Solvation effect and binding of rhaponticin with iron: a spectroscopic and DFT/TDDFT study

In this article, both experimental and computational methods are employed to investigate the photophysics of rhaponticin (RH). The bathochromic shift was observed in absorption and fluorescence spectra with increasing solvent polarity, which implied that the charge transition of RH involved was π → π*. The results showed that RH possess strong intramolecular charge transfer (ICT), and the most important parameter to characterize the photophysical behavior of RH is the intermolecular hydrogen bonding ability of the solvent. The hydrogen bonding effect occurred at the localized electron-acceptor oxygen at the glycoside bond. Density functional theory (DFT) and time dependent density functional theory (TDDFT) were used to obtain the most stable structure, electronic excitation energy, dipole moments and charge distribution. The result was found to be 2.23 and 3.67 D in ground state and excited state respectively. Fluorescence quenching of RH owing to the photoinduced electron transfer (PET) is facilitated in alkaline media. The pK _a value of RH was 6.39, which defined RH as a highly efficient "off-on" switcher. The effect of different metal ions on the fluorescence spectra of RH was also investigated, and the fluorescence quenching of RH depended on the nature of ions. The best performance was accomplished for binding with the Fe³⁺ ion. The interactions of RH with the Fe³⁺ ion were studied by FT-IR and HPLC, and the binding parameter was calculated by the Stern-Volmer equation. The results obtained reveal the binding activity of RH can make this a candidate as a good source of new agents for thalassemic patients.