Range and magnitude of the steric pressure between bilayers containing phospholipids with covalently attached poly(ethylene glycol)

Abstract

The interactive properties of liposomes containing phospholipids with covalently attached poly(ethylene glycol) (PEG-lipids) are of interest because such liposomes are being developed as drug delivery vehicles and also are ideal model systems for measuring the properties of surface-grafted polymers. For bilayers containing PEG-lipids with PEG molecular weights of 350, 750, 2000, and 5000, pressure-distance relations have been measured by X-ray diffraction analysis of liposomes subjected to known applied osmotic pressures. The distance between apposing bilayers decreased monotonically with increasing applied pressure for each concentration of a given PEG-lipid. Although for bilayers containing PEG-350 and PEG-750 the contribution of electrostatic repulsion to interbilayer interactions was significant, for bilayers containing PEG-2000 and PEG-5000 the major repulsive pressure between bilayers was a steric pressure due to the attached PEG. The range and magnitude of this steric pressure increased both with increasing PEG-lipid concentration and PEG size, and the extension length of the PEG from the bilayer surface at maximum PEG-lipid concentration depended strongly on the size of the PEG, being less than 35 A for PEG-750, and about 65 A for PEG-2000 and 115 A for PEG-5000. The measured pressure-distance relations have been modeled in terms of current theories (deGennes, 1987; Milner et al., 1988b) for the steric pressure produced by surface-grafted polymers, as modified by us to take into account the effects of polymer polydispersity and the possibility that, at low grafting densities, polymers from apposing bilayers surfaces can interpenetrate or interdigitate. No one theoretical scheme is sufficient to account for all the experimental results. However, for a given pressure regime, PEG-lipid size, and PEG-lipid surface density, the appropriately modified theoretical treatment gives a reasonable fit to the pressure-distance data.

In vitro and in vivo effects of polyethylene glycol (PEG)-modified lipid in DOTAP/cholesterol-mediated gene transfection

“…The model of PEG-surface structure being mushroom-like at concentrations below 4%, whereas brush-like structures exist at higher PEG-lipid mol-percentages (eg, 10%). 23,24 Interestingly, we found that upon mixing with DNA the size of lipoplexes increase and lipoplexes containing 2% or 4% PEG-lipid form micrometer scale aggregates, whereas the A-0 and D-10 lipoplex have sizes 784 ± 49 and 327 ± 5.3 nm, respectively. This finding is in accordance with PDI values that are close to 1 for A-0, B-2, and C-4 lipoplexes indicating high turbidity, whereas D-10 lipoplex has a medium PDI of 0.472.…”

“…[18][19][20][21][22] Furthermore the biophysical relationship between liposome surface concentration and structure of PEG has been characterized in detail. 23,24 Recent reports about the delivery of plasmid and siRNA in vivo using cationic lipoplexes that also contain a PEG-lipid for improved efficiency 19,25,26 prompted us to assess the usability of different degrees of PEGylation of DOTAP/chol/plasmid DNA lipoplexes in relation to improved systemic circulation and stability while maintaining the favorable transfection properties.…”

PEG conjugation moderately protects adeno-associated viral vectors against antibody neutralization

“…The size of a polymer near a surface may be approximated as the Flory radius (Szleifer, 1996). More recent calculations based on de Gennes theory and molecular simulation have found that for PEG chains grafted onto lipid bilayers, the cross-sectional surface area ''coverage'' is actually smaller (Kenworthy et al, 1995;Kuhl et al, 1994;Rex et al, 1998). Specifically, Rex et al found that the diameter of the cross-sectional coverage for PEG 2000 chains to be only approximately 18 Å , a more conservative number that we chose for our analysis.…”

SNARE-mediated Fusion of Single Proteoliposomes with Tethered Supported Bilayers in a Microfluidic Flow Cell Monitored by Polarized TIRF Microscopy

“…For the vSUVs to interact with the t-SBL, the v-and t-SNARE N-termini need to protrude above the PEG brushes, which seems to be the case under the conditions of the assay. Brush height can be adapted to study proteins other than SNAREs by varying the density of PEGylated lipids and the PEG chain length 37,38 . Another benefit of the PEG brushes covering the proximal surfaces of the fusing bilayers is that they mimic the crowded environment of biological membranes which are packed with 30,000-40,000 integral membrane proteins per square micron 39 .…”