PEGylation of phospholipids improves their intermembrane exchange rate

The polar headgroup of dimyristoylphosphatidylethanolamine (DMPE) was modified by attaching a hydrophilic polymer-amino acid complex and the effects of this modification on the non-protein-mediated transfer features of the lipid determined. The first step in the organic synthesis protocol was the conversion of α,ω-dicarboxy poly(ethylene glycol) into its anhydride form, followed by attachment of the molecule to the amino group of DMPE. In the second step, the remaining free –COOH group on the polymer terminus was activated consecutively with 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide and sulfo-N-hydroxysuccinimide, and then coupled to tryptophan. The purified conjugate was mixed with dipentadecanoylphosphatidylglycerol (1/9 molar ratio) and sonicated to produce small unilamellar vesicles. These vesicles (donors) were then mixed with dipentadecanoylphosphatidylglycerol magnetoliposomes (acceptors) and the transfer kinetics of the modified lipid followed, using high-gradient magnetophoresis as the fractionation technique. The transfer behavior of non-modified DMPE was also examined. The first-order kinetic plots, corrected for back exchange lipid movement, showed that hydrophilization of DMPE dramatically improved its transfer capacity. These findings are explained by considering the thermodynamical consequences of the exchange process. The possibility of using biomolecule-derivatized phospholipids to trigger physiological effects in biological cells is briefly discussed.