Two bis(triazacyclononane) ligands, 1,3-bis(1,4,7-triaza-1-cyclononyl)-p-xylene (pXTD) and 13-bis(1,4,7-triaza-1-cyclononyl)-m-xylene (mXTD), form stable dinuclear Cu(II) complexes (Cu2L). At pH 7.3, the predominant species are bis(hydroxide) complexes (Cu2L(OH)(2)(2+)) as determined by equilibrium modeling of pH-potentiometric measurements. Several dinuclear and mononuclear Cu(II) complexes and a dinuclear Zn(II) complex promote the hydrolysis of GpppG, a model for the 5'-cap of mRNA. At 0.125 mM complex, both Cu-2(pXTD) and Cu-2(mXTD) promote hydrolysis of GpppG approximately 100-fold more rapidly than does the monomeric Cu(TACN) complex (0.250 mM) at pH 7.3 and 37 degrees C (TACN = 1,4,7-triazacyclononane). The dependence of the rate constant on dinuclear Cu(II) complex concentration suggests that Cu-2(pXTD) promotes hydrolysis through both a 1:1 complex (Cu2L-GpppG) and a 2:1 complex ((Cu2L)(2)-GpppG). The 2:1 complex is 20-fold more reactive than the 1:1 complex; a first-order rate constant of 1.1 x 10(-4) s(-1) is determined for hydrolysis of the 2:1 complex. Cu-2(mXTD) effectively promotes the hydrolysis of GpppG only through a 2:1 complex which hydrolyzes with a first-order rate constant of 4.3 x 10 (-5) s(-1). Cu-2(pXTD) binds as a 1:1 complex Co m(7)GpppG with a binding constant of 27 000 M-1 as determined by use of fluorescence spectroscopy. Two Cu-2(mXTD) complexes bind stepwise to m(7)GpppG with binding constants of 5300 and 12 000 M-1 for the first complex and second complex, respectively.