Life, as we know, depends on different membrane sections of the cell where membrane proteins and their organizing scaffolding proteins maintain the flow of information and materials across the membrane. Imaging physiological pathways and providing structural insight into the molecules involved can help in the early diagnosis of various diseases. A range of novel probes, from small molecules to nanoparticles (NPs), has been explored for molecular imaging to achieve this objective. Herein, chemical techniques developed for targeted drug delivery using NPs have been exploited for cryo-electron microscopy (cryo-EM) investigations of protein structure-function relationships. A major strategy adopted is the functionalization of gold NPs (AuNPs) with targeting ligands such as glycine receptor (GlyR) antagonist (strychnine), glutathione, and biotin exhibiting high affinity towards membrane protein receptors, i.e., GlyR, glutamate receptor, and streptavidin, respectively. This was achieved by employing bi-functional linkers that attach targeting ligands at one end and bear functional groups that bind to the NPs at the other end. The length and surface density of the linker can influence the affinity of the NP-ligand construct towards the intended target and the colloidal stability of the resulting platform. Besides target specific NP-ligand constructs, positively and negatively charged end groups terminated polyethylene glycol (PEG) molecules were also conjugated to the AuNPs surface for studying non-specific interaction with the proteins. Such novel coupling strategies and the provision of precisely defining linker lengths will allow us to pinpoint the binding location on protein molecules, differentiate between channels of different compositions, and stabilize the selected conformations.

Along with this, we also fabricated monolayer bifunctional PEG via “grafting to” and polymer brushes via “grafting from” approaches to the surface of iron oxide NPs (IONPs). Ligands exhibiting specific bindings to GlyR and glutamate receptor were conjugated to these functionalized IONPs in quest of developing ligand-specific magnetic nanobeads. These proposed systems provide simple routes to purifying membrane-associated proteins when compared with the existing methods that are being used for protein purification. Moreover, a comparison between the monolayer and polymer brushes functionalized IONPs is expected to help in understanding the role of ligand density in the protein purification process. The magnetic nature of the IONPs is expected to render these materials reusable making the protein purification process cost-effective. Overall, these efforts will pave the way towards new nanotools for augmenting cryo-EM investigation and purification of membrane proteins.