Journal
CHEMISTRYOPEN
Volume 5, Issue 3, Pages 236-246Publisher
WILEY-V C H VERLAG GMBH
DOI: 10.1002/open.201500210
Keywords
6,8 flavylium rearrangement; 6-bromo-apigeninidin; 8-bromo-apigeninidin; anthocyanins; UV/Vis
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Funding
- Fundacao para a Ciencia e a Tecnologia (FCT) [SFRH/BPD/72652/2010, SFRH/BPD/84805/2012]
- FCT
- Fundo Europeu de Desenvolvimento Regional (FEDER) [007265-UID/QUI/50006/2013]
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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 pK(a)s, 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.
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