Anthocyanins and related compounds. Detecting the change of regime between rate control by hydration or by tautomerization

The network of chemical reactions involving anthocyanins and related compounds has been historically considered as the chemical transformations taking place when the flavylium cation (which is generally stable only in very acidic solutions) disappears upon a pH jump to higher pH values (direct pH jump). In this work profit is taken from the appearance of flavylium cation from equilibrated solutions at moderately acid pH values, (reverse pH jumps) as well as upon irradiation of trans-chalcone in the cases where photochemistry is available. Reverse pH jumps, flash photolysis and reaction quantum yields unequivocally show that at sufficiently acidic pH values the tautomerization becomes the rate determining step of flavylium cation formation. This is a lucky situation since gives kinetic information about tautomerization, the most difficult process to access in the anthocyanins and related compounds network of chemical reactions.

The thermodynamic and kinetic details of flavylium base photochromic systems are described. Mathematical expressions for direct and reverse pH jumps, stopped flow and flash photolysis are presented as well as the procedure to obtain all of the rate and equilibrium constants of the system. (C) 2013 Elsevier Ltd. All rights reserved.