Fluorescence regeneration as a signaling principle for choline and carnitine binding: A refined supramolecular sensor system based on a fluorescent azoalkane

The fluorescent azoalkane, 2,3-diazabicyclo[2.2.2]oct-2-ene (DBO), forms inclusion complexes with p-sulfonatocalix[4]arene (CX4). The binding constants are on the order of 10(3) M-1 in water. The addition of CX4 to DBO solutions results in an efficient fluorescence quenching (up to 90 %). This supramolecular system can be used as a truly water-soluble sensor system to signal the binding of organic ammonium ions over a large pH range. Addition of choline and carnitine derivatives and tetraalkylammonium ions results in regeneration of this fluorescence, from which the binding constants (K-C = 10(3)-10(5) M-1) are calculated by means of a competitive complexation model. Electrostatic effects are observed. namely, a more-than-one order of magnitude weaker binding of the carnitines in neutral solution.