Direct excitation luminescence spectroscopy of Eu(III) complexes of 1,4,7-tris(carbamoylmethyl)-1,4,7,10- tetraazacyclododecane derivatives and kinetic studies of their catalytic cleavage of an RNA analog

The macrocycles 1,4,7-tris(carbamoylmethyl)-1,4,7,10-tetrazacyclododecane (1), 1,4,7-tris[(N-ethyl)carbamoylmethyl]-1,4,7,10-tetraazacyclododecane (2), 1,4,7-tris[(N,N-diethyl)carbamoylmethyl]-1,4,7,10-tetraazacyclododecane (3) and their Eu(III) complexes are prepared. Studies using direct Eu(III) excitation luminescence spectroscopy show that all three Eu(III) complexes exhibit only one predominant isomer with two bound waters under neutral to mildly basic conditions (Eu(X)(H₂O)₂ for X = 1–3). There are no detectable ligand ionizations over the pH range 5.0–8.0 for Eu(3), 5.0–8.5 for Eu(2) or 5.0–9.5 for Eu(1). The three Eu(III) complexes show a linear dependence of second-order rate constants for the cleavage of 4-nitrophenyl-2-hydroxyethylphosphate (HpPNP) on pH in the range 6.5–8.0 for Eu(3), 7.0–8.5 for Eu(2) and 7.0–9.0 for Eu(1). This pH–rate profile is consistent with the Eu(III) complex–substrate complex being converted to the active form by loss of a proton and with Eu(III) water pK_a values that are higher than 8.0 for Eu(3), 8.5 for Eu(2) and 9.0 for Eu(1). Inhibition studies show that Eu(1) binds strongly to the dianionic ligand methylphosphate (K_d = 0.28 mM), and more weakly to diethylphosphate (K_d = 7.5 mM), consistent with a catalytic role of the Eu(III) complexes in stabilizing the developing negative charge on the phosphorane transition state.