Search

Proteins provide versatile mediators for assembling nanoparticles into organized composites. Here, we demonstrate that a single protein spacer (lysozyme) can be used to direct the self-assembly of gold mixed monolayer protected clusters into controlled ensembles with varied functional response. This efficient self-assembly process provides nanocomposites featuring tunable interparticle spacings, as determined through small angle X-ray scattering, varied sample morphology, as observed by transmission electron microscopy, and modular collective optical behavior, as examined through UV−visible spectroscopic measurements.

Negatively charged carboxylate-functionalized mixed monolayer protected clusters (MMPCs) effectively bind and inhibit alpha-chymotrypsin based on complementary electrostatic surface recognition. We demonstrate that this binding can be disrupted by varying the ionic strength of the medium. Enzyme activity in the presence of MMPCs increases from 5% to 97% of native activity as salt concentration is increased from 0 to 1.5 M. Variation of ionic strengths after complete binding over 13 h results only in a modest restoration of enzymatic activity (< 35%). The conformation of chymotrypsin was characterized using circular dichroism and fluorescence spectroscopy, correlating structure with enzymatic activity. This work provides a useful insight of the electrostatic influence on protein--MMPC interactions and can be applied toward MMPC-based controlled release of proteins in vivo.

Intracellular trafficking of nanoscale objects is regulated by cell membranes through interactions that are under intense investigation. These materials are typically internalized by cells into membrane-bounded endosomes and never gain access to the cell machinery located in the cytosol. While some bio-macromolecules can penetrate or fuse with cell membranes without overt membrane disruption, no synthetic material of comparable size has shown this property yet. Cationic nanoscale objects (mostly assisted by some degree of hydrophobicity) pass through cell membranes by generating transient holes, a process associated with cytotoxicity. To date, studies aimed at generating cell-penetrating nanomaterials have focused on the effect of size, shape, and composition. Here we compare membrane penetration by two nanoparticle ‘isomers’ with the same composition (same hydrophobic content), one coated with sub-nanometer striations of alternating anionic and hydrophobic groups, and the other coated with the same moieties but in a random distribution. We show that the former particles penetrate the plasma membrane without bilayer disruption while the latter were instead mostly trapped in endosomes. Our results highlight a new design rule for materials that interact with cells and offer a paradigm for analyzing the fundamental problem of cell membrane penetrating bio- and macro-molecules. We anticipate consequences for designing materials for biomedical applications and for understanding potential safety issues associated with synthetic nanomaterials.

Nanoparticles featuring highly flexible sidechains template to peptides, demonstrating substantial pre-organization of the particle monolayer.

Positively charged trimethylammonium-functionalized mixed monolayer protected clusters (MMPCs) of different chain lengths (C(8) and C(11)) have been used to bind beta-galactosidase through complementary electrostatic interactions, resulting in complete enzyme inhibition. This inhibition can be reversed in vitro by intracellular concentrations of glutathione (GSH), the main thiol component of the cell. The restoration of activity depends on the chain length of the monolayer. The activity of enzyme bound to particles with C(8) monolayer was completely restored by intracellular concentrations (1-10 mM) of GSH; however, little or no release was observed at extracellular GSH concentrations. In contrast, no restoration was observed for enzyme bound to the C(11) particles at any of the concentrations studied. Taken together, these studies demonstrate that the GSH-mediated release of enzymes bound to MMPCs can be tuned through the structure of the monolayer, a significant tool for protein and drug delivery applications.

alpha-Helices are important structural elements in proteins. To provide a scaffold for the facial recognition of peptides, we have explored the interaction of cationic mixed monolayer protected clusters (MMPCs) with a tetra-aspartate peptide in water. In these studies, substantial enhancement of peptide helicity was observed upon addition of the MMPC. Significantly, this stabilization increased with time, demonstrating templation of the monolayer to the peptide helix.

We have used mixed monolayer protected gold clusters (MMPCs) to provide flavoenzyme model systems with a high affinity and ability to modulate cofactor reduction potential.

Highly water-soluble mixed monolayer protected "rippled'' gold nanoparticles were synthesized through a one step reaction with sodium 11-mercaptoundecanesulfonate and octanethiol ligands at various ratios.