a distance of 5.2 Å (from the center).
Cruz
Luís
L
Centro de Investigação em Química, Departamento de Química, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 687, 4169-007 Porto, Portugal.
Petrov
Vesselin
V
Teixeira
Natércia
N
Mateus
Nuno
N
Pina
Fernando
F
de Freitas
Victor
V
D000872:Anthocyanins / Q000138:chemical synthesis / Q000737:chemistry*; D002392:Catechin / Q000737:chemistry*; D005960:Glucosides / Q000737:chemistry; D011714:Pyrans / Q000138:chemical synthesis / Q000737:chemistry*; D013816:Thermodynamics; D014867:Water / Q000737:chemistry
D000872:Anthocyanins; D005960:Glucosides; D011714:Pyrans; D014867:Water; D002392:Catechin