Effect of PEG homopolymer and grafted amphiphilic PEG-palmityl on the thermotropic phase behavior of 1,2-dipalmitoyl-SN-glycero-3-phosphocholine bilayer

Phospholipids covalently attached to polyethylene glycol (PEG-PE) are routi nely used for the preparation of long-circulating liposomes. The common pre paration procedure for long-circulating liposomes involves use of organic s olvent. Although there is a plethora of studies describing the interaction of PEG-PE with bilayers, little is known about the effects of PEG homopolym ers and single chain amphiphilic PEG on liposome structure. In the present investigation the interaction of PEG homopolymer and amphiphilic PEG-palmit yl conjugate with large multilamellar liposomes composed of 1,2-dipalmitoyl -sn-glycero-phosphocholine was investigated utilizing differential scanning calorimetry. Vesicle and aggregate sizes were determined by dynamic light scattering. DSC thermograms revealed interaction of PEG homopolymer with DP PC when the two are premixed in organic solvent. The data suggest that PEG interacts with the phospholipid acyl chains deep in the bilayer. Several qu estions are raised regarding the suitability of the current procedure for p reparation of long-circulating liposomes which utilizes organic solvent. In corporation of only 2 mol% 5 kDa PEG-palmityl conjugate completely solubili zed DPPC liposomes. Packing geometry of the lipid anchor, irrespective of t he polymer molecular weight, is suggested to be the primary factor for succ essful grafting of hydrophilic polymers on liposomes. Pure PEG-palmityl for med self-assembled organized structures of potential use in the delivery of pearly soluble drugs.