Head group, polar

The functional reaction center contains two quinone molecules. One of these, Qb (Figure 12.15), is loosely bound and can be lost during purification. The reason for the difference in the strength of binding between Qa and Qb is unknown, but as we will see later, it probably reflects a functional asymmetry in the molecule as a whole. Qa is positioned between the Fe atom and one of the pheophytin molecules (Figure 12.15). The polar-head group is outside the membrane, bound to a loop region, whereas the hydrophobic tail is... 

Amphipathic lipids spontaneously form a variety of structures when added to aqueous solution. All these structures form in ways that minimize contact between the hydrophobic lipid chains and the aqueous milieu. For example, when small amounts of a fatty acid are added to an aqueous solution, a mono-layer is formed at the air-water interface, with the polar head groups in contact with the water surface and the hydrophobic tails in contact with the air (Figure 9.2). Few lipid molecules are found as monomers in solution. 

Further addition of fatty acid eventually results in the formation of micelles. Micelles formed from an amphipathic lipid in water position the hydrophobic tails in the center of the lipid aggregation with the polar head groups facing outward. Amphipathic molecules that form micelles are characterized by a unique critical micelle concentration, or CMC. Below the CMC, individual lipid molecules predominate. Nearly all the lipid added above the CMC, however, spontaneously forms micelles. Micelles are the preferred form of aggregation in water for detergents and soaps. Some typical CMC values are listed in Figure 9.3. 

There are other ways in which the lateral organization (and asymmetry) of lipids in biological membranes can be altered. Eor example, cholesterol can intercalate between the phospholipid fatty acid chains, its polar hydroxyl group associated with the polar head groups. In this manner, patches of cholesterol and phospholipids can form in an otherwise homogeneous sea of pure phospholipid. This lateral asymmetry can in turn affect the function of membrane proteins and enzymes. The lateral distribution of lipids in a membrane can also be affected by proteins in the membrane. Certain integral membrane proteins prefer associations with specific lipids. Proteins may select unsaturated lipid chains over saturated chains or may prefer a specific head group over others. 

Particular phospholipids display characteristic transition temperatures (Tm). As shown in Table 9.1, increases with chain length, decreases with unsaturation, and depends on the nature of the polar head group. For pure phospholipid bilayers, the transition occurs over a narrow temperature range. The phase transition for dimyristoyl lecithin has a peak width of about 0.2°C. 

There are a few exceptions to this general rule. One of the few examples of an effect on polymer stereochemistry was provided by Dais et al.m who found that polymerization of 31 above the cmc initiated by y-irradiation at 25 °C yields polymer composed entirely of syndiolaclic dyads P(m) =0. When the double bond was distant from the polar head group in 32, the tacticity observed was similar to that observed in solution polymerization / ( )-0,18. Polymerization of 31 at higher temperatures (50 °C) initiated by AIBN also showed no sign of tacticity control. The stcrcospccific polymerization of 31 was attributed to organization of the methacrylate moiety on the surface of the micelle. 

Figure 41-3. Diagrammatic representation of a phospholipid or other membrane lipid. The polar head group is hydrophilic, and the hydrocarbon tails are hydrophobic or lipophilic. The fatty acids in the tails are saturated (S) or unsaturated (U) the former are usually attached to carbon 1 of glycerol and the latter to carbon 2. Note the kink in the tail of the unsaturated fatty acid (U), which is important in conferring increased membrane fluidity.

Figure 41-4. Diagrammatic cross-section of a micelle. The polar head groups are bathed in water, whereas the hydrophobic hydrocarbon tails are surrounded by other hydrocarbons and thereby protected from water. Micelles are relatively small (compared with lipid bilayers) spherical structures.

Figure 41-5. Diagram of a section of a bilayer membrane formed from phospholipid molecules. The unsaturated fatty acid tails are kinked and lead to more spacing between the polar head groups, hence to more room for movement. This in turn results in increased membrane fluidity. (Slightly modified and reproduced, with permission, from Stryer L Biochemistry, 2nd ed. Freeman, 1981.)...

The basic structure of all membranes is the lipid bilayer. This bilayer is formed by two sheets of phospholipids in which the hydrophilic polar head groups... 

Step 2 - A patch pipet is removed from the solution, the polar head groups of the monolayer lipids are adsorbed to the interface while the fatty acid hydrophobic tails are exposed to the air ... 

Transfer experiments of the Langmuir films onto solid substrates and the preparation of LB films were investigated for 43. The deposition of films of 43 occurred regularly on quartz sHdes or silicon wafers with a transfer ratio of 1 0.05. The diblock structure of dendrimer 43 also appeared crucial for efficient transfers of the Langmuir films in order to obtain well-ordered multilayered LB films. Effectively, the transfer of the Langmuir films of the dendrimer 42 with the small polar head group was found to be difficult with a transfer ratio of about... 

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