How do oxidized lipids behave in Langmuir monolayers and Langmuir-Blodgett films ?

Oxidation of unsaturated lipids promotes important changes in the lipid packing or phase separation in cellular membranes or membrane models. Here, we study by Atomic Force Microscopy (AFM) the behavior of oxidized lipids in Langmuir-Blodgett (LB) films by using two strategies : either following the evolution of LB films of POPC (palmitoyloleoyl-phosphatidylcholine) naturally ageing in contact of atmospheric oxygen, or incorporating a known amount of a defined oxidized derivative of POPC (ALDO-PC) in POPC monolayers before their LB transfer. AFM images of naturally ageing POPC LB films show the appearance of small circular domains after 2 days of exposure to air. These domains (not observed if the samples are kept under vacuum) are characterized by a higher thickness (+0.8 nm) as compared to the intact POPC regions, likely due to a reversal of the oxidized chain which is more polar that intact hydrophobic chains. In the second case, surface pressure measurements show that ALDO-PC induces a slight expansion of the mixed monolayers, suggesting that they are rather homogenous. This hypothesis is confirmed by their smooth and homogenous AFM images. Finally, these results confirm that oxidation in POPC LB films occurs locally in areas presenting likely a looser packing or a defect.