Paolo Pengo

Functional gold nanoparticles for recognition and catalysis

Gold nanoparticles passivated by a monolayer of functional thiolates represent a new promising tool in supramolecular chemistry since they allow the preparation of complex, self-organized, polyvalent structures with a very modest synthetic effort. These systems have sizes comparable to those of many biological molecules including proteins and nucleic acids, and to those of many cellular sub-structures. In this way they are good candidates to model recognition processes and to develop new biomimetic catalysts. Some interesting examples towards these goals are reviewed briefly.

Synthesis, characterization and properties of water-soluble gold nanoparticles with tunable core size

Robust, water-soluble gold clusters protected by monolayers of ligands containing a short alkyl chain (C7) close to the gold surface and a triethylene glycol monomethylether unit (TEG) to impart solubility in water and other polar solvents were prepared and characterized. Thiol 7 (N1-{2-[2-(2-methoxyethoxy)ethoxy]ethyl}-8-sulfanyloctanamide) constitutes a good and versatile capping agent for the preparation of these nanoparticles. By tuning the Au/thiol ratio and sodium borohydride addition rate, nanoparticles with different core diameters ranging from 1.5 to 4.2 nm, as determined by TEM analysis, could be obtained. The size distribution of the gold cores appears to become broader as the Au/thiol ratio used to prepare the nanoparticles increases. Characterization of these nanoclusters also by NMR, UV-Vis and FTIR spectroscopies is reported. Solubility properties have been studied in a large variety of solvents and different solubility behaviors were observed for nanoparticles of different sizes. Exchange reactions were carried out successfully with small (1.9 nm) and large nanoparticles (4.2 nm) using dodecanethiol as the entering thiol. This demonstrates that these materials can be used for the preparation of nanoclusters with different functional groups soluble in polar solvents including water. The synthetic procedure described represents a facile route to tailoring the size and solubility properties of Au nanoparticles.

Nanozymes: Functional Nanoparticle-based Catalysts

This short review highlights recent results on the use of nanoclusters of gold protected by a monolayer of functional ligands in both molecular recognition and catalysis. Particular emphasis is given to the multivalent properties of these systems and, in the case of catalysis, to the cooperativity between functional groups present in the ligands of the monolayer. The outstanding catalytic efficiency of some of the functional nanoparticles synthesized led us to call them nanozymes by analogy with the activity of catalytic polymers (synzymes).

Gold nanoparticles protected by fluorinated ligands for 19F MRI

Gold nanoparticles coated with fluorinated ligands (F-MPCs) present features suitable for (19)F MRI as observed from phantom experiments. Cellular uptake, by HeLa cells, and toxicity of fluorescent dye-decorated F-MPCs are presented together with their ability to bind hydrophobic molecules allowing for a potential combination of targeting, delivery and imaging features.

Anion transport across phospholipid bilayers promoted by a guanidinium calix[4]arene conjugate

The ionophoric activity of a calix[4]arene guanidinium conjugate (4G4Oct) has been investigated in bulk chloroform membrane and mainly in liposomes. In both membrane models 4G4Oct shows high activity in the transport of chloride. The mechanistic evidence suggests a carrier mechanism in which the calixarene binds the anions through electrostatic interactions and hydrogen bonds with the guanidinium moieties. Studies on anion transport in liposomes indicate a peculiar selectivity sequence inverse to the most commonly observed lyotropic series with chloride transported faster than bromide and iodide virtually non-transported. This selectivity sequence together with competitive inhibition of chloride transport by the non-transported ions suggests a strong anion binding to the guanidinium moieties in the membrane environment. The complex formed is lipophilic and is able to shuttle the ion across the membrane. The origin of the observed anion selectivity is suggested to be related to a less favourable ion exchange at the membrane interface of iodide with respect to chloride with OH− , which is the antiported ion in the transport process in liposome. These studies complement the reported ability of 4G4Oct to condensate plasmid DNA and to transfect cells, showing that the calixarene derivative is also able to efficiently transport chloride across a phospholipid bilayer.

Gold nanoparticles as drug carriers: a contribution to the quest for basic principles for monolayer design

Two structurally different water-soluble homoligand gold nanoparticle systems, one featuring a rigid fluorous monolayer in the proximity of the gold core and the other featuring a flexible fluorinated region in a distal position, were studied as putative hosting systems by determining their binding constants for a series of fluorinated and non-fluorinated radical probes by means of ESR spectroscopy. Comparison of the binding constants obtained with hydrogenated homoligand nanoparticles of similar structure used as the reference evidenced that the binding of both hydrogenated and fluorinated guests is favoured in the presence of fluorinated nanoparticles. In addition, a flexible fluorinated monolayer acts as a better hosting system than the more rigid counterpart. In the latter case decreasing the size of the nanoparticles causes a small decrease of the binding affinities for both hydrogenated and fluorinated guests. The same nanoparticle systems were analysed for their ability to retard the phase transfer of a fluorescent dye from an aqueous solution to a toluene layer. All of the nanoparticles studied produced a significant decrease of the phase transfer rate of the dye because of the efficient interaction with the monolayer. These data support the introduction of fluorinated moieties in the monolayer of gold nanoparticles as a novel design tool for the development of drug delivery systems.

The Quenching Effect of Flavonoids on 4-Methylumbelliferone, a Potential Pitfall in Fluorimetric Neuraminidase Inhibition Assays

Many assays aimed to test the inhibitory effects of synthetic molecules, and naturally occurring products on the neuraminidase activity exploit the hydrolysis of 2′-O-(4-methylumbelliferyl)-N-acetylneuraminic acid (4-MUNANA). The amount of the released product, 4-methylumbelliferone (4-MU), is then measured fluorimetrically. The authors attempted an analysis of the inhibitory properties of 35 naturally occurring flavonoids on neuraminidase N3, where only 29 of them were sufficiently soluble in the assay medium. During the analysis, the authors noticed a strong quenching effect due to the test compounds on the fluorescence of 4-MU. The quenching constants for the flavonoids were determined according to the Stern-Volmer approach. The extent of fluorescence reduction due to quenching and the magnitude of the fluorescence reduction measured in the inhibition assays were comparable: for 11 of 29 compounds, the two values were found to be coincident within the experimental uncertainty. These data were statistically analyzed for correlation by calculating the pertinent Pearson correlation coefficient. Inhibition and quenching were found to be positively correlated (r = 0.71, p(uncorr) = 1.5 × 10−5), and the correlation was maintained for the whole set of tested compounds. Altogether, the collected data imply that all of the tested flavonoids could produce false-positive results in the neuraminidase inhibition assay using 4-MUNANA as a substrate.

Synthesis and in vitro study of novel neuraminidase inhibitors against avian influenza virus

Evidences of oseltamivir resistant influenza patients raised the need of novel neuraminidase inhibitors. In this study, five oseltamivir analogs PMC-31-PMC-36, synthesised according to the outcomes of a rational design analysis aimed to investigate the effects of substitution at the 5-amino and 4-amido groups of oseltamivir on its antiviral activity, were screened for their inhibition against neuraminidase N1 and N3. The enzymes used as models were from the avian influenza A H7N1 and H7N3 viruses. The neuraminidase inhibition assay was carried out by using recombinant species obtained from a baculovirus expression system and the fluorogenic substrate MUNANA. The assay was validated by using oseltamivir carboxylate as a reference inhibitor. Among the tested compounds, PMC-36 showed the highest inhibition on N1 with an IC(50) of 14.6±3.0nM (oseltamivir 25±4nM), while PMC-35 showed a significant inhibitory effect on N3 with an IC(50) of 0.1±0.03nM (oseltamivir 0.2±0.02nM). The analysis of the inhibitory properties of this panel of compounds allowed a preliminary assessment of a structure-activity relationship for the modification of the 4-amido and 5-amino groups of oseltamivir carboxylate. The substitution of the acetamido group in the oseltamivir structure with a 2-butenylamido moiety reduced the observed activity, while the introduction of a propenylamido group was well tolerated. Substitution of the free 5-amino group of oseltamivir carboxylate with an azide, decreased the activity against both N1 and N3. When these structural changes were both introduced, a dramatic reduction of activity was observed for both N1 and N3. The alkylation of the free 5-amino group in oseltamivir carboxylate introducing an isopropyl group seemed to increase the inhibitory effect for both N1 and N3 neuraminidases, displaying a more pronounced effect against N1.

Hydrolytic metallo-nanozymes: From micelles and vesicles to gold nanoparticles

Although the term nanozymes was coined by us in 2004 to highlight the enzyme-like properties of gold nanoparticles passivated with a monolayer of Zn(II)-complexes in the cleavage of phosphate diesters, systems resembling those metallo-nanoparticles, like micelles and vesicles, have been the subject of investigation since the mid-eighties of the last century. This paper reviews what has been done in the field and compares the different nanosystems highlighting the source of catalysis and frequent misconceptions found in the literature.

Routes to the preparation of mixed monolayers of fluorinated and hydrogenated alkanethiolates grafted on the surface of gold nanoparticles

The use of binary blends of hydrogenated and fluorinated alkanethiolates represents an interesting approach to the construction of anisotropic hybrid organic-inorganic nanoparticles since the fluorinated and hydrogenated components are expected to self-sort on the nanoparticle surface because of their reciprocal phobicity. These mixed monolayers are therefore strongly non-ideal binary systems. The synthetic routes we explored to achieve mixed monolayer gold nanoparticles displaying hydrogenated and fluorinated ligands clearly show that the final monolayer composition is a non-linear function of the initial reaction mixture. Our data suggest that, under certain geometrical constraints, nucleation and growth of fluorinated domains could be the initial event in the formation of these mixed monolayers. The onset of domain formation depends on the structure of the fluorinated and hydrogenated species. The solubility of the mixed monolayer nanoparticles displayed a marked discontinuity as a function of the monolayer composition. When the fluorinated component content is small, the nanoparticle systems are fully soluble in chloroform, at intermediate content the nanoparticles become soluble in hexane and eventually they become soluble in fluorinated solvents only. The ranges of monolayer compositions in which the solubility transitions are observed depend on the nature of the thiols composing the monolayer.