RESONANCE RAMAN-SPECTROSCOPIC STUDIES OF ANTHOCYANINS AND ANTHOCYANIDINS IN AQUEOUS-SOLUTIONS

The resonance Raman spectra of the six common anthocyanidins and some of their glycosylated derivatives have been recorded from acidic aqueo us solutions. A molecular mechanics study of the three basic anthocyan idin types was undertaken in order to obtain a set of molecular parame ters to search for correlation between molecular structure and measure d resonance Raman data. The close electronical and geometrical configu rations of benzopyrylium moieties, which appear to be the chromophore of the investigated pigments, explain the great similarity in the spec tra of the six anthocyanidins. The main modifications, observed for th e inter-ring stretching mode and the in-plane deformations of the carb on skeleton, are produced by a complex mechanism which involves a chan ge in the delocalization pattern, a significant flexibility of the mol ecule around the inter-ring bond, as well as vibrational coupling betw een ring modes. However the substitution pattern can be easily recogni zed and the effect of glycosylation is clearly visible on the spectra; the substitution on position 5 seems to produce a more important pert urbation in the vibrational properties than that on position 3.