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| Untersuchte Arbeit: Seite: 59, Zeilen: 1 ff. (entire page) |
Quelle: Anézo 2003 Seite(n): 140, Zeilen: 3 ff. |
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| The solubility-diffusion theory as typically employed (i.e. Equation (2.15)) suffers from several weaknesses. It is quite obvious, both from experiment and simulation, that the membrane exhibits a structure that would not be expected at an interface between an organic solvent and water. The rough features of a lipid bilayer are clear: the polar headgroups of the lipids interface with water and the hydrocarbon interior. However, the structure of a bilayer has a much finer detail than this: a bilayer is a very heterogeneous construct, with distinct regions that may have very different affinities for a solute. Experimental and theoretical analyses clearly show that solute partitioning into bilayers differs in many respects from that into bulk solvents. Specifically, the solute partition coefficients into bilayers exhibit a strong dependence on the local lipid microstructure, a feature that cannot be accounted for based on partition coefficients in bulk fluids. Neglecting in the permeation process the role of the water/membrane interfaces, which represent about 40% of the total membrane phase, is a particularly crude approximation that often leads to erroneous estimations of permeation rates. Alkane/water partitioning systems, for instance, can only model the hydrophobic contribution of solute-membrane interactions, whereas the interactions between the solute and the polar lipid headgroups are not taken into account. Finally, the diffusion process within the membrane is not homogeneous. The lipid chains are more ordered near the water/lipid interface and become progressively less ordered as the bilayer center is approached. However, even in the center, the order parameters do not suggest the complete disorder expected in fluid hydrocarbons.
To conclude, the oversimplifications made in this homogeneous model fail to take into account the diverse and complex properties of real membranes, so that the permeation mechanism cannot be properly described by this approach. II.3.2 Inhomogeneous solubility-diffusion model Considering the limitations of the homogeneous solubility-diffusion model to predict the permeation rates of solutes in lipid membranes, Marrink and Berendsen derived an inhomogeneous solubility-diffusion model [66]. 66. Marrink, S. J.; Berendsen, H. J. C. J. Phys. Chem. 1994, 98, 4155. |
The solubility-diffusion theory as typically employed (i.e. Equation 5.1) suffers from several weaknesses. It is quite obvious, both from experiment and simulation, that the membrane exhibits a structure that would not be expected at an interface between an organic solvent and water. The rough features of a lipid bilayer are clear: the polar headgroups of the lipids interface with water and the hydrocarbon interior. However, as shown in Chapter 4, the structure of a bilayer has a much finer detail than this: a bilayer is a very heterogeneous construct, with distinct regions that may have very different affinities for a solute. Experimental and theoretical analyses clearly show that solute partitioning into bilayers differs in many respects from that into bulk solvents. Specifically, the solute partition coefficients into bilayers exhibit a strong dependence on the local lipid microstructure, a feature that cannot be accounted for based on partition coefficients in bulk fluids. Neglecting in the permeation process the role of the water/membrane interfaces, which represent about 40% of the total membrane phase, is a particularly crude approximation that often leads to erroneous estimations of permeation rates. Alkane/water partitioning systems, for instance, can only model the hydrophobic contribution of solute-membrane interactions, whereas the interactions between the solute and the polar lipid headgroups are not taken into account. Finally, the diffusion process within the membrane is not homogeneous. The lipid chains are more ordered near the water/lipid interface and become progressively less ordered as the bilayer center is approached. However, even in the center, the order parameters do not suggest the complete disorder expected in fluid hydrocarbons.
To conclude, the oversimplifications made in this homogeneous model fail to take into account the diverse and complex properties of real membranes, so that the permeation mechanism cannot be properly described by this approach. 5.2.1.2 Inhomogeneous solubility-diffusion model Considering the limitations of the homogeneous solubility-diffusion model to predict the permeation rates of solutes in lipid membranes, Marrink and Berendsen derived an inhomogeneous solubility-diffusion model [135]. [135] S. J. Marrink and H. J. C. Berendsen. J. Phys. Chem., 98:4155—4168, 1994. |
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