A comparative study of the spectral, fluorometric properties and photostability of natural curcumin, iron- and boron- complexed curcumin

Abstract

Curcumin is a yellow phenolic compound with a wide range of reported biological effects. However, two main obstacles hinder the use of curcumin therapeutically, namely its poor bioavailability and photostability. We have synthesized two curcumin complexes, the first a boron curcumin complex (B-Cur₂) and the second an iron (Fe-Cur₃) complex of curcumin. Both derivatives showed high fluorescence efficiency (quantum yield) and greater photostability in solution. The improved photostability could be attributed to the coordination structures and the removal of β-diketone group from curcumin. The fluorescence and ultra violet/visible absorption spectra of curcumin, B-Cur₂ and Fe-Cur₃ all have a similar spectral pattern when dissolved in the same organic solvent. However, a shift towards a lower wavelength was observed when moving from polar to non-polar solvents, possibly due to differences in solvent polarity. A plot of Stokes' shift vs the orientation polarity parameter (Δf) or vs the solvent polarity parameter (E_T 30) showed an improved correlation between the solvent polarity parameter than with the orientation polarity parameter and indicating that the red shift observed could be due to hydrogen-bonding between the solvent molecules. A similar association was obtained when Stokes' shift was replaced by maximum synchronous fluorescence. Both B-Cur₂ and Fe-Cur₃ had larger quantum yields than curcumin, suggesting they may be good candidates for medical imaging and in vitro studies.

Keywords: Boron-complexed curcumin; Curcumin; Fluorescence intensity; Fluorometry; Iron-complexed curcumin; Photostability; Quantum yield; Spectroscopy.