An investigation into the effect of the structure of bile salt aggregates on the binding interactions and ESIHT dynamics of curcumin: A photophysical approach to probe bile salt aggregates as a potential drug carrier

This work demonstrates the utilization of bile salt aggregates as a potential biological host system for studying the binding interactions and dynamics of the poorly-water-soluble drug curcumin by means of photophysical techniques. We found that the level of degradation of curcumin is greatly suppressed upon encapsulation into the nanocavities of three different bile salt aggregates. However, NaTC aggregates are more effective to suppress the level of degradation of curcumin than NaCh and NaDC aggregates. We also report the modulation of the photophysical and dynamical properties of curcumin into the nanocavities of bile salt aggregates using steady-state and time-resolved fluorescence spectroscopy. The reduced level of interaction of curcumin with water upon incorporation into the different binding sites of bile salt aggregates results in an enhanced fluorescence intensity along with the blue shift in the emission maxima of curcumin. However, the observation of higher fluorescence quantum yield as well as longer fluorescence lifetime in NaTC aggregates compared to that in NaCh and NaDC aggregates clearly indicates a more effective decrease in the excited-state intramolecular hydrogen atom transfer (ESIHT) mediated nonradiative deactivation of curcumin by the interaction with the anionic headgroup of NaTC. The binding and location of curcumin into the bile salt aggregates has been further confirmed from the steady-state fluorescence anisotropy measurements. In addition, we have shown the effect of addition of salt on the photophysical properties of curcumin in the confined environments of bile salt aggregates. Our results indicate that on addition of salt the time scale of ESIHT process of curcumin in bile salt aggregates is markedly increased.