Formation and stability of mononuclear and dinuclear Eu(III) complexes and their catalytic reactivity toward cleavage of an RNA analog

The complex between Eu(III) and 1,7-diaza-4,10,13-trioxacyclopentadecane-N, N′-diacetic acid (L4) was characterized by pH potentiometric titration and ¹H NMR spectroscopy. The conversion of the monomer to a dimeric complex is observed as the pH is increased from 7 to 10 in a reaction that releases one mol/HO^- per dimer formed. The dimeric complex undergoes a further ionization with a pK_a of 10.7. Kinetic parameters are reported for the cleavage of the simple phosphodiester 2-hydroxypropyl-4- nitrophenyl phosphate catalyzed by both the monomeric and the dimeric Eu(III) complexes. These data show that the monomer and dimer stabilize their bound reaction transition states with similar free energies of 7.1 and 7.6 kcal/mol, respectively. Clearly, a bridging hydroxide is not an optimal linker to promote cooperative catalysis between Eu(III) centers in macrocycles with multiple polyaminocarboxylate pendent groups.