ANTHOCYANIN INTRAMOLECULAR INTERACTIONS - A NEW MATHEMATICAL APPROACHTO ACCOUNT FOR THE REMARKABLE COLORANT PROPERTIES OF THE PIGMENTS EXTRACTED FROM MATTHIOLA-INCANA
P. Figueiredo et al., ANTHOCYANIN INTRAMOLECULAR INTERACTIONS - A NEW MATHEMATICAL APPROACHTO ACCOUNT FOR THE REMARKABLE COLORANT PROPERTIES OF THE PIGMENTS EXTRACTED FROM MATTHIOLA-INCANA, Journal of the American Chemical Society, 118(20), 1996, pp. 4788-4793
In the last few years, a series of investigations has brought to light
a mechanism of stabilization of the colorant properties of certain an
thocyanins. Intramolecular interactions take place between the chromop
hore moiety of the anthocyanin and one of its aromatic acid residues,
which folds over the chromophore and thus confers protection against h
ydration and subsequent formation of colorless forms. In our continuin
g study of the physicochemical properties exhibited by acylated natura
l anthocyanins, we report here on a series of five structurally relate
d pigments extracted from the violet flowers of Matthiola incana. Thes
e pigments all bear the same chromophore moiety, i.e., the cyanidin ag
lycon, but differ in the degree of glycosylation and acylation. Acidit
y constants for the deprotonation and hydration equilibria of the flav
ylium cation, together with rate constants for the hydration step alon
e were determined from UV-visible absorption measurements. The data su
pport the existence of intramolecular, noncovalent interactions that s
trongly stabilize the colored forms of the pigments. However, none of
the four more heavily substituted anthocyanins follows the above-menti
oned mechanism that was previously successfully applied to the study o
f acylated anthocyanins. Consequently, a new mechanism with a differen
t mathematical treatment is here developed to account for the differen
t behavior exhibited by these distinctive pigments.