Dendrimer Cleaves Cooperatively

The functional units of enzymes often cooperate with each other to carry out catalytic tasks efficiently. Chemists in Italy now show that metal complexes dotting the periphery of a dendrimer also can work cooperatively. Paolo Scrimin of the University of Padova and colleagues functionalized a dendrimer with Zn²⁺-bound triazacyclononane ligands. They found that it cleaves a phosphate diester model of RNA cooperatively, exhibiting a 270-fold rate acceleration compared with the Zn²⁺-triazacyclononane complex alone and a 9,900-fold acceleration compared with the uncatalyzed process (J. Am. Chem. Soc. 2007, 129, 6982). The metallodendrimer also shows up to 3.7-fold greater activity compared with gold nanoparticles functionalized with Zn²⁺ triazacyclononane. The nonlinear dependence of reaction rate on the amount of Zn²⁺ indicates that two metal ions cooperate to cleave the substrate, the researchers say. The catalytic activity of the functionalized dendrimer is also pH dependent, which the researchers interpret as the two zinc ions stabilizing the transition state and facilitating deprotonation of the substrate.