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 aqueous solutions. A molecular mechanics study of the three basic anthocyanidin types was undertaken in order to obtain a set of molecular parameters to search for correlation between molecular structure and measured resonance Raman data. The close electronical and geometrical configurations of benzopyrylium moieties, which appear to be the chromophore of the investigated pigments, explain the great similarity in the spectra of the six anthocyanidins. The main modifications, observed for the inter-ring stretching mode and the in-plane deformations of the carbon skeleton, are produced by a complex mechanism which involves a change in the delocalization pattern, a significant flexibility of the molecule around the inter-ring bond, as well as vibrational coupling between ring modes. However the substitution pattern can be easily recognized and the effect of glycosylation is clearly visible on the spectra; the substitution on position 5 seems to produce a more important perturbation in the vibrational properties than that on position 3.