Q-mode curve resolution of UV-vis spectra for structural transformation studies of anthocyanins in acidic solutions

Chemometric analysis of ultraviolet-visible (UV-vis) spectra for pH values 1.0, 3.3, 5.3, and 6.9 was used to investigate the kinetics and the structural transformations of anthocyanins in extracts of calyces of hibiscus flowers of the Hibiscus acetosella Welw. ex Finicius for the first time. Six different species were detected: the quinoidal base (A), the flavylium cation (AH(+)), the pseudobase or carbinol pseudobase (B), cis-chalcone (C-C), trans-chalcone (C-t), and ionized cis-chalcone (Cc(-)). Four equilibrium constant values were calculated using relative concentrations, hydration, pK(h) = 2.60 +/- 0.01, tautomeric, K-T = 0.14 +/- 0.01, acid-base, pK(a) = 4.24 +/- 0.04, and ionization of the cis-chalcone, pKC(Cc) = 8.74 +/- 1.5 x 10(-2). The calculated protonation rate of the tautomers is KH+ = 0.08 +/- 7.6 x 10(-3). These constants are in excellent agreement with those measured previously in salt form. From a kinetic viewpoint, the situation encountered is interesting since the reported investigation is limited to visible light absorption in acid medium. These models have not been reported in the literature. (c) 2006 Elsevier B.V. All rights reserved.