Chromatographic investigation on acyl migration in betacyanins and their decarboxylated derivatives

Chemopreventive and antioxidant action of betalain pigments can differ in dependence on their stereoselective properties, therefore, it is necessary to use relevant methods for monitoring of their possible stereoisomers. Chromatographic characterisation of a group of new isomers of various 6′-O-acylated betacyanins and decarboxylated betacyanins which were generated at low concentration by intramolecular pH-dependent acyl migration was studied in aqueous solutions by HPLC separation with diode-array and mass spectrometric detection. Under alkaline conditions (pH 10.5) the rate of migration was dramatically accelerated, however, always favouring the 6′-O-position and it was much less prominent at lower pH (under 7.0). The possible products of the partial rearrangement were tentatively identified as the 3′-O- and 4′-O-acylated forms and their relative retention times were provided. In malonylated betacyanins and 17-decarboxy-betacyanins the 4′-O-forms were characterised in RP-HPLC by higher retention than the 6′-O forms, whereas the 3′-O-forms were always the most polar. In contrast, the isomerisation of hylocerenin and 17-decarboxy-hylocerenin resulted in different chromatographic profiles of the migration products. In 2-decarboxy- and 2,17-bidecarboxy-betacyanins the 3′-O- and 4′-O-acylated forms eluted always before the 6′-O-acylated betacyanins. The investigations on acyl migration in isolated 4′-O-malonyl-betanin confirmed the strong tendency of reverse acyl migration (4′ → 6′) and also partial 4′ → 3′ rearrangement which were leading to the final monoester regioisomeric distribution (%) close to 87:7:6 (6′-O-, 4′-O-, 3′-O-).