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

• At very high pH values hydration becomes faster than tautomerization, change of regime.
• Flash photolysis, quantum yields and reverse pH jumps permit to assess the change of regime.
• A step-by-step procedure to obtain all the rate and equilibrium constants is presented.

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.

Determination of the rate and equilibrium constants of the tautomerization reaction involving hemiketal and cis-chalcone, (ring opening/closure) in flavylium networks lacking of a cis-trans isomerization barrier is very difficult since these species are elusive. The change of regime between hydration and tautomerization allows access to all these constants by means of reverse pH jumps, flash photolysis and isomerization quantum yields.Figure optionsDownload as PowerPoint slide