Premium
Catalytic Self‐Assembled Monolayers on Au Nanoparticles: The Source of Catalysis of a Transphosphorylation Reaction
Author(s) -
Zaupa Giovanni,
Mora Claudia,
Bonomi Renato,
Prins Leonard J.,
Scrimin Paolo
Publication year - 2011
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201002590
Subject(s) - catalysis , monolayer , nanoparticle , chemical engineering , nanotechnology , materials science , chemistry , organic chemistry , engineering
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 .