Schematic representation of tmFRET experimental strategy. The membrane-resident dye R18 is depicted by a red sphere atop an acyl chain. Bound transition metal is depicted as a blue sphere. The inset structure is that of R18. If R18 and membrane-bound metal ions are sufficiently close, excitation of R18 (lightning bolt) will excite the donor fluorophore whose emission will be quenched by tmFRET with the acceptor transition metal.