An efficient synthesis of sialic-acid-terminated glycerol dendron to chemically functionalize 2 nm and 14 nm gold nanoparticles (AuNPs) is described. These nanoparticles are highly stable and show high activity towards the inhibition of influenza virus infection. As the binding of the viral fusion protein hemagglutinin to the host cell surface is mediated by sialic acid receptors, a multivalent interaction with sialic-acid-functionalized AuNPs is expected to competitively inhibit viral infection. Electron microscopy techniques and biochemical analysis show a high binding affinity of the 14 nm AuNPs to hemagglutinin on the virus surface and, less efficiently, to isolated hemagglutinin. The functionalized AuNPs are nontoxic to the cells under the conditions studied. This approach allows a new type of molecular-imaging activity-correlation and is of particular relevance for further application in alternative antiviral therapy.

Nanomedicine is a rapidly growing field that has the potential to deliver treatments for many illnesses. However, relatively little is known about the biological risks of nanoparticles. Some studies have shown that nanoparticles can have an impact on the aggregation properties of proteins, including fibril formation. Moreover, these studies also show that the capacity of nanoscale objects to induce or prevent misfolding of the proteins strongly depends on the primary structure of the protein. Herein, light is shed on the role of the peptide primary structure in directing nanoparticle-induced misfolding by means of two model peptides. The design of these peptides is based on the alpha-helical coiled-coil folding motif, but also includes features that enable them to respond to pH changes, thus allowing pH-dependent beta-sheet formation. Previous studies showed that the two peptides differ in the pH range required for beta-sheet folding. Time-dependent circular dichroism spectroscopy and transmission electron microscopy are used to characterize peptide folding and aggregate morphology in the presence of negatively charged gold nanoparticles (AuNPs). Both peptides are found to undergo nanoparticle-induced fibril formation. The determination of binding parameters by isothermal titration calorimetry further reveals that the different propensities of both peptides to form amyloid-like structures in the presence of AuNPs is primarily due to the binding stoichiometry to the AuNPs. Modification of one of the peptide sequences shows that AuNP-induced beta-sheet formation is related to the structural propensity of the primary structure and is not a generic feature of peptide sequences with a sufficiently high binding stoichiometry to the nanoparticles.

The nanoparticle-peptide interaction described here is based on electrostatic forces and the pH value can act as a trigger to direct the organization of functionalized nanoparticles in a reversible and repeatable manner. The ability of the peptide to interact with the charged gold nanoparticles is directly related to its helical structure and was not found for a random coil peptide with the same net charge. Interestingly, the interaction with nanoparticles seems to induce a fibrillation of the coiled coil peptide.

Colloidal gold particles with functionalized organic shells were applied as novel selectin binders. The ligand shell was terminated with different monocyclic carbohydrate mimetics as simplified analogs of the sLe x unit found in biological selectin ligands. The multivalent presentation of the sulfated selectin binding epitopes on the gold particles led to extremely high binding affinities towards L-and P-selectin and IC 50 values in the subnanomolar range. Depending on the ring size of the sulfated carbohydrate mimetic, its substitution pattern and its configuration, different selectivities for either L-selectin or P-selectin were obtained. These selectivities were not found for gold particles with simple acyclic sulfated alcohols, diols and triols in the ligand shell. In addition, the influence of the particle size and the thickness of the hydrophobic organic shell were systematically investigated.

Gold colloids with terminally functionalized sulfated thiol shells were found to bind to P- and L-selectins with IC(50) values in the picomolar range; branched acyclic epitopes show the highest affinity, whereas a sulfated carbohydrate mimetic provides the best selectivity.