Direct evidence for a lipid alkyl chain ordering transition in poly(ethylene oxide) lipopolymer monolayers at the air-water interface obtained from infrared reflection absorption spectroscopy

Lipopolymers are lipids with a polymer chain covalently attached to the lipid. At the air-water interface such lipopolymers may undergo up to two monolayer phase transitions. One is correlated with desorption of the polymer group from the surface (the so-called pancake-mushroom transition) and a second is "native" to lipopolymers in the sense that it can only be observed when lipid and polymer are present in the same molecule. In this study we present direct evidence from infrared spectroscopy showing that the "native" transition, although requiring the presence of the polymeric headgroup, is a transition solely within the lipid alkyl chains. This transition correlates to a strong reduction of the number of gauche isomers within the lipid alkyl chains. We used two poly(ethylene oxide) lipopolymers which differed only in the lipid alkyl chains. These were either fully deuterated or fully protonated. The polymer chains were protonated in both cases. Within the protonated lipopolymer, a strong ordering of the CH2 groups was observed during the "native" transition. Within the deuterated lipopolymer, the exclusive deuteration of the alkyl chains allowed us to distinguish directly between lipid and polymer. We observed that the polymer CH2 spectra remained unchanged when the lipopolymer monolayer was compressed. As it was possible to build the subtraction spectra between the two lipopolymers, we could unambiguously identify the lipid component of the protonated lipopolymer in the spectra. It was found that the strong ordering of the methylene groups occurs solely within the lipid alkyl chains.