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 fluo rescent N-epsilon-dansyl lysine moiety, which is attached to a hydrophilic poly(ethylene glycol) (PEG) spacer that is linked to a cationic headgroup m ade of lysine residues. Introduction of the dansyl moiety allows rapid and accurate quantification of CPLs within lipid bilayers using fluorescence te chniques. The synthetic scheme is straightforward, using repeated amino-car boxyl 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 w ith one, two, four, and eight distal positive charges, respectively. The st ructures of the CPLs were confirmed by H-1 NMR spectroscopy and chemical an alysis. CPLs provide a means of introducing positive charge to a bilayer th at is localized some distance from the membrane surface, and are of particu lar interest for nonviral gene delivery applications. The usefulness of CPL s is demonstrated by the enhanced in vitro cellular binding and uptake of l iposomes containing CPL4.