Distal phenylalanine modification for enhancing cellular delivery of fluorophores, proteins and quantum dots by cell penetrating peptides.

For cell penetrating peptides (CPPs) to fulfil their promise as effective delivery vectors we need a better understanding of their mechanisms of cell binding and uptake. This is especially the case when they are linked to different types of cargo. Here we describe new studies based on our previous findings suggesting that, for peptide-CPP chimeras, distal hydrophobic residues upstream of the CPP sequence can have profound effects on the way they interact with cells. We studied peptides bearing an N-terminal Glycine or Phenylalanine linked via a neutral and flexible bridging group, SGSGSGSG, to three well-studied CPPs: octaarginine, penetratin and TP10. Using a combination of flow cytometry, live-cell imaging and image analysis we examined the effects of this single amino acid change on binding and uptake of Alexa488-fluorophore, bovine serum albumin and quantum dot cargoes. The influence of the glycine-phenylalanine switch for fluorophore delivery was most dramatic in TP10, increasing cellular uptake by 4.4 and 9.9 fold in non-adherent and adherent cells, respectively. Only penetratin showed effective uptake of bovine serum albumin with the phenylalanine variant showing an increase of 1.6 fold over the glycine variant. The uptake of quantum dots was most efficiently demonstrated by octaarginine, with the glycine variant increasing uptake 4.8 fold and the phenylalanine variant increasing uptake 9.5 fold over quantum dots alone. Overall the data demonstrate that hydrophobicity distal to the CPP could be utilised to enhance their capacity to bind to the cell membrane and deliver a range of macromolecules to the insides of cells.