UV-induced reaction kinetics in dilinoleoylphosphatidylcholine monolayers with incorporated photosensitizers

Cholesterol-nitrone conjugates as protective agents against lipid oxidation: A model membrane study

Free radical scavengers such as α-phenyl-N-tert-butylnitrone (PBN) have been widely used as protective agents in several biological models. We recently designed two PBN derivatives by adding a cholesterol moiety to the parent nitrone to increase its lipophilicity. In addition to the cholesterol, a sugar group was also grafted to enhance the hydrophilic properties at the same time. In the present work we report on the synthesis of a third derivative bearing only a cholesterol moiety and the physical chemical and antioxidant characterization of these three derivatives. We demonstrated they were able to form stable monolayers at the air/water interface and with the two derivatives bearing a sugar group, repulsive interactions with 1,2-dilinoleoyl-sn-glycero-3-phosphocholine (DLPC) were observed. We next investigated the interaction with DLPC on a liposome model. Fluorescence spectroscopy experiments showed the addition of a cholesterol moiety causes an ordering effect whereas the presence of the sugar group led to a disordering effect. The protective effect against lipid oxidation was then investigated using dynamic light scattering and the formation of conjugated dienes was quantified spectrophotometrically. Two oxidizing systems were tested, i.e. the AAPH-thermolysis which generates peroxyl radicals and the Fenton reagent which is responsible of the formation of hydroxyl radicals. Due to their membrane localization, the three cholesteryl-PBN derivatives are able to prevent lipid oxidation with the two types of radical inducers but with a different mode of action.

Study of PEGylated lipid layers as a model for PEGylated liposome surfaces: molecular dynamics simulation and Langmuir monolayer studies

We have combined Langmuir monolayer film experiments and all-atom molecular dynamics (MD) simulation of a bilayer to study the surface structure of a PEGylated liposome and its interaction with the ionic environment present under physiological conditions. Lipids that form both gel and liquid-crystalline membranes have been used in our study. By varying the salt concentration in the Langmuir film experiment and including salt at the physiological level in the simulation, we have studied the effect of salt ions present in the blood plasma on the structure of the poly(ethylene glycol) (PEG) layer. We have also studied the interaction between the PEG layer and the lipid bilayer in both the liquid-crystalline and gel states. The MD simulation shows two clear results: (a) The Na(+) ions form close interactions with the PEG oxygens, with the PEG chains forming loops around them and (b) PEG penetrates the lipid core of the membrane for the case of a liquid-crystalline membrane but is excluded from the tighter structure of the gel membrane. The Langmuir monolayer results indicate that the salt concentration affects the PEGylated lipid system, and these results can be interpreted in a fashion that is in agreement with the results of our MD simulation. We conclude that the currently accepted picture of the PEG surface layer acting as a generic neutral hydrophilic polymer entirely outside the membrane, with its effect explained through steric interactions, is not sufficient. The phenomena we have observed may affect both the interaction between the liposome and bloodstream proteins and the liquid-crystalline-gel transition and is thus relevant to nanotechnological drug delivery device design.