On the intrinsic stability of curcumin.

Curcumin is the primary polyphenol and active ingredient in turmeric, which has demonstrated anti-cancer, anti-microbial, anti-inflammatory and anti-oxidant properties, leading to a significant effort to produce therapeutic variants, with, for example, improved solubility. However, numerous questions remain about the biological activity of curcumin, including whether curcumin, or a metabolite, is responsible for these therapeutic properties, the nature of the mechanism(s) of biological activity for the active species and the potential role of photochemistry. Underpinning these questions is a lack of fundamental understanding about the electronic (and nuclear) structure of curcumin, which, in part, limits the development of curcumin-based therapeutics. Here we isolate the gas-phase deprotonated curcumin anion, which is a significant charge state for biological settings and study its intrinsic structure and light-driven dynamics using mass-selected ion spectroscopy. Our measurements show the presence of a bound anion state, efficient internal conversion from electronically excited states to the ground state and indicate relatively high-lying dissociation products. Taken together, our results suggest that fragmentation of curcumin is unlikely to be a key route to biological activity, explain its potential as a photodynamic therapeutic and provide a rationale for understanding its thermal and photochemical stability.