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| Both concepts will be clarified in the next sections. The notion of membrane fluidity will be then redefined and emphasis will be laid on its biological relevance.
Motional order. The motional or orientational order of a phospholipid molecule or a portion of the molecule refers to the number of degrees of freedom expressed by the motion experienced by the molecule or a part from it [10]. The order of motion is usually described by an order parameter. Depending on the method of measurment [sic], different order parameters can be defined and calculated. Generally, small values of the order parameter correspond to a high degree of freedom, i.e. a disordered state. In phospholipid bilayers, the region between the carbonyl groups and the middle of the hydrocarbon chains is usually characterized by relatively high order parameters, which are roughly constant independently on the position considered. Beyond the middle of the chains, the order parameters drop significantly towards the chain termini. The motions of the phospholipid molecules are thus restricted in the region of the hydrocarbon chains near the glycerol backbone, while the molecules exhibit more motional freedom towards the chain extremities. The motional order profile across the bilayers is characteristic of the physical state of the membrane. Motional rates. Phospholipid motions occur on different time scales. The rates of motion are characterized by correlation times – the motion being described by an exponential correlation function – and are not directly related to the motional order. Phospholipids exhibit various motional modes, which all account for the dynamic properties of the membrane. Intramolecular reorientational motions comprise torsional oscillations around single bonds and free or slightly hindered rotations of certain groups or segments (such as methyl groups or fragments of the headgroup). The rotational motion of the phospholipids constitutes an important contribution to the membrane dynamics. The overall rotational motion can be characterized by a spinning motion, which is a rotation around the long molecular axis parallel to the bilayers normal, and by a so-called wobbling motion, in which the long axis changes its orientation – the molecule describing thereby a cone of finite dimensions. “Wobble” is a restricted motion, [since all possible orientations of the long axis are not allowed within the bilayers structure and large deviations from the bilayers normal are unlikely.] 10. Yeagle, P. L. The Membranes of Cells, 2nd ed.; Academic Press: San Diego, 1993. |
Both concepts will be clarified in the next sections. The notion of membrane fluidity will be then redefined and emphasis will be laid on its biological relevance.
1.2.4.1 Motional order The motional or orientational order of a phospholipid molecule or a portion of the molecule refers to the number of degrees of freedom expressed by the motion experienced by the molecule or a part from it [2]. The order of motion is usually described by an order parameter. Depending on the method of measurement, different order parameters can be defined and calculated (see Chapter 4, Section 4.3.4.3, page 118, for more details). Generally, small values of the order parameters correspond to a high degree of freedom, i.e. a disordered state. In a phospholipid bilayer, the region between the carbonyl groups and the middle of the hydrocarbon chains is usually characterized by relatively high order parameters, which are roughly constant independently on the position considered. Beyond the middle of the chains, the order parameters drop significantly towards the chain termini. The motions of the phospholipid molecules are thus restricted in the region of the hydrocarbon chains near the glycerol backbone, while the molecules exhibit more motional freedom towards the chain extremities. The motional order profile across the bilayer is characteristic of the physical state of the membrane. [page 37:] 1.2.4.2 Motional rates Phospholipid motions occur on different time scales. The rates of motion are characterized by correlation times - the motion being described by an exponential correlation function - and are not directly related to the motional order. Phospholipids exhibit various motional modes, which all account for the dynamic properties of the membrane. The different types of motion are illustrated in Figure 1.11. Intramolecular reorientational motions comprise torsional oscillations around single bonds and free or slightly hindered rotations of certain groups or segments (such as methyl groups or fragments of the headgroup). The rotational motion of the phospholipids constitutes an important contribution to the membrane dynamics. The overall rotational motion can be characterized by a spinning motion, which is a rotation around the long molecular axis parallel to the bilayer normal, and by a so-called wobbling motion, in which the long axis changes its orientation - the molecule describing thereby a cone of finite dimensions. "Wobble" is a restricted motion, since all possible orientations of the long axis are not allowed within the bilayer structure and large deviations from the bilayer normal are unlikely. [2] P. L. Yeagle. The Membranes of Cells. Academic Press, San Diego, second edition, 1993. |
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