Peptide-Conjugated Ultrasmall Gold Nanoparticles (2 Nm) For Selective Protein Targeting

Ultrasmall gold nanoparticles with a metallic core diameter of 2 nm were surface-conjugated with peptides that selectively target epitopes on the surface of the WW domain of the model protein hPin1 (hPin1-WW). The binding to the gold surface was accomplished via the thiol group of a terminal cysteine. The particles were analyzed by NMR spectroscopy, high-resolution transmission electron microscopy, and differential centrifugal sedimentation. The surface loading was determined by conjugating a FAM-labeled peptide, followed by UV-vis spectroscopy, and by quantitative H-1 NMR spectroscopy, showing about 150 peptide molecules conjugated to each nanoparticle. The interaction between the peptide-decorated nanoparticles with hPin1-WW was probed by H-1-N-15-HSQC NMR titration, fluorescence polarization spectroscopy (FP), and isothermal titration calorimetry (ITC). The particles showed a similar binding (K-D = 10-20 mu M) compared to the dissolved peptides (K-D = 10-30 mu M). Small-angle X-ray scattering (SAXS) showed that the particles were well dispersed and did not agglomerate after the addition of hPin1-WW (no cross-linking by the protein). Each nanoparticle was able to bind about 20 hPin1-WW protein molecules. An unspecific interaction with hPin1 was excluded by the attachment of a nonbinding peptide to the nanoparticle surface. The uptake by cells was studied by confocal laser scanning microscopy. The peptide-fiinctionalized nanoparticles penetrated the cell membrane and were located in the cytosol. In contrast, the dissolved peptide did not cross the cell membrane. Peptide-functionalized nanoparticles are promising agents to target proteins inside cells.