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Permeation of Organometallic Compounds through Phospholipid Membranes

von Dr. Raycho Yonchev

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[1.] Ry/Fragment 084 01 - Diskussion
Zuletzt bearbeitet: 2016-03-19 12:52:32 WiseWoman
Anézo 2003, Fragment, Gesichtet, Ry, SMWFragment, Schutzlevel sysop, Verschleierung

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Untersuchte Arbeit:
Seite: 84, Zeilen: 1 ff.
Quelle: Anézo 2003
Seite(n): 49, 182, 183, Zeilen: 49:18-26; 182:2-10; 183:12-24
[Because of the anisotropic properties of membranes, their complex structural organization and chemical composition, and their asymmetry, the transport cannot be correctly described] by single parameters. A detailed knowledge of membrane characteristics is fundamental for a proper estimation of permeation rates. This is especially true for amphiphilic molecules, having a hydrophobic and a hydrophilic gravity center: their orientation within the membrane may be strongly affected by the structural organization of the bilayer as well as by the possible electrostatic and van der Waals interactions with the membrane components.

The simulations performed show that the permeation process of organometallic compounds through a phospholipid membrane is essentially determined by the free energy barrier that results from the breakage of H-bonds between the solutes and water and between the solutes and the polar lipid headgroups and from the loss of electrostatic interactions. In the DPPC membrane, the carbonyl groups of the glycerol-ester linkages seem to play an important role in the permeation process: they have the polar and hydrogen-bonding function closest to the bilayer center and attract water as well as polar solutes close to the hydrophobic hydrocarbon region of the bilayer. They also play a determinant role in the adsorption, as is shown in the simulations with trimethylbismuthane.

The full description of permeation processes requires not only the knowledge of the underlying free energy behavior, but also of the local diffusion coefficient, both quantities contributing to the global permeation rate. With the simulations based on the average force method on constrained particle is possible to estimate free energy profile, local diffusion coefficient and permeation coefficient, however, corresponding force curves are very noisy and further work with them is particularly difficult. The reasons could be several.

The theoretical derivation of the permeability coefficient described in Section II.4.2, rests on the assumption that the thermodynamic gradient can be considered constant over the correlation distance of the particle. In the strict sense, this method is only valid for permeants of relative small size and for energy barriers, which do not exhibit too steep slopes. The molecules, on which the method was experimented, are probably too big for this method, and their permeation cannot be treated as an equilibrium process. A permeant of the size of the molecules studied does not get to the top of the energy barrier [slowly and in constant equilibrium, but is driven over it quickly by its momentum, inertia effects becoming important in the case of a large permeant.]

[page 49]

Because of the anisotropic properties of membranes, their complex structural organization and chemical composition, and their asymmetry, drug transport cannot be correctly described by single parameters such as octanol/water partition coefficients. A detailed knowledge of membrane characteristics is fundamental for a proper estimation of permeation rates. This is especially true for amphiphilic drugs, having a hydrophobic and a hydrophilic gravity center: their orientation within the membrane may be strongly affected by the structural organization of the bilayer as well as by the possible electrostatic and van der Waals interactions with the membrane components.

[page 182]

5.6 Discussion

The simulations performed show that the permeation process of polar solutes through a phospholipid membrane is essentially determined by the free energy barrier that results from the breakage of H-bonds between the solutes and water and between the solutes and the polar lipid headgroups and from the loss of electrostatic interactions. In the DPPC membrane, the carbonyl groups of the glycerol-ester linkages seem to play an important role in the permeation process: they have the polar and hydrogen-bonding function closest to the bilayer center and attract water as well as polar solutes close to the hydrophobic hydrocarbon region of the bilayer. They also play a determinant role in the adsorption of amphiphilic solutes, as shown in the simulations with salicylic acid.

[page 183]

The full description of permeation processes requires not only the knowledge of the underlying free energy behavior, but also of the local diffusion coefficient, both quantities contributing to the global permeation rate. Unfortunately, the simulations based on the average force method on constrained particle, aimed at calculating permeability coefficients, were not successful. The theoretical derivation of the permeability coefficient described in Section 5.2.2.2, page 144, rests on the assumption that the thermodynamic gradient can be considered constant over the correlation distance of the particle. In the strict sense, this method is only valid for permeants of relative small size and for energy barriers which do not exhibit too steep slopes [135, 194]. The molecules, on which the method was experimented, are probably too big for this method, and their permeation cannot be treated as an equilibrium process. A permeant of the size of the molecules studied does not get to the top of the energy barrier slowly and in constant equilibrium, but is driven over it quickly by its momentum, inertia effects becoming important in the case of a large permeant.


[135] S. J. Marrink and H. J. C. Berendsen. J. Phys. Chem., 98:4155–4168, 1994.

[194] S. J. Marrink and H. J. C. Berendsen. J. Phys. Chem., 100:16729–16738, 1996.

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