Curcumin/Turmeric as an Environment-Friendly Protein Gel Stain

Curcumin, the main curcuminoid in food spice turmeric, is insoluble in water at room temperature. We showed that curcumin can be solubilized in water with the application of heat (100 °C). Here we demonstrate that heat-solubilized curcumin can serve as a nontoxic and environment-friendly fluorescent/colorimetric reversible protein stain. Curcumin, the yellow pigment found in the rhizomes of the perennial herb Curcuma longa (turmeric), is insoluble in aqueous solvents. However, heat solubilization in water renders 1.5% of curcumin soluble. Curcumin solubilized by ethanol or alkali is ineffective in staining proteins. Heat-solubilized curry spice turmeric also stains proteins. Staining is achieved in 30 min, with a sensitivity almost equaling that of Coomassie Brilliant Blue (CBB). Destaining is not required and excess curcumin/turmeric can be discarded into the sink. Binding of proteins by silver inhibits curcumin binding, suggesting similarity of protein binding by silver and curcumin. It costs $1.5-2.0 to stain a mini-gel with curcumin, while turmeric costs less than 0.005 cent. CBB staining/destaining costs about two cents. Curcumin/turmeric, thus, can serve as an ideal nontoxic protein stain.