Hydrolysis of a model for the 5′-cap of mRNA by dinuclear copper(II) and zinc(II) complexes. Rapid hydrolysis by four copper(II) ions

Two bis(triazacyclononane) ligands, 1,3-bis(1,4,7-triaza-1-cyclononyl)-p-xylene (pXTD) and 1,3-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)₂²⁺) 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 Cu₂(pXTD) and Cu₂(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°C (TACN = 1,4,7-triazacyclononane). The dependence of the rate constant on dinuclear Cu(II) complex concentration suggests that Cu₂(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 × 10^-4 s^-1 is determined for hydrolysis of the 2:1 complex. Cu₂(mXTD) effectively promotes the hydrolysis of GpppG only through a 2:1 complex which hydrolyzes with a first-order rate constant of 4.3 × 10^-5 s^-1. Cu₂(pXTD) binds as a 1:1 complex to m⁷GpppG with a binding constant of 27 000 M^-1 as determined by use of fluorescence spectroscopy. Two Cu₂(mXTD) complexes bind stepwise to m⁷GpppG with binding constants of 5300 and 12 000 M^-1 for the first complex and second complex, respectively.