The catalytic activity of a series of Au monolayer protected colloids (Au MPCs) containing different ratios of the catalytic unit triazacyclononane⋅Zn(II) (TACN⋅Zn(II) ) and an inert triethyleneglycol (TEG) unit was measured. The catalytic self-assembled monolayers (SAMs) are highly efficient in the transphosphorylation of 2-hydroxy propyl 4-nitrophenyl phosphate (HPNPP), an RNA model substrate, exhibiting maximum values for the Michaelis-Menten parameters k(cat) and K(M) of 6.7×10(-3) s(-1) and 3.1×10(-4) M, respectively, normalized per catalytic unit. Despite the structural simplicity of the catalytic units, this renders these nanoparticles among the most active catalysts known for this substrate. Both k(cat) and K(M) parameters were determined as a function of the mole fraction of catalytic unit (x(1)) in the SAM. Within this nanoparticle (NP) series, k(cat) increases up till x(1) ≈0.4, after which it remains constant and K(M) decreases exponentially over the range studied. A theoretical analysis demonstrated that these trends are an intrinsic property of catalytic SAMs, in which catalysis originates from the cooperative effect between two neighboring catalytic units. The multivalency of the system causes an increase of the number of potential dimeric catalytic sites composed of two catalytic units as a function of the x(1) , which causes an apparent increase in binding affinity (decrease in K(M)). Simultaneously, the k(cat) value is determined by the number of substrate molecules bound at saturation. For values of x(1) >0.4, isolated catalytic units are no longer present and all catalytic units are involved in catalysis at saturation. Importantly, the observed trends are indicative of a random distribution of the thiols in the SAM. As indicated by the theoretical analysis, and confirmed by a control experiment, in case of clustering both k(cat) and K(M) values remain constant over the entire range of x(1) .

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 (kcat/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 kcat and KM 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.

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 CuII and ZnII), three with the polyazamacrocycles and one with the Tren platform. Some of the ZnII 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 ZnII 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 ZnII-azacrown complexes provide the cooperative, catalytic site. The allosteric role of the ZnII ion located in the Tren binding site was unambiguously demonstrated by studying the catalytic activity of a derivative unable to complex ZnII in that site. In this case, the cooperativity between the three ZnII 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.

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Multivalent systems are well known for their enhanced ability to bind multivalent counterparts. This contribution addresses the question whether they can also behave as cooperative catalysts. Analyzing examples from our own laboratory we show that self-assembled systems obtained by covering gold nanoclusters with thiol-terminated amino acids and peptides behave indeed as cooperative catalysts. By comparing their activity profiles with those of discrete, multivalent systems we show what are minimal conditions to elicit cooperativity in multivalent systems. Reactions taken into considerations for our analysis are the hydrolyses of carboxylate- and phosphate esters.