Fluorescent-labeled poly(ethylene glycol) lipid conjugates with distal cationic headgroups

The synthesis of a new class of fluorescent cationic poly(ethylene glycol) lipid conjugates (CPLs) is described. These lipids consist of a hydrophobic distearoyl-phosphatidylethanolamine (DSPE) anchor coupled to a highly fluorescent N(epsilon)-dansyl lysine moiety, which is attached to a hydrophilic poly(ethylene glycol) (PEG) spacer that is linked to a cationic headgroup made of lysine residues. Introduction of the dansyl moiety allows rapid and accurate quantification of CPLs within lipid bilayers using fluorescence techniques. The synthetic scheme is straightforward, using repeated amino-carboxyl coupling reaction steps, with purification by precipitation. A series of dansylated CPLs was synthesized with zero, one, three, and seven lysine residues located at the distal end of the PEG chain, giving rise to CPLs with one, two, four, and eight distal positive charges, respectively. The structures of the CPLs were confirmed by (1)H NMR spectroscopy and chemical analysis. CPLs provide a means of introducing positive charge to a bilayer that is localized some distance from the membrane surface, and are of particular interest for nonviral gene delivery applications. The usefulness of CPLs is demonstrated by the enhanced in vitro cellular binding and uptake of liposomes containing CPL(4).