Pressure and temperature dependence of enantioselective excited-state quenching of chiral Tb(III) and Eu(III) tris (pyridine-2,6-dicarboxylate) chelates by various C-type ferricytochromes

For mitochondrial ferricytochrome c from horse, cow and tuna and for bacterial cytochrome c-550 from Paracoccus versutus, the pressure and temperature dependence of their quenching of racemic Tb(DPA)₃^3- and Eu(DPA)₃^3- (DPA = pyridine-2,6-dicarboxylate) luminescence in aqueous solution is investigated. Of these energy transfer reactions the activation volumes (ΔV(@?)) and energies (E(a)) are determined for the ranges P = 0-3 kbar and T= 15-40°C. For the Λ enantiomers of Tb(DPA)₃^3- and Eu(DPA)₃^3-, ΔV(@?) and E(a) are almost the same for all proteins: 0.4 ≤ ΔV(@?) ≤ 1.1 cm³/mol and O ≤ E(a) ≤ 2 kJ/mol. For the Δ enantiomer, whose luminescence is quenched faster, the activation parameters vary significantly with the origin of the protein and they are different for Tb(DPA)₃^3- and Eu(DPA)₃^3-. Values for ΔV(@?,Δ) range from +4.9 ± 0.5 (Eu/horse) to +0.5 ± 0.5 cm³/mol (Tb/tuna); E(a) ranges from +8 ± 1 (Tb/cow) to -1 ± 1 kJ/mol (Tb/tuna). The degree of enantioselectivity in the quenching (E(q)) by cytc from horse, cow, chicken and pigeon is almost the same and markedly higher than from tuna. The differing quenching characteristics of the tuna protein are ascribed to the amino acid variations near the exposed heme edge: Ile9→Thr, Thr28→Val and/or Phe46→Tyr. Other variations in this region, Thr47→Ser and Ala15→Ser, do not affect E(q).