Catalytic Self‐Assembled Monolayers on Au Nanoparticles: The Source of Catalysis of a Transphosphorylation Reaction

Abstract

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) .

Gold Nanoparticle-Biological Molecule Interactions and Catalysis

“…The produced supramolecule was also found to be able to catalyze the same cleavage of actual RNA dinucleotides. A more in-depth analysis [124] using triazacyclononane·Zn (II) as the catalytic unit along with unreactive ligands, found maximum k cat and K M values of 6.7 × 10 −3 s −1 and 3.1 × 10 −4 M respectively. The maximum k cat value was reached when the mole fraction of the catalytic unit was 0.4 of the total ligands.…”

Chiral Nanozymes–Gold Nanoparticle‐Based Transphosphorylation Catalysts Capable of Enantiomeric Discrimination

“…Recently, gold nanoparticles (AuNPs) passivated by thiols which terminate with metal complexes have been shown to possess many features of metalloenzymes . These include the cooperativity between neighboring metal ions and Michaelis–Menten kinetics in the cleavage of phosphodiester bonds . These AuNPs have thus emerged as an attractive platform for the development of synthetic enzymes and, in particular, as a model for nucleases.…”

Manipulating the Monolayer: Responsive and Reversible Control of Colloidal Inorganic Nanoparticle Properties

“…Coordination of Zn 2+ to the NP‐bound macrocycles affords active transphosphorylation catalysts that mimic phosphodiester cleavage by ribonucleases. A sigmoidal increase in observed reaction rates with increasing metal ion loading indicates a cooperative multinuclear catalytic site, which is facilitated by the multivalent nature of the NP‐bound monolayer, and leads to catalytic activities over 600‐fold greater than mononuclear molecular analogues . The monolayer composition was characterized by a combination of NMR analyses and spectrophotometry.…”