Caffeine interaction with synthetic flavylium salts. A flash photolysis study for the adduct involving 4′,7-dihydroxyflavylium

The adducts formed between caffeine and three hydroxyflavylium salts (4-methyl-7-hydroxyflavylium, 4′-hydroxyflavylium, and 4′,7-dihydroxyflavylium) are described and the last studied by flash photolysis. A consistent evidence of the caffeine effect on the rate constants of this system is given for the first time. A net hyperchromic effect was only observed for the adduct involving 4′-hydroxyflavylium. The results confirm previous observations that co-pigmentation is essentially a phenomenon that changes the molar fraction distribution of the several species in equilibrium, and not a result of the formation of highly colored charge transfer bands. A simple mathematical model is used to account for the experimental results.