Enantioselective quenching of luminescence: Molecular recognition of chiral lanthanide complexes by biomolecules in solution

Principles, experimental techniques, and applications of enantioselective quenching of lanthanide luminescence are addressed. Upon the addition of chiral, enantiomerically resolved quencher molecules to an aqueous, racemic solution of the tris (pyridine-2,6-dicarboxylate) chelate of Eu(III) or Tb(III), strong quenching of the lanthanide luminescence is observed, with a quenching rate that depends on the chirality of the lanthanide species. Quenching by c-type cytochromes and vitamin B-12 derivatives is discussed in detail. For the latter quenchers, the energy transfer reaction held responsible for the quenching proceeds via formation of an encounter complex between donor and quencher in which the actual energy transfer takes place. A structural model for this transient pair has been constructed on the basis of data on complexation between B-12 and ground-state lanthanide chelate obtained by measurements of lanthanide induced shifts and relaxation of the NMR signals of protons of Bit and from the induced circular dichroism spectra. The enantioselectivity in the quenching results mainly from selective binding of the two lanthanide enantiomers to B-12. A smaller contribution to the selectivity comes from differences in the rate of the quantum mechanical energy transfer within the two diastereomeric encounter complexes. Finally, the use of the enantiodifferential quenching to study complexation of B-12 with two B-12 binding proteins (haptocorrin and a monoclonal antibody) is addressed.