On the Nature of Conformational Transition in Poly(ethylene glycol) Chains Grafted onto Phospholipid Monolayers

Abstract

This study was aimed at understanding the nature of conformational transition(s) that occur in poly(ethylene glycol) chains with molecular weight 5000 (PEG5000) grafted onto phospholipid monolayers. The study was performed with monolayers of PEG−phospholipid, DSPE−PEG5000, at the air/water interface and on solid substrates. Surface pressure and surface potential measurements together with ellipsometry and Brewster angle and atomic force microscopy were used to assess changes in the conformation and hydration of PEG5000 chains with increasing PEG grafting density. A comparative analysis of our experimental data suggests that neither the concept of the first-order pancake-to-brush conformational transition nor the model considering two subsequent transitions, namely, pancake-to-mushroom and mushroom-to-brush transitions, are adequate in describing conformational changes in the DSPE−PEG5000 monolayer at the air/water interface. At low grafting densities, PEG5000 chains did behave as grafted polymeric chains in a good solvent forming true 2D pancakes at the air/water interface. However, the conformation developed at high grafting densities was found to differ significantly from the PEG5000 brush in a good solvent. Its extension into the subphase was in far better agreement with the theoretical predictions for the height of PEG5000 brush in a theta solvent. This finding together with the decreasing absolute value of ψ 0 potential of DSPE−PEG5000 monolayer implies that conformational transition(s) in PEG5000 are accompanied by changes in interactions of PEG monomers with water molecules and by a substantial dehydration of PEG5000 chains. Conformational changes in grafted PEG5000 chains at the air/water interface are therefore interpreted as a transition from pancake to poorly hydrated brush conformation.

Quantifying the Coverage Density of Poly(ethylene glycol) Chains on the Surface of Gold Nanostructures

“…This phenomenon, however, was not observed for PEG of 20,000 in molecular weight probably due to its randomly-coiled conformation. 31,32 …”

Liposomes in tissue engineering and regenerative medicine

“…It is known that PEG chains grafted onto lipid membranes act like polymeric chains grafted onto solid surfaces, stretching into brushes with increasing surface density. Based on this concept, it is believed that the PEGylated surface provides a steric barrier that prevents the adsorption of proteins onto the liposome surface [31,42]. Numerous formulations of SSLs have been described for the systemic delivery of bioactive agents [25].…”

“…The liposome formulations composed of phospholipids, Chol and PEG are defined as 'sterically stabilized liposomes' (SSLs), also called 'stealth' liposomes, or PEGylated liposomes (figure 8b) [31,42]. It is known that PEG chains grafted onto lipid membranes act like polymeric chains grafted onto solid surfaces, stretching into brushes with increasing surface density.…”

Air–water interfacial behavior of linear-dendritic block copolymers containing PEG and azobenzene chromophores

“…During the compression, the ethylene oxide (EO) segments interact each other and water molecules are expelled from the film [52]. Under further compression, steric repulsion between stretched polymeric chains appears.…”