Tripodal, cooperative, and allosteric transphosphorylation metallocatalysts

Three artificial amino acids derived from L-serine by replacing the hydroxyl moiety with 1,4,7-triazacyclononane, 1,5,9-triazacyclododecane, and 1,4,7,10-tetraazacyclododecane, respectively, have been connected to the three arms of the tetraamine tris(2-aminoethyl)amine, Tren, to obtain tripodal ligands. They are able to bind up to four metal ions (like Cu-II and Zn-II), three with the polyazamacrocycles and one with the Tren platform. Some of the Zn-II complexes of these tripodal ligands proved to be good catalysts for the cleavage of the RNA model substrate 2-hydroxypropyl-p-nitrophenylphosphate (HPNP). Studies of the catalytic activity in the presence of increasing amounts of Zn-II show that the complexes represent minimalist examples of metallocatalysts with cooperativity between the metal centers and allosteric control by a metal ion. The Tren binding site constitutes the allosteric regulation unit, while the three Zn-II-azacrown complexes provide the cooperative, catalytic site. The allosteric role of the Zn-II ion located in the Tren binding site was unambiguously demonstrated by studying the catalytic activity of a derivative unable to complex Zn-II in that site. In this case, the cooperativity between the three Zn-II ions bound to the peripheral azacrowns was totally suppressed. The kinetic analysis has shown that cooperativity is due to neither the occurrence of general-acid/general-base catalysis nor a decreased binding of the substrate because of the deprotonation of a water molecule bound to the complex but, rather, stabilization of the complexed substrate in its transformation into the transition state.