Comparative Study of the Photophysical Behavior of Fisetin in Homogeneous Media and in Anionic and Cationic Reverse Micelles Media†

Abstract

The 3,3', 4',7 tetrahydroxiflavone (fisetin) is a natural therapeutically active and fluorescent polyhydroxyflavone, with important spectroscopic and biological behavior. Fisetin shows dual emission, with a normal band (N) from the S1 --> S0 transition and the one generated in the excited state (phototautomer; PT) from the intramolecular proton transfer (ESIPT) process. The influence of different interfaces on the ESIPT process of fisetin was investigated in reverse micelles media (RMs) made of the anionic sodium 1,4-bis (2-ethylhexyl) sulfosuccinate (AOT) and cationic benzyl n-hexadecyl dimethylammonium chloride (BHDC) surfactants, in benzene. The studies were carried out by absorption, emission spectroscopy, steady-state anisotropy and time-resolved fluorescence measurements. Fisetin behavior was also investigated in homogeneous media with special emphasis in water and benzene, which are the polar core and the organic pseudofase in the RMs, respectively. In addition, the effect of concentration in benzene and the variation of the pH in water were studied. Fluorescence lifetime measurements show that in water the ESIPT process is independent on the concentration, while in benzene it was possible to detect fluorescent aggregate species (Nas) formed in the ground state. The effect of the pH in water allowed us to identify the anionic fisetin (A-) emission. The studies in RMs show that fisetin interacts specifically with the head of the surfactants, which always results in diminishing the emission of the PT. Also the formation of A- is detected particularly at W0 > 0. Appreciable high anisotropy values are obtained in RMs, as compared with those in fluid homogeneous media, which are independent of the water content confirming that fisetin molecules are anchored in the anionic as well as in the cationic interfaces.

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

“…18,19 So far, there has been just one report of emission peak at this region and it is assigned to the anion form of fisetin in the homogeneous medium. 6 Further evidence was obtained from a methanolic solution of fisetin in 0.1 M sodium methoxide, showing excitation peak at 414 nm and corresponding emission peak at 490 nm. Thus the peak observed at 490 nm (l ex 418 nm) in NaC is due to the emission of (FA) GS .…”

“…19 Moreover the excited state phototautomer does form by intramolecular proton transfer from this hydroxyl group. 6 Upon excitation at their characteristic wavelengths, FN species (l ex 369 nm) gives rise to FT, (FA*) ES and FN emission species, whereas the (FA) GS (l ex 418 nm) gives rise to (FA*) GS and FT emission. Scheme 1 also describes the prototropic equilibria of the various forms in the excited state, which leads to the highly overlaid emission spectra of FN and (FA) GS and hence there is no clear description of the emission peaks to each of the prototropic forms.…”

“…It exhibits intramolecular excited state proton transfer (ESIPT) and shows two emission peaks in organized media like liposome and cyclodextrins; one peak is at 400 nm due to S 1 → S 0 emission and the other peak at a red-shifted wavelength, 540 nm due to phototautomerization of the neutral molecule under excitation. [5][6][7][8][9][10][11] Recently the presence of anionic species of fisetin was observed in alkaline pH. 6 Bile salts are biologically important molecules, synthesized from cholesterol within the liver.…”

“…[5][6][7][8][9][10][11] Recently the presence of anionic species of fisetin was observed in alkaline pH. 6 Bile salts are biologically important molecules, synthesized from cholesterol within the liver. Bile salts have a hydrophobic steroidal backbone with one to three hydroxyl groups and a carboxyl side chain lying along in the same plane of hydroxyl groups (Chart 1B and Chart 1C).…”

Fluorescence behavior of (selected) flavonols: a combined experimental and computational study

“…6 This was confirmed also with the help of deuteration effects and it was found that ESIPT can even occur under cryogenic conditions via a tunneling mechanism 7 or in semi-biological environments. [8][9][10][11] Besides experimental work, effort has also been put into the theoretical description of excited states of biomolecular chromophores. 12,13,[18][19][20][21][22][23][24] We use densityfunctional theory (DFT) and time-dependent density functional theory (TDDFT) as efficient semi-quantitative methods to investigate the fluorescence emission behavior.…”