Investigation of the Effect of Oxy Bridging Groups in Dinuclear Zn(II) Complexes that Catalyze the Cleavage of a Simple Phosphate Diester RNA Analogue

Two sets of dinuclear Zn(II) complexes were prepared to determine the effect of the presence of oxyanionic bridging groups between the metal centers on the catalytic activity toward the methanolysis of the RNA analogue 2-hydroxypropyl-4-nitrophenyl phosphate (HPNPP, 2). The Zn(II)(2) complexes of bis(di-(2-pyridylmethyl)amino)-m-xylene (6) and 2,6-bis(di-(2-pyridylmethyl)amino)-4-methylphenol (7) were compared to assess the effect of a bridging phenoxide ligand, while the Zn(II)(2) complex of 1,3-bis-N-1-(1,5,9-triazacyclododecyl)-propan-2-ol (8) was prepared to determine the effect of the 2-propoxy group compared to the previously studied complex of 1,3-bis-N-1-(1,5,9-triazacyclododecyl)-propane (4). Detailed kinetic studies of the cleavage of 2 including k(obs) VS [catalyst] plots and (s)(s)pH-rate profiles were performed for each system along with potentiometric titration experiments to determine the acid dissociation constants for the catalytically relevant groups. The results show that inclusion of the phenoxy bridging group in 7:Zn(11)2 reduces the second-order catalytic rate constant (k(2)(cat)) for cleavage of 2 by a factor of 160 relative to that of 6:Zn(II)(2), while the incorporation of a propoxy group in 8:Zn(II)(2) reduces its efficacy by 3.7 x 10(4) times relative to 4:Zn(II)(2). Energetics calculations reveal that 6:Zn(II)(2) offers a 3.7 kcal/mol greater stabilization of the reaction transition state for the cleavage of 2 than does 7:Zn(II)(2) and that 4:Zn(II)(2) affords 6.5 kcal/mol greater transition state stabilization than does 8:Zn(II)(2). The analyses show that the reduction in the transition state stabilization experienced with the complexes having permanently bridging oxyanion groups stems almost entirely from a weaker binding of the phosphate and catalyst, and a reduced catalytic rate constant. These results indicate that the presence of a bridging oxyanion ligand between the metal centers, a common structural element required for the successful formation of many small molecule dinuclear catalysts that show cooperative activity in water, significantly impairs the catalytic efficiency for cleavage of 2.