Phospholipid vesicles cross-linking

Various azido alkyl and aryl fatty acids and lipids Membrane components Biosynthetic incorporation into phospholipids. Some cross-linking on irradiation of vesicles 36, 69... 

The so-called coated vesicles are an example of an enveloped membrane3). A phospholipid vesicle within a cell is coated by a polypeptide and resembles a foot-ball in a net. A comparable feature of membrane coating is found in cell walls of bacteria4 . These cell walls consist of polysaccharides which are cross-linked by oligopeptides. It is remarkable that this extreme stabilization by an exogenous support is found in bacteria. As parasites in foreign tissues, they have to be especially resistant. 

Tew and Amt [92-94] have prepared amphiphilic meta-PPEs 58 and investigated their behavior at the air-water interface where they prefer an extended conformation. These PPEs are active in the lysis of phospholipid vesicles and might find application as bactericidal substances. While the derivative 58 with R = H forms clear solutions in water, the alkoxy-substituted congeners aggregate and precipitate out upon addition of water to their solution in DMSO. The structure of 58 in the solid state is assumed to be helical rather than extended. If acrylic ester substituents are placed on 59, it is possible to capture the helical intermediates by a photochemical 2-1-2 cycloaddition (Figure 6.2) [95,96]. The presence of a fixated helical structure was evident, because addition of chloroform to the cross-linked form did not... 

In an attempt to optimize the lower limit concentration of AAPD, vesicles formed from phospholipid 1, alone, were tested for the extent of cross-linking by increasing the molar ratio of (phospholipids initiator) to 100 1. Vesicles produced from phospholipid 1 showed that, after 5 min, only 79% of the phospholipid in the vesicles was cross-linked, as determined H NMR analysis. An additional 10 min of UV exposure increased the cross-linking extent to 92%. Thereafter, it took an additional 10 min of UV irradiation for complete disappearance of the proton signals at 6.6, 5.8, and 5.3 ppm, indicating complete cross-linking of the individual phospholipids in the vesicles. Therefore, the 25 1 molar ratio of (phospholipids initiator) was taken as the lower concentration limit of AAPD in order to obtain 100% cross-linking in 5 min at ambient temperature for phospholipids 1-3. 

Differential scanning calorimetry was performed to measure the chain melting temperature for non-cross-linked phospholipids (NCP) 1-3. Non-cross-linked vesicle dispersions (NCVD) and cross-linked vesicle dispersions (CVD) of 1 and 2 were also measured along with the chain melting transition of CVD-1 with a hydrophobic dye entrapped. Parent phospholipids DPPE and DLPE were used as references for NCP 1 and 2. Parent phospholipid EGGPE was used as reference for phospholipid 3. NCPs were weighed directly into the DSC pans. Vesicle dispersions were used for the non-cross-linked vesicle samples and freeze-dried powder was used for the cross-linked vesicle dispersion samples. 

NCP is an abbreviation for non-cross-linked phospholipids, NCVD is an abbreviation for vesicle dispersion non-cross-linked, CVD is an abbreviation for cross-linked vesicle dispersion. 

The first approach in using vesicle membranes in nanoparticle synthesis was to polymerize vesicles formed from 9 1 and 1 1 mixtures of 1 and 2 in the presence of Pd(NH3)4Cl2. Palladium ions bound to negatively charged phospholipids in bilayer membranes had been previously demonstrated to serve as catalytic sites for electroless metallization (47). Light scattering revealed that bimodal populations of vesicles were formed both from the 9 1 (31 + 15 nm and 114 + 40 nm) and 1 1 (38 + 13 nm and 109 + 41 nm) mixtures. Exposure of the vesicles to UV radiation (254 nm) resulted in the reaction of 33 % + 6% of the lipid monomer when 10 % of 2 was present and 50% + 4% for vesicles containing 50% 2. The differences in the amount of reaction of monomer are probably due to both ion and pH effects. Non-cross linked polymerized vesicle membranes consist of many individual... 

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