Type I blue copper proteins as enantioselective quenchers of the photoluminescence of Δ,Λ-Eu(pyridine-2,6-dicarboxylate)₃ ^3–: azurin from Pseudomonas aeruginosa and its Met44→Lys mutant, amicyanin from Paracoccus versutus and parsley plastocyanin

Abstract

 The dynamic quenching of the luminescence of racemic Eu(III)(pyridine-2,6-dicarboxylate=dpa)₃ ^3– by the title proteins is investigated and the enantioselectivity of the proteins in the quenching of the Δ and Λ enantiomers of Eu(dpa)₃ ^3– is determined. The two diastereomeric quenching rate constants pertaining to azurin (k _q ^Δ=3.3×10⁶, k _q ^Λ=2.7×10⁶ M^–1s^–1, pH 7.2, ionic strength I=22 mM) are lower than for its Met→44Lys mutant (k _q ^Δ=1.9×10⁷, k _q ^Λ=1.4×10⁷ M^–1 s^–1, same pH and I), indicating that energy transfer occurs from Eu(dpa)₃ ^3– to the Cu(II) centre when the luminophore is bound to the hydrophobic patch of the protein near residue 44. The enantioselectivity remains unaltered by the mutation: k _q ^Δ/k _q ^Λ=1.27±0.04, so Lys44 is probably not in direct contact with the Eu chelate. The I and pH dependence of k _q indicate that the lysine residue interacts electrostatically with Eu(dpa)₃ ^3–. For plastocyanin the quenching rates are of the order of 10⁶ M^–1 s^–1; for amicyanin they are two orders of magnitude larger (k _q ^Δ=12×10⁷, k _q ^Λ=11×10⁷ M^–1 s^–1, pH 7.2, I=22 mM). The variation of k _q is attributed to differences in the charge distribution on the proteins, which influences the binding of the luminophore to the protein surface. For amicyanin the anion binding site near Lys59 and Lys60 may be involved in the energy transfer.