about a single C-C bond results in a large bend in the chain. Lower down the chain, lateral displacements resulting from rotations about single C-C bonds are very much smaller, and, therefore, steric restrictions on motion become less important. The order parameter profile for the palmitoyl chain in (C16:0, C18:1)PC is very similar to that observed in di(C16:0)PC, but with order being slightly higher in bilayers of di(C16:0)PC than in bilayers of (C16:0,C18:1)PC, suggesting that introduction of a cis double bond results in a slight disordering of the bilayer (Seelig and Seelig 1980). This effect could follow from the effect of the "bent" cisdouble bond on motion in the bilayer, but it could also reflect the fact that at 42°C, the temperature of the experiment shown in Fig. 6.4, (C16:0,C18:1)PC is about 50°C above its phase transition temperature, whereas di(C16:0)PC with a phase transition temperature of 42°C would only just have entered the liquid crystalline phase.
Interpretation of the order parameter profile for the unsaturated chain in (C16:0,C18:1)PC is more complex than for the saturated chain (Fig. 6.4). The experimental order parameters for carbons atoms 10 and 11 are low in the oleoyl chain. This does not, however, indicate a high degree of motional disorder for these carbons, but rather follows from effects of the cis double bond on the orientation of C-D bonds with respect to the bilayer normal. Since the measured spectra for a C-D bond depend not only on the true molecular order parameter but also on the angle between the C-D bond and the applied magnetic field direction (or bilayer normal), this will have a large effect on the measured spectra; the dip in order parameter following the cis double bond shown in Fig. 6.4 follows largely from the special geometry of the cis double bond (Heller et al. 1993). Nevertheless, molecular dynamics simulations do show an increased motion for the C=C double bond and the methylene groups next to it, particularly for that on the terminal methyl side (Heller et al. 1993; Huang et al. 1994). Increased disorder in the region of the double bond could follow from a cooperative effect of the double bond on the way that lipid molecules pack within the bilayer or could be a result of the shallow energy barriers for rotation about C-C bonds adjacent to a double bond, as described above (Fig. 6.3).
In conclusion we can say that the effects of a single cis C=C double bond on motion of fatty acyl chains in a bilayer are rather modest. It might have been thought that the presence of a single cis double bond towards the middle of a fatty acyl chain would have led to a significant packing problem because of the sharp bend in the chain at the double bond. This indeed is the basis of the large decrease in phase transition temperature observed on introduction of a single double bond into phosphatidylcholines or phosphatidylethanolamines. However, the data in Fig. 6.4 suggest that in a liquid crystalline bilayer the oleoyl chain is unlikely to contain a sharp bend; by C12 the order parameter has a value very similar to that observed in the corresponding position of a saturated chain.
Larger effects of unsaturation can be expected for polyunsaturated chains. As for monounsaturated chains, description of chain motion for a polyunsaturated chain is more difficult than for a saturated chain. The effects of polyunsaturation have been studied in a series of phosphatidylcholines with a deuterated stearoyl chain at the sn-l position and an unsaturated chain at the sn-2 position. Effects are rather small, with the sn-l chain becoming slightly more disordered as the unsaturation of the sn-2 chain is increased, the effect of unsaturation reaching a maximum at three double bonds (Holte et al. 1995). The largest changes in order occur around the centre of the chain, with relatively little change in the top part of the chain close to the glycerol backbone. The effects of a polyunsaturated chain have been shown to depend slightly on the position of unsaturation (McCabe et al. 1994).
It had been suggested that chains such as linoleic acid or docosahexaenoic acid (DHA) containing a 1,4-pentadiene structure might have unique conformational properties and that the presence of six cz's-double bonds in DHA might reduce chain flexibility (Applegate and Glomset 1986). However, the DHA chain in fact shows considerable flexibility with a greater lateral compressibility than a saturated chain because the presence of the cz's-double bonds leads to increased rates of interconversion between torsional states, because of a decrease in energy barriers (Feller et al. 2002). It has been suggested that the extreme flexibility for the DHA chain could be important for interaction with membrane proteins. A molecular dynamics simulation of rhodopsin in a bilayer of l-stearoyl-2-docoso-hexaenoyl-phosphatidylcholine showed that the DHA chains penetrate deeper into the protein interface than do the stearic acid chains (Feller et al. 2003). It was suggested that the extreme flexibility of the DHA chain could allow it to adapt better to the rugged surface of the protein (Feller et al. 2003).
Profiles of chain order are almost unaffected by the lipid headgroup, although absolute values of order parameters can be affected. Thus order parameters for phosphatidylethanolamines in the liquid crystalline phase are almost constant for the first part of the chain, but decrease rapidly towards the terminal methyl group, as for the phosphatidylcholines, but the order parameters are higher for phosphatidylethanolamines than for phosphatidylcholines, at all positions of the chain (Perly et al. 1985; Lafleur et al. 1990). The higher order parameters in phosphatidylethanolamines can be attributed to the smaller headgroup of the phos-phatidylethanolamine and to strong intermolecular hydrogen bonding between the headgroups, both factors leading to a greater packing density throughout the bilayer. However, differences in packing density in the chain region between phosphatidylcholines and phosphatidylethanolamines must be quite small since the thicknesses of bilayers of di(C18:l)PC and di(C18:l)PE are equal (Fenske et al. 1990).
The chain order parameter profile for dipalmitoylphosphatidylserine [di(C16:0)PS] is also very similar to that for di(C16:0)PC along most of the length of the chain; order parameters are, however, slightly less in di(C16:0)PS than for
Zero Curvature nor
Fig. 6.5. The forces present in a lipid bilayer. At the top Is shown the distribution of lateral pressures and tensions across a lipid monolayer. The repulsive lateral pressure Fc In the chain region Is due to thermally activated bond rotational motion. The Interfacial tension y, tending to minimize the interfacial area, arises from the hydrophobic effect (unfavourable hydrocarbon-water contacts). Finally, the lateral pressure Fh In the headgroup region arises from sterlc, hydrational and electrostatic effects; It is normally repulsive, but may contain attractive contributions from, for example, hydrogen bonding Interactions. After Seddon (1990). Below Is shown the tendency for spontaneous curvature of a lipid monolayer arising from an Imbalance In the distribution of lateral forces across the monolayer. The arrows show the direction of observation used In the definition ofnegatlve and positive curvature
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