Fatty Acyl Chain Region of the Bilayer

One characteristic of the liquid crystalline phase is the considerable intramolecular motion of the lipid fatty acyl chains, due to rotation about C-C bonds in the chains. The change in steric energy that results from rotation about the C6-C7 single bond in dodecane is shown in Fig. 6.3. The minimum potential energy occurs when the two neighbouring methylene groups are related by a dihedral angle of 0°, in the trans conformation. Two other minima are obtained at torsion angles of 115 and 245°, corresponding to the gauche+ and gauche- conformations, respectively. These conformations have energies 3.7 kj mol"1 above that of the all-trans conformation, and to reach the gauche+ or gauche- conformations from the trans conformation the molecule has to move over an energy barrier of 14.0 kj mol"1. The relative sharpness of the minima and the quite high barriers to rotation mean that the molecule will remain for most of the time in the vicinity of the minima, carrying out torsional oscillations.

Introduction of a cz's-double bond into a fatty acyl chain has a significant effect on motion in the chain (Rich 1993; Li et al. 1994). Figure 6.3 also shows the steric energy as a function of the torsion angle ai describing rotation about the C7-C8 bond adjacent to the carbon-carbon double bond in cis-dodecene-6. The energy profile is characterized by a very broad peak from 120 to 240° and a narrower and smaller peak centred at 0°. Two broad minima are observed centred at 65 and 295°. The energy barrier to rotation between these two minima is just 8.1 kj mol"1, 5.9 kj mol"1 less than the corresponding energy in a saturated chain. Thus the car-

Fig. 6.3. Steric energy changes as a function ofthe torsion angle about the C6-C7 bond in dodecane (solidline) or about the C7-C8 bond in cis-dodecene-6 (broken line). Calculations were performed usingMM3forcefieids (Aiiinger et ai. 1989)

60 120 180 240 Dihedral angle

Fig. 6.3. Steric energy changes as a function ofthe torsion angle about the C6-C7 bond in dodecane (solidline) or about the C7-C8 bond in cis-dodecene-6 (broken line). Calculations were performed usingMM3forcefieids (Aiiinger et ai. 1989)

60 120 180 240 Dihedral angle

bon-carbon single bond adjacent to a rigid cis double bond has more freedom of motion than the corresponding bond in a saturated chain. The shallowness of the potential energy wells around 65° suggests that the C-C bond adjacent to a double bond can readily adopt a wide range of torsion angles (Li et al. 1994).

The extent or range of motion in a fatty acyl chain can be described by an order parameter that defines the time-averaged disposition in space of each group of atoms in the fatty acyl chain. The rate of motion can be described in terms of a correlation time, a measure of the rate of movement of a group of atoms between its various possible positions in space. Formally, fluidity (and its inverse, viscosity) corresponds solely to rate of motion. The most powerful technique for measuring order parameters is 2H-NMR, studying the motion of C-D groups introduced at specific positions in the chains (Seelig and Seelig 1980; Bloom et al. 1991). Order parameter profiles for all phospholipid bilayers in the liquid crystalline phase are remarkably similar; that for the saturated palmitoyl chain in l-palmitoyl-2-ole-oylphosphatidylcholine [(C16:0, C18:1)PC] is shown in Fig. 6.4 (Seelig and Seelig 1980). The magnitudes of the order parameters are observed to lie between the values expected for an all-trans chain rotating about its long axis (Scd = -0.5) and for complete orientational disorder, as found in an isotropic liquid (Scd = 0). Thus the fatty acyl chain region exists in a state of intermediate order, with some order persisting despite the liquid-like state of the chains. The degree of order varies along the chain; an initial plateau region of constant order is followed by a region of rapidly decreasing order towards the centre of the bilayer. The plateau region has its origin in the intermolecular restrictions on chain motion. In the upper part of the chain excluded volume effects are very important since rotation

Fig. 6.4. The experimental order parameters (-Scd) for the palmitoyl (O) and oleoyl (□) chains of: (a) (C16:0,C18:1)PC (POPC); (b) lipids of E. coli labelled in the palmitoyl(0)andoleoyl (□) chains. Modified from Seelig and Seelig (1980)

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Fig. 6.4. The experimental order parameters (-Scd) for the palmitoyl (O) and oleoyl (□) chains of: (a) (C16:0,C18:1)PC (POPC); (b) lipids of E. coli labelled in the palmitoyl(0)andoleoyl (□) chains. Modified from Seelig and Seelig (1980)

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