Synthesis and characterization of a new fluorescent probe for measuring potassium.

A new fluorescent probe, 6,7-(4-methyl)coumaro-[2.2.2] cryptand, has been developed for measuring K+. This compound was made by fusing [2.2.2] cryptand with 4-methylcoumarin through 2 ethoxy bridges at the 6 and 7 positions. The probe has a fluorescent excitation peak at 340 nm and an emission peak at 420 nm. In aqueous solutions, increasing the K+ concentration from 0 to 10 mM causes the fluorescence intensity to increase by 143%. The dissociation constant (Kd) for K+ in aqueous solutions is 1.9 mM. In 100% methanol, the Kd for K+ decreases to 0.012 mM, making it possible to measure K+ concentrations in the micromolar range. Na+, tetramethylammonium, NH4+, Ca2+, and Mg2+ have a minimal affect on the fluorescence of the probe in the absence of K+. The coefficient of variation for the measurement of K+ by use of this new dye is less than 1%. In this report, the synthetic procedure is described and the spectral properties of the probe are characterized. Experiments are described demonstrating the use of this probe 1) in measuring K+ in aqueous solutions from 0 to 10 mM and in a microfluorometric assay to measure K+ from 0.0005 to 0.003 mM and 2) in monitoring K+ transport in rabbit proximal tubule brush-border membrane vesicles.