TAC-Limedex synthesis and K+ sensitivity. (a) Synthesis procedures: (i) Methyl-3-(2,4-dimethyl-1H-pyrrol-3-yl) propanoate, TFA, DCM, rt, 18 h followed by addition of DDQ, 4 h; (ii) BF3 Et2O, DIEA, DCM, 0 °C, 2 h; (iii) 0.2 M aq NaOH, 100 °C, 2 h; (iv) O-(N-succinimidyl)-N,N,N′,N′-tetramethyluronium tetrafluoroborate, DIEA, DMF; (v) amino-dextan, TEA, H2O/DMSO. (b) Fluorescence spectra of TAC-Limedex.K+ titrations done at 500 nm excitation wavelength (for emission spectra) and 510 nm emission wavelength (for excitation spectrum). Solutions contained 5 μM TAC-Limedex (5 mM HEPES, pH 7) in balanced KCl/NaCl to maintain constant ionic strength at 200 mM.