Gold nanoclusters protected by conformationally constrained peptides

The preparation and properties of a series of gold nanoclusters protected by thiolated peptide based on the -aminoisobutyric acid (Aib) unit are described. The peptides were devised to form 0-3 C=O•••H-N intramolecular hydrogen bonds, as required by their 310-helical structure. The monolayer-protected clusters (MPCs) were prepared, using a modified version of the two-phase Brust-Schiffrin preparation, and fully characterized with 1H NMR, IR and UV-Vis absorption spectroscopies, transmission electron microscopy (TEM), thermogravimetric analysis (TGA), and X-ray photoelectron spectroscopy (XPS). The MPCs were obtained with core diameters in the range 1.1 - 2.3 nm, depending on the reaction conditions. Structured peptides formed smaller clusters. The smallest MPC obtained is in agreement with the average formula Au38Pep18. The results showed that the chemical integrity of the peptide is maintained upon monolayer formation and that the average number of peptide ligands per gold cluster is typically 75-85% the value calculated for alkanethiolate MPCs of similar sizes. The IR and NMR spectra indicated that in the monolayer the peptides are involved in both intra- and interligand C=O•••H-N hydrogen bonds.