Abstract:

Functionalization of multivalent structures such as dendrimers and monolayer passivated nanoparticles with catalytically active groups results in very potent catalysts, a phenomenon described as the positive dendritic effect. Here, we describe a series of peptide dendrons and dendrimers of increasing generation functionalized at the periphery with triazacyclononane, a ligand able to form a strong complex with Zn^II. Kinetic studies show that these metallodendrimers very efficiently catalyze the cleavage of the RNA model compound HPNPP, with dendrimer D₃₂ exhibiting a rate acceleration of around 80 000 (k_cat/k_uncat) operating at a concentration of 600 nM. A theoretical model was developed to explain the positive dendritic effect displayed by multivalent catalysts in general. A detailed analysis of the saturation profile and the Michaelis–Menten parameters k_cat and K_M shows that it is not necessary to ascribe the positive dendritic effect to, for instance, changes in the catalytic site, increased substrate binding constant, or changes in the microenvironment. Rather it appears that the efficient catalytic behavior of multivalent catalysts is mainly determined by two factors: the number of catalytic sites occupied by substrate molecules under saturation conditions, and the efficiency of the multivalent system to generate catalytic sites in which multiple catalytic units act cooperatively on the substrate.