The enantioselective quenching of the luminescence of racemic Ln(DPA)(3)(3-) (Ln = Tb, Eu; DPA = 2,6-pyridinedicarboxylate) by ferricytochrome c proteins in aqueous solution has been studied by time-resolved luminescence and circular-polarization-of-luminescence spectroscopy. All title proteins appear to be very efficient quenchers with quenching rate constants on the order of 10(7)-10(9) M(-1) s(-1) depending on the nature of the protein and Ln, on the absolute configuration of the Ln complex (Delta or Lambda), and on the solution’s ionic strength I. The ratio of the individual diastereomeric quenching rate constants, k(q)(D)elta/k(q)(L)ambda, ranges from 1.2 up to 3 depending on the nature of the protein and Ln ion, but hardly on I. The average quenching rate, k(q)(av) (=(k(q)(D)elta + k(q)(L)ambda)/2), also depends on the ground stare lanthanide concentration, which allows for an estimate of the binding constant of ground state Ln complex to the active site of the protein : 7 x 10(2) (cytc) and 4 x 10(2) M(-1) (cytc-550) at I = 0.022 M. The quenching process is modeled by a two-step reaction pathway : (i) formation of an encounter complex between protein and excited Ln(DPA)(3)(3-), at sites which are primarily determined by the electrostatic potential V around the protein molecules in the aqueous salt solutions, and (ii) transfer of electronic energy transfer from the excited Ln(DPA)(3)(3-) to the heme chromophore. The potentials V calculated at positions near the exposed heme edge account qualitatively for the observed I-dependence of the quenching rates. The large enantioselectivities show that energy transfer takes place in conformations where the Ln complex and the protein are in intimate contact, and the large magnitude of k(q)(av) shows that the contact takes place at the solvent exposed heme edge. While the enantioselectivities for the wild type and Lys14 --> Glu cytc-550 are almost equal, that for the Lys99 --> Glu mutant is considerably lower, This indicates that the quenching site is closer to Lys99 than to Lys14. Molecular modeling was used to optimize geometries of Delta- and Lambda-Ln(DPA)(3)(3-)/cytc-550 complexes in which a hydrogen bond between the epsilon-amino group of Lys99 and the carboxyl group of a DPA ligand is present, yielding tentative structures of the diastereomeric complexes in which the energy transfer occurs.