Multiple prototropism of fisetin in sodium cholate and related bile salt media

Fisetin, a bioflavonoid, has important biological relevance. It exhibits intramolecular excited state proton transfer (ESIPT), analogous to the structurally similar flavonoids. The presence of multiple prototropic forms of fisetin was observed at various concentrations of different bile salt molecules. The presence of ground state fisetin anion (FA)(GS) (λ(ex) 418 nm; λ(em) 490 nm) in alcohols and bile salt micellar media is a novel observation. The interaction of fisetin with sodium cholate (NaC) and some other bile salts has been studied in detail, using the intrinsic fluorescence of different prototropic forms of fisetin: neutral form (FN, λ(ex) 369 nm, λ(em) ~ 400 nm), ground state anion form ((FA)(GS), λ(ex) 418 nm, λ(em) 490 nm) and phototautomer (FT, λ(ex) 369 nm, λ(em) 540 nm). The hypsochromic shift of (FA*)(ES) emission and bathochromic shift of FT emission with increasing bile salt concentration suggests the progressive reduction of polarity of the bile salt media, which could be resulting from the neutralization of bile salt molecules as their concentration increases.