Hydration of Polyethylene Glycol-Grafted Liposomes

Abstract

This study aimed to characterize the effect of polyethylene glycol of 2000 molecular weight (PEG2000) attached to a dialkylphosphatidic acid (dihexadecylphosphatidyl (DHP)-PEG2000) on the hydration and thermodynamic stability of lipid assemblies. Differential scanning calorimetry, densitometry, and ultrasound velocity and absorption measurements were used for thermodynamic and hydrational characterization. Using a differential scanning calorimetry technique we showed that each molecule of PEG2000 binds 136 +/- 4 molecules of water. For PEG2000 covalently attached to the lipid molecules organized in micelles, the water binding increases to 210 +/- 6 water molecules. This demonstrates that the two different structural configurations of the PEG2000, a random coil in the case of the free PEG and a brush in the case of DHP-PEG2000 micelles, differ in their hydration level. Ultrasound absorption changes in liposomes reflect mainly the heterophase fluctuations and packing defects in the lipid bilayer. The PEG-induced excess ultrasound absorption of the lipid bilayer at 7.7 MHz for PEG-lipid concentrations over 5 mol % indicates the increase in the relaxation time of the headgroup rotation due to PEG-PEG interactions. The adiabatic compressibility (calculated from ultrasound velocity and density) of the lipid bilayer of the liposome increases monotonically with PEG-lipid concentration up to approximately 7 mol %, reflecting release of water from the lipid headgroup region. Elimination of this water, induced by grafted PEG, leads to a decrease in bilayer defects and enhanced lateral packing of the phospholipid acyl chains. We assume that the dehydration of the lipid headgroup region in conjunction with the increase of the hydration of the outer layer by grafting PEG in brush configuration are responsible for increasing thermodynamic stability of the liposomes at 5-7 mol % of PEG-lipid. At higher PEG-lipid concentrations, compressibility and partial volume of the lipid phase of the samples decrease. This reflects the increase in hydration of the lipid headgroup region (up to five additional water molecules per lipid molecule for 12 mol % PEG-lipid) and the weakening of the bilayer packing due to the lateral repulsion of PEG chains.

Modulating Mucin Hydration and Lubrication by Deglycosylation and Polyethylene Glycol Binding

“…The protein solutions were pumped to fi ll the reservoir above the crystal surface, and then the fl ow was stopped to allow the coating to form. The crystal vibration was followed at its fundamental frequency (5 MHz) and six overtones (15,25,35,45,55, and 65 MHz). Changes in the resonance frequencies and in dissipation of the vibration once the excitation was stopped were followed at the seven frequencies.…”

“…We here developed an alternative strategy by substituting the glycans with PEG polymer chains ( Figure 3 A). PEG was chosen for its known biocompatibility and the well-characterized hydration [ 34,35 ] and lubrication [36][37][38][39] capacity. Deglycosylation can decrease the molecular weight of mucins by up to 80%, [ 40,41 ] thus relatively large 40 kDa PEG chains were chosen to compensate for that loss.…”

Enhancement of Air Filter with TiO2 Photocatalysis for Mycobacterium Tuberculosis Removal

“…Water molecules orient to surround the polymer chain to create a hydration shell with a maximum of one water molecule per ethylene glycol unit (-CH 2 -CH 2 -O-), making PEG's solubility unlimited (Allen et al, 2002). Tirosh et al (1998) used a differential scanning calorimeter to measure water in each molecule of PEG-2000 and found 136 ± 4 molecules of water binding with it. Thus, it is possible that there is less space for TiO 2 bonding which leads to the greatest loss of coating materials, resulting in lower photocatalysis than with other binders.…”

Aggregation of PEGylated liposomes driven by hydrophobic forces

“…The observation that high AS concentration results in rapid fusion of the vesicles may be explained by excessive structural alterations of PEG chains due to their hydrophobic modification. Modification of structure and hydration of PEG chains may lead to steric imbalance of the vesicles as it was shown earlier [15,29].…”

“…We identify them as bilayers, although they are somewhat thinner than a lipid bilayer covered with a PEG polymer brush on both sides (approx. 12 nm) calculated with a different method [14,15]. The bilayer patches probably emerge because as osmolality increases, vesicles exhibit a greater propensity to burst on the substrate [16,17].…”