Unsaturated lipids, effects

Whereas the main challenge for the first bilayer simulations has been to obtain stable bilayers with properties (e.g., densities) which compare well with experiments, more and more complex problems can be tackled nowadays. For example, lipid bilayers were set up and compared in different phases (the fluid, the gel, the ripple phase) [67,68,76,81]. The formation of large pores and the structure of water in these water channels have been studied [80,81], and the forces acting on lipids which are pulled out of a membrane have been measured [82]. The bilayer systems themselves are also becoming more complex. Bilayers made of complicated amphiphiles such as unsaturated lipids have been considered [83,84]. The effect of adding cholesterol has been investigated [85,86]. An increasing number of studies are concerned with the important complex of hpid/protein interactions [87-89] and, in particular, with the structure of ion channels [90-92]. 

The second mechanism is realized when organic or inorganic compounds are reduced by endogenous reductants (for example, by NADH or NADPH and the other components of mitochondrial or microsomal respiratory chains). The typical compounds are anthracycline antibiotics and carbon tetrachloride. CC14 is easily reduced by microsomes to the free radical CC13 , which is able to abstract a hydrogen atom from unsaturated lipids and initiate lipid peroxidation. Because of this, the CCl4-initiated lipid peroxidation is a reliable and frequently applied model system for the study of in vitro iron-independent lipid peroxidation and the effects of antioxidants (see for example Ref. [54]). 

From the modelling results for bilayers composed of unsaturated lipids one can begin to speculate about the various roles unsaturated lipids play in biomembranes. One very well-known effect is that unsaturated bonds suppress the gel-to-liquid phase transition temperature. Unsaturated lipids also modulate the lateral mobility of molecules in the membrane matrix. The results discussed above suggest that in biomembranes the average interpenetration depth of lipid tails into opposite monolayers can be tuned by using unsaturated lipids. Rabinovich and co-workers have shown that the end-to-end distance of multiple unsaturated acyl chains was significantly less sensitive to the temperature than that of saturated acyls. They suggested from this that unsaturated... 

We have also undertaken MD simulations to examine the effect of cholesterol content on the thermodynamics of DPPC desorption [54], We found that DPPC had a lower affinity for bilayers with high cholesterol content (Figure 3B). This suggests that while cholesterol prefers to interact with saturated lipid tails, the saturated tails might not prefer to interact with cholesterol. It would be interesting to repeat this study on unsaturated lipid tails. 

The effect of double bonds on both transfection and phase behavior is well illustrated by the pair C18 1/C10-EPC and C18 0/C10-EPC. These two lipids differ by one double bond only. However, the unsaturated lipid is over ten times more efficient as transfection agent than the saturated one [112]. The superior efficiency of C18 1/C10-EPC relative to C18 0/C10-EPC is also implied by the phases that evolve in membrane lipid formulations upon mixing with these two cationic lipids. A biomembrane-mimicking lipid formulation DOPC/DOPE/DOPS/Chol 45 20 20 15 remained lamellar in mixtures with C18 0/C10-EPC in contrast, the more efficient C18 1/C10-EPC induced a lamellar-nonlamellar phase conversion in this mixture, which was taking place at physiological temperature (Fig. 27). 

Ivanov (1929) found a close inverse relationship between the value of lipid unsaturation of plants (determined by iodine value) and the latitude of their habitat. It was found that, with lower environmental temperatures, the iodine numbers of the lipids were greater (i.e. there was more unsaturation). The effect of increasing unsaturation is to lower the temperature at which the lipids change from liquids to a liquid-crystalline state, and so to lower the temperature at which the cells remain flexible. 

Therefore, volatile phenols play a minor role in the aroma of most wines, and when their influence is significant in certain wines, they have mostly a negative effect, which can definitely depreciate their aroma in limit cases (phenolic off-flavors). Thus, the corresponding precursors in grape, phenolic acids, as well as the above-mentioned unsaturated lipids, are hardly taken into account to capture an essential characteristic of the varietal aroma, but to avoid their transformation into off-flavors. 

Hsu ER, Gebert MS, Becker NT, Gaermer AL. Effects of plasticizers and titanium dioxide on the properties of poly(vinyl alcohol) coatings. Pharm Dev Technol 2001 6(2) 277-284. Kakinoki K, Yamane K, Teraoka R, et al. Effect of relative humidity on the photocatalytic activity of titanium dioxide and photostability of famotidine. J Pharm Sci 2004 93(3) 582-589. Sayre RM, Dowdy JC. Titanium dioxide and zinc oxide induce photooxidation of unsaturated lipids. Cosmet Toilet 2000 115 75-80, 82. 

The radical formed may add directly on the unsaturated hpid bonds or initiate an unsaturated lipid peroxidation or undergo another one-electron reduction. The last reaction yields a carbene that can form a complex with the iron of the reduced form of CYP. Reduction of polyhalogenated compounds gives rise to several reactive intermediates, such as radicals, carbenes and peroxides, whose participation in the toxic effect varies greatly. 

Radical-Catalyzed Cis-Trans Isomerization of Unsaturated Lipids and its Effect on Biological Membranes... 

Finally, one also has to consider, that hydrogenation of live cells creates un unnatural composition and physical state (increased rigidity) of the membranes. In case the cells survive this treatment, they mobilize all their reserves to restore the original conditions, optimal for life at the given temperature, pressure, culture medium, etc. Detection of de novo synthetized unsaturated lipids during hydrogenation of protoplasts from tobacco leaf (Nicotiana plumbaginifolid) proved that such compensation of unfavourable outside effects can follow the stress very fast (immediately) and can be very effective [214],... 

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