Hydrophobically Self-Assembled Nanoparticles as Molecular Receptors in Water

Abstract

Biomolecular and artificial receptors are typically designed to exploit the hydrophobic effect in order to enhance the stability of receptor-ligand complexes in water. For example, artificial receptors are often built around hydrophobic cavities. These receptors exploit the hydrophobic effect toward ligand recognition, but the structure of the binding site requires a rigid framework to overcome the hydrophobic effect-driven tendency to collapse. Here we present an artificial receptor that exploits the hydrophobic effect to define its structure in water. The receptor is based on amphiphilic building blocks that assemble into micelle-like aggregates of a very high stability, attributed to the unusual shape of the amphiphile: a relatively rigid molecule composed of a large hydrophobic segment, based on the cholesterol molecule, and a very large headgroup build around a Zn-metalloporphyrin moiety. The assemblies, persistent down to the nanomolar range, are better described as self-assembled nanoparticles. Within the nanoparticle-water interface, Zn-metalloporphyrin moieties form multiple binding sites that specifically bind ligands bearing basic nitrogen atoms. The nanoparticles show enhanced binding affinity relative to a model receptor that does not self-assemble. Structurally related ligands show a correlation between the enhancement of binding and the octanol/water partition coefficient, log P, suggesting that the desolvation of binding sites is the main driving force for the enhancement of binding affinity at the nanoparticle-water interface. In addition, the highest affinity observed for the ditopic ligands relative to the monotopic ligands is evidence of a multivalent effect operating within this type of receptors. The nanoparticle readily deassembles upon addition of water-miscible organic solvents, such as methanol, or in the presence of detergents. This approach toward self-assembled receptors can be easily adapted to the development of differential receptors by the simple expedient of mixing slightly different amphiphiles (i.e., different metals in the porphyrin ring for the amphiphiles described here) in variable proportions.