Structural characterization of the poly(ethylene glycol) layer of sterically stabilized liposomes by means of FTIR spectroscopy

Attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) combined with curve fitting procedures was used to characterize the poly(ethylene glycol) (PEG) layer of a pharmacologically relevant liposomal drug delivery system. The hydration of the polymer layer was described based on the ratio of the trans and gauche configurations of the C–O–C bond of the PEG molecules, which show a minimum at ∼6 mol% PEG-lipid molar ratio. The latter corresponds to highest bilayer stability, and coincides with the transition from the mushroom to the brush configuration of the PEG structure. This preliminary study showed that the ATR-FTIR technique is capable of qualitatively describing the hydration of the PEG-layer of stealth liposomes, which is in connection with the structure of the polymer on the liposomal surface.