Thiol-terminated polyethylene glycol (PEG) is commonly used to functionalize the surface of gold nanoparticles (AuNPs) in order to improve their in vivo stability and to avoid uptake by the reticular endothelial system. Although it has been reported that AuNPs functionalized with tethered PEG are stable in biological media, the influence of chain density remains unclear. This study investigates the influence of PEG capping density on the stability of washed and dried AuNPs in: water, phosphate-buffered saline solution (PBS), phosphate-buffered saline solution containing bovine serum albumin (PBS/BSA), and dichloromethane (DCM). PEG coating had a dramatic effect on stability enabling stable suspensions to be produced in all the media studied. A linear relationship was observed between capping density and stability in water and DCM with a somewhat lower stability observed in PBS and PBS/BSA. A maximum PEG loading level of ∼14 wt.% was achieved, equivalent to a PEG surface density of ∼1.13 chains/nm 2 .

Recent studies have shown that antimicrobial peptides (AMPs) can self-assemble into supramolecular structures, but this has been overlooked as causative of their antimicrobial activity. Also, the higher antimicrobial potency of D-enantiomers compared to L-enantiomers of AMPs cannot always be attributed to their different resistance to protease degradation. Here, we tested all L- and D-amino acid versions of GL13K, an AMP derived from a human protein, to study structural links between AMPs secondary structure, supramolecular self-assembly dynamics, and antimicrobial activity. pH dependence and the evolution of secondary structures were related to a self-assembling process with differences among these AMPs. The two GL13K enantiomers formed analogous self-assembled twisted nanoribbon structures, but D-GL13K initiated self-assembly faster and had notably higher antimicrobial potency than L-GL13K. A non-antimicrobial scrambled amino acid version of LGL13K assembled at a much higher pH to form distinctively different self-assembled structures than L-GL13K. Our results support a functional relationship between the AMPs self-assembly and their antimicrobial activity.