Selective fluorescence quenching of biologically important acidic and non-acidic indoles by trivalent lanthanide ions and analysis of mixtures by a fluorimetric quenching method

The effects of several lanthanide(II)(Ln³⁺)ions(Dy³⁺,Er³⁺,Eu³⁺,Nd³⁺,Yb³⁺) on the room-temperature fluorescence spectra of 2-indolecarboxylic acid, 5-indolecarboxylic acid, 5-methoxy-2-indolecarboxylic acid and 3-indoleacetic acid were investigated. It was found that the fluorescence quenching by Ln³⁺ ions is much more efficient for the acidic compounds than for non-acidic indoles. Stern-Volmer relationships were obtained for most acidic indoles in the Ln³⁺ concentration range 10^?7–10^?4 M, with quenching constants ranging between 3 × 10³ and 4.6 × 10⁵ l mol^?1. A fluorimetric quenching method was developed for the quantitative analysis of binary mixtures of acidic and non-acidic indoles.