Factors affecting T cell responses induced by fully synthetic glyco-gold-nanoparticles.

We have synthesized and characterized nearly monodisperse and highly pure gold nanoparticles (2 and 5 nm) coated with non-immunoactive mono- and disaccharides, modelled after the capsular polysaccharide of serogroup A of the Neisseria meningitidis bacterium. We have used them to test their ability to induce immune cell responses as a consequence of their multivalency. The results indicate that they are indeed immunoactive and that immunoactivity is strongly dependent on size, and larger, 5 nm nanoparticles perform far better than smaller, 2 nm ones. Immune response (activation of macrophages) initiates with the whole nanoparticle recognition by the surface of antigen-presenting cells, independent of the saccharide oligomerization (or charge) on the nanoparticle surface. The induction of T cell proliferation and the increase of IL-2 levels, a consequence of the expression of MHC II involved in antigen presentation, require the presence of a disaccharide on the nanoparticle, not just a monosaccharide. A possible explanation is that, at this stage, the saccharides are detached from the gold surface. These results may provide leads for designing new saccharide-based, nanoparticle-conjugate vaccines.

Self-assembly and selective exchange of oligoanions on the surface of monolayer protected Au nanoparticles in water.

Self-assembled monolayers on Au nanoparticles terminating with TACN·Zn(II) head groups are attractive scaffolds for the formation of multivalent supramolecular structures at submicromolar concentrations in water.

Catalytic self-assembled monolayers on Au nanoparticles: the source of catalysis of a transphosphorylation reaction.

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) .

Assessment of the morphology of mixed SAMs on Au nanoparticles using a fluorescent probe.

The distribution of thiols in mixed SAMs can be determined in a straightforward manner from spectrophotometric titrations using a fluorescent probe. A plot of saturation concentration as a function of mole fraction provides information on the number of headgroups involved in binding.

Phosphate diesters cleavage mediated by Ce(IV) complexes self-assembled on gold nanoparticles.

Monolayer protected gold nanoparticles (MPGN) coated with metal chelating groups were prepared. In the presence of Ce(IV), the MPGN proved to be highly active in promoting the hydrolytic cleavage of the DNA model bis-p-nitrophenyl phosphate (BNP). A 2.5 million-fold rate acceleration of the BNP hydrolysis was measured at a concentration of Ce(IV) as low as 120 microM (pH 7, 25 degrees C). This corresponds to a reduction of the substrate half-life time from 2000 years to few hours. The MPGN-Ce(IV) system is 1 to 2 orders of magnitude more reactive than any mono- and binuclear Ce(IV) complex so far reported. The source of such reactivity is the cooperation between the metal ions self-organized on the surface of the nanoparticles.

Insights on nuclease mechanism: the role of proximal ammonium group on phosphate esters cleavage.

The role of remotely located ammonium groups in the active site of many phosphate-cleaving enzymes is investigated by using model systems thus providing mechanistic hints to a still unsolved problem.

Phosphate diester and DNA hydrolysis by a multivalent, nanoparticle-based catalyst.

2-nm gold nanoclusters coated with Zn(II) complexes bearing auxiliary hydrogen bond donors act as multivalent catalysts capable of promoting the hydrolysis of model phosphate diesters with exceptional activity and inducing DNA double strand cleavage.