Probing solute–solvent hydrogen bonding with fluorescent water-soluble curcuminoids

Glycosylated water-soluble curcuminoids (C1–C3, first scheme of this article) differing in the 3,3-ringsubstituents (–OH, –OCH3 and H) equilibrate between the di-keto and the keto–enol forms. The formerare well observable in the absorption spectra in water, but their emissions are always negligible. Theketo–enol forms of C1–C3 exhibit a broad range of fluorescence quantum yields in different solvents,organic and water: formation of solute–solvent hydrogen bonds through the 3,3-ring substituents maychange the radiationless S1-state decay constant by up to a factor 200. Such a fluorescence quenchingmechanism is extremely efficient in water and, for C1, in accepting organic media. On the contrary, noeffects traceable to intermolecular hydrogen bonds involving the central -dicarbonyl moiety have beenobserved. So, fluorescence of these curcuminoids may probe the hydrogen bonding ability, particularly asacceptor, of their microenvironments, including hydrophilic/hydrophobic domains in complex biologicalsystems. Interaction of C1 and C2 with bovine serum albumin results in emission enhancements inverseto the quantum yields of the curcuminoids in water. The observations support the idea that, although thecurcuminoid microenvironment within its complex with the protein is less polar and hydrogen bondingthan water itself, residual water/ligand hydrogen bonds are active in enhancing radiationless transitions.Finally, fluorescence confocal images of HCT116 cells treated with C1–C3 suggest the apparently smallstructural differences to affect, besides their fluorescence behaviour, their interactions and fate withinliving cells.