Extending the Study of the 6,8 Rearrangement in Flavylium Compounds to Higher pH Values: Interconversion between 6-Bromo and 8-Bromo-apigeninidin.

The rearrangement between isomers 6- and 8-bromo-apigeninidin (6 and 8) was studied by pH jumps using stopped flow, UV/Vis, NMR, and HPLC analysis. The system constitutes a pH-dependent network of chemical reactions involving up to 18 different species. The dynamic network is equivalent to a single diprotic acid exhibiting two pKas, 2.55 and 5.4. Similar to other flavylium derivatives, the mole fraction of the species hemiketal and cis-chalcone in both multistate isomers are negligible at the equilibrium. At pH 1, the pure isomers are slowly converted in a mixture containing about 50 % of isomers 6 and 8, while at pH 4, the system evolves to mixture of 10 % trans-chalcone and 90 % of quinoidal bases. A series of pH jumps from pure isomer 6 at pH 1 to pH 6 and back to pH 1 leads to the same initial absorption spectra of the pure isomer 6. The same occurs for pure isomer 8, showing the lack of communication between the cis-chalcones, at least in the time scale of few minutes. A pH jump from the equilibrated mixture of the isomers at pH 1.0 to 5.8 permits to follow a very slow isomerization.