Lipids monomeric

Figure 3. IR spectra of Lipid 1, (a) monomeric lipid, (b) After UV irradiation, (c) hydrolyzed lipid.

This polymeric lipid can first be polymerized by free radical initiator in organic solutions before making the vesicles. The proton NMR spectrum of the polymerized lipid shows that vinyl protons of the cyclic acrylate between 85.00 ppm and 86.00 ppm disappeared from the spectrum, compared with that of monomeric lipid. Also in the IR spectrum (Figure 6) the absorption peak at 1670 cm"1 for the cyclic acrylate carbon carbon double bond disappeared as the result of polymerization. The carbonyl absorptions of the esters at 1740 cm 1 and the lactone at 1805 cm"1 still remain in the spectrum. 

The procedure for the formation of vesicles from this prepolymerized lipid was similar to that for the monomeric lipid. However, the concentration of the lipid in this system was lower than in the case of the monomeric lipid. Also the time of sonication for this polymerized lipid was longer than that for the monomeric lipid because of the decreased freedom of motion of the amphiphilic structure in the polymerized system. The electron microscope pictures (Figure 7) show the formation of tiny and very homogeneous vesicles. 

The DSC spectra confirm that the fluid phase of the polymerized vesicles remains and the phase transitions are retained with the introduction of the spacer group. As can been seen in Figure 8 of the DSC spectrum of the monomeric lipid, there is a peak around 28°C which corresponds to the phase transition of monomeric lipid. As the result of the presence of the spacer group, a similar phase transition can also be observed clearly in the spectrum of the polymerized lipid as shown in Figure 9, but the transition temperature is increased to 36°C by the presence of the polymer chains. 

Figure 8. DSC spectrum of the monomeric Lipid 2 in an aqueous system.

As mentioned in the previous paragraphs, the striking relationship between the chirality of the individual molecular components and the corresponding helical handedness of the DC8,9PC tubules, has led scientists to believe that the tubule formation is driven by the molecular chirality. However, according to more recent studies the process of tubule formation is more complex than previously thought. It initially involves the formation of enantiopure La -phase vesicles which are then transformed to -phase helical ribbons composed of a nearly racemic mixture of left and right handed helices.90 In the few minutes following the sphere-to-tubule transition, monomeric lipids from the saturated... 

Therefore a detailed study of the influence of dimerization of well-defined synthetic lipids seems to be the most direct approach to achieve an understanding of the role of these complex amphiphilic molecules in living cells. In addition, new lipidic materials based on dimerization might be of practical value. Based on these expectations, several reports describing the design, synthesis and characterization of a number of new dimeric lipids have appeared in recent literature. Most of these lipids are made up of two identical monomeric lipid moieties connected at the level of the head groups or very close to these by a spacer unit, which can be either hydrophilic or hydrophobic, or even be either rigid or flexible. 

The findings described above with unrelated systems reveal the widespread effects of lipid dimerization. Clearly the covalent connection of two monomeric lipids brings about an effect at the membrane level that is much more pronounced than one would expect from seemingly trivial structural modifications at the lipid level. These findings emphasize the continuing needs for newer design of synthetic lipid structures to expand our understanding of their behavior upon membrane formation. 

A Protein Example Phoshpholipase A2 Pancreatic phospholipase A2 is an enzyme of molecular weight 14,000, which catalyses the hydrolysis of 2-acyl ester bonds in a variety of naturally occurring phospholipids. The enzyme is secreted as a zymogen, which is activated by tryptic cleavage of the N-terminal heptapeptide. Both the enzyme and its precursor show catalytic activity towards monomeric lipids, but in contrast to the precursor the active enzyme shows a tremendous rate increase when it acts upon organized lipid structures such as micelles and bilayers (23). 

The perturbation of the vibrational structure of hydrocarbon chains by the diacetylenic unit persists even when the motional restrictions of the crystalline state are removed The spectrum of the monomeric lipid above its phase transition temperature is shown... 

Finally, note that the overspill effect is not specific to rod-coil aggregates. It is expected to play a role in the phase separation of membranes incorporating both monomeric lipids and polymerized lipids or lipids attached to a hydrophilic chain. 

---