546 C. Santos-Buelga and V. de Freitas
vinylpy-Mv-Cat (575 nm)Mv3glc (λmax520 nm)Wavelength (nm)Absorbancevinylpy-Mv-Cat (575 nm)Mv3glc (λmax520 nm)Wavelength (nm)AbsorbanceFig. 9D.7UV-visible spectra of malvidin 3-glucoside (Mv3glc) and vinylpyrano-Mv3glc-catechin
(vinylpy-Mv-Cat) (adapted from Mateus et al. 2003)
a higher stability to their molecules. These pigments also have increased resistance
to sulfites and pH-induced discoloration (Oliveira et al. 2006b) and their formation
involves the reaction between pyranoanthocyanins and vinyl-flavanols, thus consti-
tuting a further step in the evolution of wine pigments, in which anthocyanins are
no longer the main precursor. Although these blue pigments were only detected in
very small quantities in fortified wines, they present unique spectroscopic features
that may somehow contribute to the changing color of aged wines.
9D.2.3.3 Anthocyanin-ethyl-flavanol Pigments
These pigments were described by Timberlake and Bridle (1976) as resulting from
the condensation between anthocyaninsand flavanol mediated by acetaldehyde.
They have absorption spectra with maximum wavelengths in the visible region
about 15 nm bathochromically shifted with regard to that of the parent anthocyanins
and show a more violet hue in wine-like solutions. Despite having the positions
C2 and C4 of the anthocyanin moiety free, these pigments are partially resistant
to pH-induced discoloration and sulfite bleaching (Fig. 9D.8), which is explained
by their folded spatial conformation where the flavylium and catechin nuclei form
a cavity that could accommodate part of a second molecule of adduct. This would
favour the formation of non-covalent dimers where two pigments are stacked on one
another in a way that protects their flavylium nuclei from the nucleophilic attack of
water and SO 2 (Escribano-Bailon et al. 1996). A hydration constant of 4.17 was
calculated for catechin-ethyl-malvidin 3-glucoside, higher than its proton transfer