The interaction of three Eu(III) macrocyclic complexes Eu(THED)(3+), Eu(ATHC)(3+), and Eu(S-THP)(3+) with two 5'-cap model compounds, GpppG and m(7)GpppG 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 10(4) 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 10(3) and 4.4 x 10(4) 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 degrees 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 degrees C, and 0.30 mM complex.