Water between DPPC bilayers

Figure 8. Data points that represent the orientational polarization of water between DPPC bilayers. The dashed line is a jit to a functional form given by...

Chapter 7 is an interesting review by Kodama and Aoki (Japan) on the behavior of water in phospholipid bilayer systems. The authors distinguish between nonfreezable interlamellar water and freezable intralamellar and bulk water, and estimate the number of molecules of water in each category. They also examine the relationship between lipid phase transitions and ice-melting behavior in lipid-water systems. The behavior of water is also discussed in the gel phase of systems such as DPPC, DMPE, and DPPG. 

Biological membranes are so complicated that a lipid membrane is often used as a model sample for their studies. The author and coworkers investigated the interface between a DPPC bilayer and PBS buffer solution. At the lipid-water interface, water molecules interact with lipid head groups to show nonuniform distribution. Since an AFM tip interacts with water molecules as well as the lipid head groups, the force distribution detected by AFM should be influenced by the water distribution as well as by the surface corrugation. 

Figure 3 A schematic overview of the structure of a DPPC bilayer in the context of the four-region model. Dashed lines are used for water molecules and bold lines for choline and phosphate groups. Vertical lines indicate the boundaries between the different regions. Crosses result from bonds cut by the boundary planes. Reproduced with permission from Ref. 38...

The stmcture of the water-lipid interface plays an important role in the hydration force, a repulsive force that acts between two bilayers at distances of 5-15 A. A similar repulsion was found between biological macromolecules in solution. Three different mechanisms have been proposed for the origin of this force in bilayers the ordering of water (a solvent effect), a combination of lipid and solvent, or purely a lipid effect caused by direct interactions of protruding lipids. Berkowitz and coworkers have studied the hydration force in a series of simulations of gel and liquid-crystalline phases of DPPC... 

DSC studies of lipid-surfactant mixtures in the regime of low surfactant concentrations provide some insight into the partition coefficient of the surfactant molecule between water and the bilayer. We have systematically studied the be-havior of DMPC/octylglucoside (DMPC/OG) mixtures by ITC and DSC. The results of the ITC experiments will be described below. Figure 31 shows as an example some DSC curves of DPPC/OG mixtures as a function of total OG concentration. First, a decrease of the transition temperature due to preferential OG partitioning into the L,t-phase bilayers is observed. 

Several additional studies were carried out to obtain information about the precise behavior of the various components in the model system. The interplay between the manganese porphyrin and the rhodium cofactor was found to be crucial for an efficient catalytic performance of the whole assembly and, hence, their properties were studied in detail at different pH values in vesicle bilayers composed of various types of amphiphiles, viz. cationic (DODAC), anionic (DHP), and zwitterionic (DPPC) [30]. At pH values where the reduced rhodium species is expected to be present as Rh only, the rate of the reduction of 13 by formate increased in the series DPPC < DHP < DODAC, which is in line with an expected higher concentration of formate ions at the surface of the cationic vesicles. The reduction rates of 12 incorporated in the vesicle bilayers catalyzed by 13-formate increased in the same order, because formation of the Rh-formate complex is the rate-determining step in this reduction. When the rates of epoxidation of styrene were studied at pH 7, however, the relative rates were found to be reversed DODAC DPPC < DHP. Apparently, for epoxidation to occur, an efficient supply of protons to the vesicle surface is essential, probably for the step in which the Mn -02 complex breaks down into the active epoxidizing Mn =0 species and water. Using a-pinene as the substrate in the DHP-based system, a turnover number of 360 was observed, which is comparable to the turnover numbers observed for cytochrome P450 itself. 

Fig. 6.13 Distribution of the angle between P-N vector and bilayer normal in pure DPPC membrane (solid line) and in membranes containing cholesterol 11 mol% (dotted line), 50 mol% structure A (dash dot line), and structure B (dashed line). When cosine is positive, the P—N vector points into the water layer.

Before looking at the reconstituted membranes in more detail, we shall first discuss a simpler question concerning the distribution of ions in the aqueous phase of bilayer structures. Is the ion distribution in the water phase of typically 15 A width between the lipid surfaces homogenous, or is there a preferential binding to the polar head groups of the lipids These studies have been started in collaboration with G. Biildt and some preliminary results from dipalmitoyl phosphatidyl cholin (DPPC) membranes will be reported here. ... 

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