Lipid bilayer melting

Lipofullerenes such as 35-37 self-assemble within lipid bilayers into rod-like structures of nanoscopic dimensions [61, 62]. These anisotropic superstructures may be important for future membrane technology. Significantly, lipofullerenes 35 and 37 have very low melting points, 22 and 67 °C (DSC, heating scan), respectively, with 35 being the first liquid fuUerene derivative at room temperature. 

The chemical compositions and isomeric structures of the fatty acid chains of phospholipids is well known to have large effects on the physical properties of lipid bilayers, such as the temperatures of endothermic chain melting phase transitions. Lipid vesicles sensitized with lipid haptens can be agglutinated with specific antibodies directed against the haptens (see Fig. 1). 

The phase behavior of a synthetic lecithin, dipalmitoyllecithin, as analyzed by Chapman and co-workers (5), is diagrammed in Figure 3. The main features are the same as in the phase diagram of egg lecithin a mixture of numerous homologs. As a consequence of the variation in fatty acid chain length, the chain melting point is lowered which means that the critical temperature for formation of liquid crystalline phases is reduced. This temperature is about 42 °C for dipalmitoyllecithin, and, if the lamellar liquid crystal is cooled below this temperature, a so-called gel phase is formed. The hydrocarbon chains in the lipid bilayers of this phase are extended, and they can be regarded as crystalline. The gel phase and the transitions between ordered and disordered chains are considered separately. 

The newly synthesized phenothiazine derivative 2-trifluoromethyl-10-[4-(methanesulfonylamido)butyl]-phenothiazine (FPhMS, see Fig. 1 for chemical structure) was also extensively studied in the context of its interaction with lipid bilayers. DSC was used to study the influence of this compound on model membranes formed from DMPE [80], DPPC [81], DMPC, and DMPG [82]. In all the studied lipid systems FPhMS (16) lowered Tm, caused broadening of transition peaks, and induced the decrease of AH. Melting temperatures were found to be reduced by the phenothiazine derivative to a similar extent when different lipids possessing acyl chains of the same length were compared. 

Fig. 9. Force-distance relationships for lipid bilayers. Data for repulsion between dilau-roylphosphatidylcholine bilayers at 25°C. At high pressures (0) the bilayers bave been forced into a frozen-chain gel phase, a response that shows the structural importance of forces exerted by osmotic stress. The exponential part of the melted liquid-crystalline samples ( ) is best fit by an exponential decay constant of 2.6 A. From Parsegian et al. (1986).

An obvious hypothesis is that this unusual membrane lipid composition is related directly to membrane function in some way. Within the restricted area of lipid bilayers, lipid composition is known to be an important determinant of physical properties. There are several prominent examples. First, the temperature at which the hydrocarbon chains melt when assembled in bilayers (the gel-to-liquid-crystalline transition temperature, marks an abrupt change in many of the physical properties of such bilayer systems for example, water permeability through such bilayers increases by several orders of magnitude above the transition. Second, the presence of cholesterol within bilayers composed of amphipathic lipids has a profound effect on lipid motion, mechanical properties (such as resistance to shear), and permeability to water. 

The dependence of the chain-melting or main phase transition temperature in a lipid bilayer, Tm, on the nature of the lipid head group as well as the length of the acyl chains and the degree and type (cis- or trans-) of unsaturation of the acyl chains has been studied exhaustively for some diacyl phospholipid classes (21). Bulkier head groups result in lower values of... 

The lamellar gel-lamellar liquid-crystalline (L - L ) phase transition, frequently also referred to as (chain-)melting, order-disorder, solid-fluid, or main transition, is the major energetic event in the lipid bilayers and takes place with a large enthalpy change. It is associated with rotameric disordering of the hydrocarbon chains, increased headgroup hydration, and increased... 

The phase transition of lipid bilayers which comprise phospholipid mixtures, or phospholipids with different lengths of acyl chains, are of great importance due to their similarities with biomembranes or with lipid drug carriers such as liposomes, it is important to study the phase transitions and to detennine the exact crystalline mesophases of the mixed system. Thermal analysis studies indicated that the mi.xed lipids provide higher melting temperatures compared to those of pure lipids. This phenomenon occurrs w hen the lengths of the acyl chains are substantially different. 

Lee JCM, Santore M, Bates FS, Discher DE (2002) From membranes to melts, rouse to reptation diffusion in polymersome versus lipid bilayers. Macromolecules 35 323-326... 

Freezing and melting of lipid bilayers greatly alters the conductance mediated by a carrier, because it influences its mobility in the membrane. In contrast, the conductance induced by a channel former is not influenced158. Similarly, an increase of the membrane viscosity, by addition of cholesterol, reduces carrier-, but not channel- mediated conductance278,217. ... 

Reflect and. / ply In lipid bilayers, there is an order-disorder transition similar to the melting of a crystal. In a lipid bilayer in which most of the fatty acids are unsaturated, would you expect this transition to occur at a higher temperature, a lower temperature, or the same temperature as it would in a lipid bilayer in which most of the fatty acids are saturated Why ... 

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