Eu(III) macrocyclic complexes promote cleavage of and bind to models for the 5'-cap of mRNA. Effect of pendent group and a second metal ion

The interaction of three Eu(III) macrocyclic complexes Eu(THED)³⁺, Eu(ATHC)³⁺, and Eu(S-TH)³⁺ with two 5'-cap model compounds, GpppG and m⁷GpppG is studied (THED = 1,4,7,10-tetrakis(2-hydroxyethyl)-1,4,7,-10-tetraazacyclododecane, ATHC = 1-(carbamoylmethyl)-4,7,10-tris(2-hydroxyethyl)- 1,4,7,10-tetraazacyclododecane, S-THP = 1S,4S,7S,10S-tetrakis(2-hydroxypropyl)- 1,4,7,10-tetraazacyclododecane). Laser-induced excitation luminescence spectroscopy is used to study the binding of Eu(S-THP)³⁺ to GpppG (K = 5.9 x 10⁴ M^-1) and to characterize the Eu(S-THP)-GpppG complex. Both Eu(THED)³⁺ and Eu(S-THP)³⁺ bind to m⁷GpppG as monitored by use of fluorescence spectroscopy with binding constants of 5.9 x 10³ and 4.4 x 104 M^-1, respectively. The kinetics of cleavage of GpppG by two macrocyclic complexes is studied. Cleavage of GpppG by Eu(THED)³⁺ is accelerated by 15-fold in the presence of an equivalent of Zn(NO₃)₂ at pH 7.3, 37 °C, suggesting that two metal ions accelerate the cap cleavage reaction. Eu(ATHC)³⁺ promotes cleavage of GpppG with a pseudo-first-order rate constant of 2.6 x 10^-5 s^-1 at pH 7.3, 37°C and 0.30 mM complex.