Lanthanide-dependent perturbations of luminescence in indolylethylenediaminetetraacetic acid-lanthanide chelate

Using a synthetic EDTA analog built out of tryptophan, we have investigated the photophysics of the interaction of indole with various lanthanides chelated in close enough proximity to the indole to permit dramatic quenching and energy transfer to the metals. We investigate the strength of a number of interactions and determine means to separate the most significant contributions (dipole-dipole energy transfer, intersystem crossing) to the overall quenching rate. This allows more quantitative agreement in molecular parameters between the various lanthanides than might otherwise be obtained. In those lanthanides which are readily reduced in aqueous solution, electron transfer apparently dominates other quenching processes. The electron-transfer process, which is treated by Marcus' theory, shares some features in common with an additional energy-transfer process observable in the indolyl-EDTA-Ln complex (especially for Tb³⁺), involving an electronic state of the indole distinct from its first excited singlet. Metal luminescence consequent upon this energy transfer is quenched by added acrylamide or oxygen in a manner distinct from the effect of iodide, which suggests the involvement of the indole triplet in the additional transfer process. The overall analysis reveals a number of pitfalls to the facile generalization of results from this or any other particular single system to the interpretation of results of similar studies on tryptophyl residues in proteins.