Fluidity, phospholipid monolayers

It has been mentioned previously that sterols were readily incorporated into lecithin or mixed phospholipid monolayers [194—196]. The area of sterol-containing films was smaller than that calculated for the separate components, indicating some kind of interaction between the two molecules [221,222]. Sterols have also been shown to reduce the phase transitions of pure lecithin dispersions [214]. It has been suggested that cholesterol modifies the fluidity of the hydrocarbon chains of the phospholipid molecules by disrupting the crystalline chain lattice of the gel phase and by inhibiting the flexing of the chains in the dispersed liquid-crystalline phase [221—226]. 

However, hybrid bilayers are not suitable for protein incorporation because water is needed at the inner part of the bilayer to avoid protein denaturalization. In order to avoid this problem, Au surfaces were functionalized with a short hydroxylated dithiol (dithiothreitol, DTT) which adopts a lying down configuration with the OH groups exposed to the environment. Vesicle fusion on these DTT surfaces allows phospholipid bilayer formation with a water layer between the DTT SAM and the inner phospholipid monolayer. The phospholipid bilayer exhibits good fluidity as has been shown by in situ AFM (atomic force microscopy) imaging. These bilayers have been formed on both planar and nanostructured [SERS (surface enhanced Raman spectroscopy) active] gold surfaces. ... 

Langmuir-Blodgett films (LB) and self assembled monolayers (SAM) deposited on metal surfaces have been studied by SERS spectroscopy in several investigations. For example, mono- and bilayers of phospholipids and cholesterol deposited on a rutile prism with a silver coating have been analyzed in contact with water. The study showed that in these models of biological membranes the second layer modified the fluidity of the first monolayer, and revealed the conformation of the polar head close to the silver [4.300]. 

The HBM consists of two differing leaves normally generated on a gold or silica surface. The lower leaf is a fixed long chain alkyl self-assembled monolayer covalently bound to a solid support while the upper leaf is a phospholipid mono-layer [82, 83]. This type of bilayer may show increased stability due to the covalent nature of the lower leaf fixation however it would present a system further removed in structure from the biological condition given this fixed nature and more limited fluidity [84]. 

Since the solidity or fluidity of the bilayer membrane is likely to depend on the alkyl chain interactions and consequently their length, an understanding of the relationship between chain order and chain length for tightly packed monolayers of phospholipids are important. As an example of how VSFS can be employed to study phospholipids at a liquid surface, a series of saturated symmetric chain phosphatidylcholines (PCs) were examined at the air/water and CCfr/water interfaces [49]. At the air/D20 interface, chain order within the monolayer was found to increase as the length of the chains increased (Figure 2.9a) under conditions of constant phospholipid head group area. 

Studies of the physical properties of UC, reviewed in Chapter 6 of this volume, have contributed much to our understanding of the role of this Upid in membranes and lipoprotein surfaces. The shape and polarity of UC promote its association with the phosphoUpids of membranes and Upoproteins, and this association has important effects on membrane fluidity and permeability. The physical properties of long-chain fatty acid esters of cholesterol, on the other hand, differ strikingly from those of UC, and cause these esters to be largely excluded from phospholipid bilayers and monolayers and to aggregate instead in oil droplets. 

Important factors when considering the enhanced hydrolysis at interfaces are the substrate environment in the monolayer and the need to transfer a substrate molecule from this monolayer to the active site. Interfacial disorder may provide an important parameter that facilitates such transfer of substrate to the active site. Phospholipase activity is enhanced under conditions that affect phospholipid fluidity, packing density of the phospholipids, and polymorphism of the aggregate. A highly ordered structure seen with phosphatidylcholine either above or below the transition temperature tends to give low rates of hydrolysis. Discontinuities in such ordered structures occur at temperatures close to the transition temperatures and the presence of other lipids such as anionic lipids or non-bilayer-forming phospholipids promote catalysis by perturbing the interface. 

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