Manipulation of the reactivity of a synthetic anthocyanin analogue in aqueous micellar media

The effects of anionic, cationic, and nonionic micelles on the dynamics of proton transfer in the ground and excited state of anthocyanins have been examined employing the 4-methyl-7-hydroxyflavylium ion (HMF) as a model compound. Unlike other anthocyanins and anthocyanin analogues, HMF does not hydrate in water, allowing study of the acid-base (AH(+)/A) equilibrium in the absence of competing hydration or tautomerism reactions. Anionic sodium dodecyl sulfate (SDS) micelles are found to stabilize the acid form of HMF, AH(+), relative to A, while both cationic hexadecyltrimethylammonium chloride and nonionic Triton X-100 and Brij 35 micelles stabilize A more than AH(+). Stabilization in SDS is reflected in the decrease of the deprotonation rate constant (ca. 40-fold relative to water), while destabilization in cationic and nonionic micelles is evidenced by a decrease in the protonation rate of A. Surprisingly, HMF is no longer resistant to hydration in cationic and nonionic micelles. Thus, one can manipulate the reactivity of anthocyanins by changing the detergent type in such a way as to selectively stabilize or destabilize AH(+) relative to the neutral forms. In the case of HMF, destabilization of AH(+) by cationic and nonionic micelles is sufficient to elicit chemistry that does not occur in the absence of the detergent.