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Permeation of Organometallic Compounds through Phospholipid Membranes

von Dr. Raycho Yonchev

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[1.] Ry/Fragment 058 01 - Diskussion
Zuletzt bearbeitet: 2016-01-29 15:00:35 Schumann
Anézo 2003, Fragment, Gesichtet, KomplettPlagiat, Ry, SMWFragment, Schutzlevel sysop

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[The other model, the defect model, involves the occurrence of transient pore-like defects such] as water pores in the lipid bilayer, which allow ions and small polar molecules to bypass the partitioning energy barrier.

II.3.1 Homogeneous solubility-diffusion model

The most generally accepted model to describe the permeation of small neutral permeants across lipid bilayer membranes is the homogeneous solubility-diffusion model [63-65], which was originally applied by Overton [3] to permeation across cell membranes. This model depicts the bilayer membrane as a thin static slab of bulk organic solvent, representing the permeation barrier and separating two aqueous phases. The bilayer is considered to be isotropic and homogeneous, and the water/membrane interface is treated as a sharp boundary between the two phases. The permeation process is described in three steps: the permeating molecule has first to partition into the membrane, then to diffuse through the membrane interior, and finally to dissolve again into the second aqueous phase. In this model, permeation through the membrane is assumed to be the rate-limiting step in the transport process and interfacial barriers for membrane entry or exit are neglected.

Thus, the permeation resistance R, which is equivalent to the inverse of the permeability coefficient P, can be expressed as follows:


R = \frac{1}{P} = \frac{d_b}{K_{b/w} D_b}     (2.15)

The permeation resistance is proportional to the bilayer thickness of the barrier db (which does not include the two interfaces and is assumed to be constant), and inversely proportional to the partition and diffusion coefficients of the permeant in the barrier, Kb/w and Db respectively. In practice Kb/w is associated to the partition coefficient of the permeant between water and bulk organic solvent resembling membrane interior, such as octanol, olive oil, or a liquid hydrocarbon like decane or hexadecane, and Db is approximated by the diffusion coefficient of the permeant in a bulk hydrocarbon solvent.


3. Overton, E. Vierteljahrsschr. Naturforsch. Ges. Zurich [sic] 1899, 44, 88.

63. Finkelstein, A. J. Gen. Physiol. 1976, 68, 127.

64. Reevs, [sic] J. B.; Dowben, R. M. J. Membr. Biol. 1970, 3, 123.

65. Hanai, T.; Haydon, D. A. J. Theor. Biol. 1966, 11, 370.

The other model, the defect model, involves the occurrence of transient pore-like defects such as water pores in the lipid bilayer which allow ions and small polar molecules to bypass the partitioning energy barrier.

5.2.1.1 Homogeneous solubility-diffusion model

The most generally accepted model to describe the permeation of small neutral permeants across lipid bilayer membranes is the homogeneous solubility-diffusion model [166―168], which was originally applied by Overton [13] to permeation across cell membranes. This model depicts the bilayer membrane as a thin static slab of bulk organic solvent, representing the permeation barrier and separating two aqueous phases. The bilayer is considered to be isotropic and homogeneous, and the water/membrane interface is treated as a sharp boundary between the two phases. The permeation process is described in three steps: the permeating molecule has first to partition into the membrane, then to diffuse through the membrane interior, and finally to dissolve again into the second aqueous phase. In this model, permeation through the membrane is assumed to be the rate-limiting step in the transport process and interfacial barriers for membrane entry or exit are neglected.

Thus, the permeation resistance R, which is equivalent to the inverse of the permeability coefficient P, can be expressed as follows:


R = \frac{1}{P} = \frac{d_b}{K_{b/w} \cdot D_b}     (5.1)

The permeation resistance is proportional to the bilayer thickness of the barrier db (which does not include the two interfaces and is assumed to be constant), and inversely proportional to the partition and diffusion coefficients of the permeant in the barrier, Kb/w and Db, respectively. In practice, Kb/w is associated to the partition coefficient of the permeant between water and a bulk organic solvent resembling the membrane interior, such as octanol,

[page 140]

olive oil, or a liquid hydrocarbon like decane or hexadecane, and Db is approximated by the diffusion coefficient of the permeant in a bulk hydrocarbon solvent.


[13] E. Overton. Vierteljahrsschr. Naturforsch. Ges. Zürich, 44:88—135, 1899

[166] T. Hanai and D. A. Haydon. J. Theor. Biol., 11:370—382, 1966.

[167] J. B. Reeves and R. M. Dowben. J. Membr. Biol., 3:123—141, 1970.

[168] A. Finkelstein. J. Gen. Physiol., 68:127—135, 1976.

Anmerkungen

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