Phase Transition in Polymeric Liposomes

So far, it has been shown that the stability of a model membrane can be tremendously increased by polymerization. This increased stability however, is associated with the presence of a polymer chain in the membrane itself or on its surface, bringing about increased viscosity and thus reduced flexibility. How does the reduced membrane mobility affect one of the most vital properties of biomembranes, the phase transition  

In contrast, the phase transition of polymeric liposomes is retained if the polymer chain is more flexible or located on the surface of the vesicles instead within the hydrophobic core. Polymerized vesicles of methacrylamide (29) show a phase transition temperature which is slightly lower than the one for the corresponding monomeric vesicles (Fig. 26). This can be explained by a disordering influence of the polymer chain on the head group packing 15). 

Deuteron -NMR measurements on the same system (labelled in the methyl group) also show that the mobility of the hydrophilic ammonium head group is reduced by 

Methacryloylic lipid (5) is polymerizable in the hydrophobic part of the molecule. The phase transition temperature of the polymeric vesicle is again lowered compared to the non-polymerized vesicle (Fig. 27). The difference between the phase transition temperatures of monomer and polymer is somewhat larger than in the case of acrylamide (29). This might indicate that a saturated polymer chain in the hydrophobic core of a membrane decreases membrane order to a higher extent than a polymer chain on the membrane surface 15). 

Polycondensation reactions in oriented monolayers and bilayers proceed without catalysis, and simply occur due to the high packing density of the reactive groups and their orientation in these layers. Bulk condensation of the a-amino acid esters at higher temperatures does not lead to polypeptides but to 2,5-diketopiperazines. No diketopiperazines are found in polycondensed monolayers or liposomes. Polycondensation in monolayers and liposomes leading to oriented polyamides represents a new route for stabilizing model membranes under mild conditions. In addition, polypeptide vesicles may be cleavable by enzymes in the blood vessels. In this case, they would represent the first example of stable but biodegradable polymeric liposomes. 

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