Labeling spHCN ^R332C with single isomers MTS-5-TAMRA and MTS-6-TAMRA recapitulates the mixed isomer response. Response of representative oocytes expressing spHCN^R332C and labeled with either MTS-5-TAMRA (left) or MTS-6-TAMRA (right) to the voltage step protocol shown in the inset. Upper traces: membrane current; lower traces: fluorescence. Oocytes were labeled under the same conditions as for the mixed isomer (see Materials and methods) and were from the same donor frog.