Hydrocarbon chains, thermal motion

Studies of the effect of permeant s size on the translational diffusion in membranes suggest that a free-volume model is appropriate for the description of diffusion processes in the bilayers [93]. The dynamic motion of the chains of the membrane lipids and proteins may result in the formation of transient pockets of free volume or cavities into which a permeant molecule can enter. Diffusion occurs when a permeant jumps from a donor to an acceptor cavity. Results from recent molecular dynamics simulations suggest that the free volume transport mechanism is more likely to be operative in the core of the bilayer [84]. In the more ordered region of the bilayer, a kink shift diffusion mechanism is more likely to occur [84,94]. Kinks may be pictured as dynamic structural defects representing small, mobile free volumes in the hydrocarbon phase of the membrane, i.e., conformational kink g tg ) isomers of the hydrocarbon chains resulting from thermal motion [52] (Fig. 8). Small molecules can enter the small free volumes of the kinks and migrate across the membrane together with the kinks. 

The positive current which increases rapidly on heating above 40 °C may be caused by the increase in the conformational disorder and thermal motion of the hydrocarbon chains. 

Although the lipid bilayer structure is quite stable, its individual phospholipid and sterol molecules have some freedom of motion (Fig. 11-15). The structure and flexibility of the lipid bilayer depend on temperature and on the kinds of lipids present. At relatively low temperatures, the lipids in a bilayer form a semisolid gel phase, in which all types of motion of individual lipid molecules are strongly constrained the bilayer is paracrystalline (Fig. ll-15a). At relatively high temperatures, individual hydrocarbon chains of fatty acids are in constant motion produced by rotation about the carbon-carbon bonds of the long acyl side chains. In this liquid-disordered state, or fluid state (Fig. 11—15b), the interior of the bilayer is more fluid than solid and the bilayer is like a sea of constantly moving lipid. At intermediate temperatures, the lipids exist in a liquid-ordered state there is less thermal motion in the acyl chains of the lipid bilayer, but lateral movement in the plane of the bilayer still takes place. These differences in bilayer state are easily observed in liposomes composed of a single lipid,... 

Trauble [193] made an interesting attempt to take into account the influence of the membrane molecular structure on the transmembrane transfer of small molecules. The transmembrane motion of these molecules was considered under the assumption that the thermal motion of the hydrocarbon chains of the membrane lipid leads to the appearance of the mobile structural defects (so called kinks ) in the membrane. The kinks are small free volumes in the hydrocarbon phase of the membrane diffusing in the membrane. The molecules from the aqueous phase may be captured by these kinks and, moving together with them, transferred to the other side of the membrane. Such a model was shown to describe satisfactory the translocation of small neutral molecules such as water. 

To elucidate the role ofhydrophobic bonding, a detailed study on the kinetics ofin-testinal absorption has been performed on sulfonamides. It was concluded that transport across the microvillus membrane occurs via kinks in the membrane which are pictured as mobile structural defects representing mobile free volumes in the hydrocarbon phase of the membrane and whose diffusion coefficient is fairly fast ( 10-5 cm2/s) [1]. The thermal motion of the hydrocarbon chain leads to the formation of kinks. It was also postulated that a transient association of the drug molecules with proteins on the surface of the microvillus membrane is involved in the formation of the activated complex in the absorption process [1]. 

Figure 8. Schematic of the thermal motion of the upper segments of hydrocarbon chains at the air-water interface. The cones shown by broken lines represent the time-average space occupied by the thermal motion of the segments.

Surfactants. Some compounds, like short-chain fatty acids, are amphiphilic or amphipathic that is, they have one part that has an affinity for the nonpolar media (the nonpolar hydrocarbon chain), and one part that has an affinity for polar media, that is, water (the polar group). The most energetically favorable orientation for these molecules is at surfaces or interfaces so that each part of the molecule can reside in the fluid for which it has the greatest affinity (Figure 4). These molecules that form oriented monolayers at interfaces show surface activity and are termed surfactants. As there will be a balance between adsorption and desorption (due to thermal motions), the interfacial condition requires some time to establish. Because of this time requirement, surface activity should be considered a dynamic phenomenon. This condition can be seen by measuring surface tension versus time for a freshly formed surface. 

A different thermal behavior is observed for those examples in which the hydrocarbon chain length exceeds the length of the fluorocarbon part. Here, no phase transitions other than that to the isotropic liquid occur at elevated temperatures. Indeed, in case of 6, F(CF2)i2(CH2)ir)H, the thermally induced molecular motion of the long hydrocarbon chain is comparable to that of short chained examples at room temperature, shown by Raman spectra, for... 

Figure 4.20 shows a typical Jt- oisotherm (dashed line) and the phase behavior inferred from isotherms determined at various temperatures for long-chain fatty acids and similar compounds. For very large values of o (e.g., several hundred square nanometers per molecule), the surfactant molecules have their hydrocarbon tails lying along the surface and move independently, owing to random thermal motion. For these conditions, the two-dimensional ideal gas equation applies ... 

A hydrocarbon chain is in a constant thermal motion, and without external force field, the chains fluctuate around the most stable position given by the distribution of possible conformations at the temperature. The action of external forces at the ends of a molecule causes displacements of chains from their equilibrium conformations and evokes retractive forces. For a hydrocarbon chain of M = 14,000, extended length 125.5 nm, and the end-to-end distance r = 1 mn, the maximum exerted force is 10 MPa. The level of forces exerted by the random coil macromolecules are much lower than the theoretical strength of the primary bonds. The presence of strong intermolecular interactions, such as hydrogen bonds in polyamides, affects the retractive force substantially, causing a restriction of the number of possible chain conformations. In addition, the transitions... 

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