Spectroscopy of fisetin in solution, confined media, and in protein.
Author
Al-Rasbiyah, Ghaliya Salim Said.
English abstract
The spectroscopy of fisetin was examined in different solvents and in the drug carriers cyclodextrins (CDs) and human serum albumin (HSA). The steady state absorption and fluorescence spectra of fisetin were studied in solvents of varying polarity and hydrogen bonding ability. The absorption peak shows a significant red shift upon solvating fisetin in protic solvents. The observed shift in protic solvents drives from hydrogen bond interaction between fisetin and the protic solvent. When the polarity of the solvent increases, a red shift in the normal fluorescence band is observed. At the same time, the tautomer band exhibits a moderate shift to high energy (blue shift) with increasing solvent polarity. As a result, a superposition of a single broad emission band is observed in water due to the gradual enhancement of the overlapping between the normal and tautomer fluorescence.
The inclusion of the fisetin molecule inside the cavities of different CDs was studied using absorption and fluorescence spectroscopy. The change in the absorption spectra of fisetin as a function of CD concentration indicates different caging effects due to the change in the cavity size and hydrophobicity. The results indicate that dimethyl- B-CD (DMB-CD) has the largest caging effect among the other CDs. The binding constant for fisetin:DMB-CD complex is estimated to be larger than that for fisetin:B-CD which indicates that the binding affinity of fisetin is higher and the caging effect is stronger in fisetin:DMB-CD. Also, the results suggest that the stiochiometry of fisetin:CD is dominated by 1:1.
Interaction of fisetin with HSA was investigated to determine the nature of binding, The fluorescence intensity and lifetime of HSA was quenched upon the addition of fisetin. This indicates that subdomain IIA binding site is the host of the fişetin probe. Fisetin was found to selectively quench the fluorescence intensity of Trp-214, which results in partial unmasking of the tyrosine fluorescence. The reduction in the fluorescence of Trp-214 is due to energy transfer from this residue to fisetin. The distance between Trp-214 and fisetin was calculated using Förster's theory of energy transfer. The quenching rate constant (kg) was estimated by using a Stern-Volmer plot. The calculated distance and ką values both indicate a major contribution from static quenching in the fisetin:HSA complex. The binding constant and the number of binding sites of the complex were also estimated. One binding site in the complex was found from the calculations which indicates that the probe binds only in subdomin IIA.
Chemical unfolding of HSA was studied in 6 M of GuHCI. By comparing the lifetimes of tryptophan in native HSA with that in denatured HSA, the shorter lifetimes in the latter case is a clear indication of the exposure of Trp-214 residue to the buffer which results in enhancement of the nonradiative decay at the expense of the radiative decay.
