Structural study of Langmuir monolayers containing lipidated poly(ethylene glycol) and peptides

The structure of Langmuir monolayers containing either a lipidated poly(ethylene glycol) (PEG-lipid) or a lipidated peptide (peptide-amphiphile) or a binary mixture of both was studied using neutron reflectivity. The PEG portion of the PEG-lipid extends into the water, forming dense polymer "brushes". The PEG volume fraction profiles and the brush height were evaluated from the reflectivity curves for monolayers containing PEG-lipids with PEG molecular masses of 120, 750, 2000, and 5000 Da at various grafting densities. At relatively low surface densities, the segmental concentration profile for DSPE-PEG5000 (1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-[(ethylene glycol)_n], DSPE) and DSPE-PEG2000 was well-described by the parabolic profile predicted by the analytical self-consistent mean field theory. An increase in the surface density produced "flattening" of the profile, which became more pronounced as the chain length decreased. The dependence of the brush height on the surface density and the chain length was in close agreement with the power laws predicted by the self-consistent mean field and the scaling theories. Unlike the flexible PEG chains, the peptide-amphiphile that was used in this study has a stiff conformation. The headgroup is oriented perpendicular to the air-water interface, and this configuration is nearly unaffected by changes in the surface density. Incorporation of the peptide-amphiphile into a PEG-lipid monolayer results in perturbation of the brush structure, due to the enhanced configuration constraints. These studies enable us to gauge how the tethered peptide in the monolayer can be exposed or masked when mixed with tethered PEG chains.