Effects of Poly(ethylene glycol) (PEG) Chain Length of PEG-Lipid on the Permeability of Liposomal Bilayer Membranes

Abstract

The effects of poly(ethylene glycol) (PEG) chain length of PEG-lipid on the membrane characteristics of liposomes were investigated by differential scanning calorimetry (DSC), freeze-fracture electron microscopy (FFEM), fluorescence polarization measurement and permeability measurement using carboxyfluorescein (CF). PEG-liposomes were prepared from mixtures of dipalmitoyl phosphatidylcholine (DPPC) and distearoyl phosphatidylethanolamines with covalently attached PEG molecular weights of 1000, 2000, 3000 and 5000 (DSPE-PEG). DSC and FFEM results showed that the addition of DSPE-PEG to DPPC in the preparation of liposomes caused the lateral phase separation both in the gel and liquid-crystalline states. The fluidity in the hydrocarbon region of liposomal bilayer membranes was not significantly changed by the addition of DSPE-PEG, while that in the interfacial region was markedly increased. From these results, it was anticipated that the CF leakage from PEG-liposomes is accelerated compared with DPPC liposomes. However, CF leakage from liposomes containing DSPE-PEG with a 0.060 mol fraction was depressed compared with regular liposomes, and the leakage decreased with increasing PEG chain length. Furthermore, the CF leakage from liposomes containing DSPE-PEG with a 0.145 mol fraction was slightly increased compared with that of liposomes containing DSPE-PEG with a 0.060 mol fraction. It is suggested that the solute permeability from the PEG-liposomes was affected by not only properties of the liposomal bilayer membranes such as phase transition temperature, phase separation and membrane fluidity, but also the PEG chain of the liposomal surface.

Effects of Poly(ethylene glycol) (PEG) Concentration on the Permeability of PEG-Grafted Liposomes

“…This shows that the fluidity of the liposomal bilayer membrane is not the main factor in CF leakage from PEG-liposomes which consist of this lipid composition. In our previous paper, 7) we confirmed that the addition of DSPE-PEG to the DPPC liposomal bilayer membrane causes lateral phase separation in the gel and liquid-crystalline states, as seen by DSC measurements and freeze-fracture electron microscopy observations. Solute leakage from liposomes rapidly accelerates when the components which constitute the liposomes cause phase separation.…”

“…In general, when the solute leakage from the liposomes is discussed, differences in the liposomal bilayer thickness and particle sizes should be taken into consideration. 15) In our previous study, 7) we confirmed that the addition of DSPE-PEG to the DPPC liposomal bi- layer membrane caused lateral phase separation, and it seems that the thickness of liposomal bilayer membranes is not uniform. Furthermore, it is necessary to calculate the liposomal bilayer thickness for PEG-liposomes considering the thickness of the PEG chain layer on the liposomal surface.…”

“…Many studies have reported that the conjugation of amphipathic polyethylene glycol (PEG) with liposomes (PEG-liposomes) significantly increases the blood circulation liposome half-life compared with those without PEG. [2][3][4][5] In our previous studies, 6,7) we reported on the effect of PEG-lipid PEG chain length on the membrane characteristics of liposomes. The addition of PEG-lipid to liposomes caused lateral phase separation in the gel and liquid-crystalline states, and the fluidity in the interfacial region of liposomal bilayer membranes was markedly increased by the addition of PEG-lipids.…”

Calorimetry and Cryo-Transmission Electron Microscopic Studies of PEG2000-Grafted Liposomes

“…Furthermore, the reason why PEG-lipids localize to the edge portion of the disk micelle was considered. In our previous paper, 11) we confirmed that the addition of PEG-lipid to the DPPC liposomal bilayer membrane caused lateral phase separation, as seen by freeze-fracture electron microscopic observation. Bedu-Addo et al 21) suggested that the occurrence of the phase separation in the PEG-liposomes due to the property of the PEG chain of the PEG-lipid, that is, the phase separation of the PEG-liposome, is generated by the PEG chain-chain entanglement due to the van der Waals forces, and also inter-and intra-chain hydrogen bonds that act in the PEG chains.…”

Hybrid, Nanoscale Phospholipid/Block Copolymer Vesicles

“…The effect of PB-PEO on vesicle permeability is reminiscent of the effect that PEG-ylation exerts on the self-assembly and permeability of liposomes [30][31][32][33]. It was observed that addition of increasing concentrations of DSPE-PEG (DSPE: distearoylphosphatidylethanolamine; M W(PEG) > 2000) decreases the CF permeability of the PEG-ylated liposomes [32].…”