Chimie des pigments anthocyaniques. 4. Spectre d’absorption UV-visible de la forme chalcone de la glucoside-3 malvidine

Institut de Topologie et de Dynamique des Systèmes de l'Université Paris VII, associé au CNRS, 1, rue Guy-de-la-Brosse, 75005 Paris.

Résumé

Les anthocyanes existent, en milieu aqueux acide, sous plusieurs formes en équilibre : la base quinonique, le cation flavylium, le carbinol et la chalcone. Du tait de sa faible abondance et du fait de la présence des trois autres structures, l'observation spectroscopique de la forme chalcone est diffi- cile. Nous donnons ici une méthode permettant d'obtenir le spectre d’absorption UV-visible de la forme chalcone de la glucoside-3 malvidine. Ce spectre présente des caractéristiques semblables à celles des spectres de chalcones de pigments flavylium, en général d’origine non naturelle. Toutefois, au maximum d’absorption (340-350 nm), la valeur du coefficient d’extinction moléculaire (≈ 10 000 M^-1 .cm^-1) reste faible devant celles des pigments précités (20 000 à 30 000 M^-1.cm^-1).

Abstract

Anthocyanins are the principal pigments of flowers and fruits and thus constitute one of the most important families of natural coloring matters. Electronic absorption spectroscopy bas been widely used in chemical and biochemical studies of these pigments. In particular, Harborne has given a very useful method for identifying anthocyanins (³). This method is based on absorption spectra measurement in acidic methanol solutions. In aqueous acid solutions at room temperature, anthocyanins exist in four forms in equilibrium : the quino- noïdal base, the flavylium cation, the carbinol and the chal- lone. Due to the presence of the three other structures and to its low concentration, the chalcone form is rather difficult to observe. We give here a method for obtaining the UV-visible absorption spectrum of the chalcone. This method is based on the theoretical evaluation of the relaxation ampli- tude of the carbinol-chalcone equilibrium induced by an appropriate pH-jump and on its measurement by UV-visible spectrophotometry. The spectra characteristics of the chalcone form of malvidin 3-glucoside are similar to those of the chalcone forms of synthetic flavylium cations. It is well known that the chalcone is the stable form of these latter. However, the extiction coefficient at the absorption maximum (10 000 M^-1.cm^-1), is two to three times lower than those generally observed for the synthetic pigments (¹⁰). Finally, the spectrum of the chalcone differs greatly from the spectra of the three other structures.