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Permeation of Organometallic Compounds through Phospholipid Membranes

von Dr. Raycho Yonchev

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[1.] Ry/Fragment 083 01 - Diskussion
Zuletzt bearbeitet: 2016-03-19 13:04:43 WiseWoman
Anézo 2003, Fragment, Gesichtet, Ry, SMWFragment, Schutzlevel sysop, Verschleierung

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Verschleierung
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Klgn
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Untersuchte Arbeit:
Seite: 83, Zeilen: 1 ff. (entire page)
Quelle: Anézo 2003
Seite(n): 48, 49, Zeilen: 48: 9-17,26-31; 49: 1-7,15-21
III.3. Discussion

The absorption of compounds by passive diffusion can be reasonably well predicted on the basis of their chemical properties. The presence of polar and charged functional groups a substantial number of hydrogen-bonding sites, high molecular weight, and large polar surface area are generally associated with poor membrane permeability. In many cases, however, the consideration of permeant properties alone proves to be not sufficient for a proper evaluation of permeation data: permeability coefficients may be overestimated or, on the contrary, underestimated. Another important factor to take into account is, indeed, the nature and the permeability of the membranes that have to be crossed.

Independently on the type of membranes, membrane fluidity is generally correlated with membrane permeability and the degree of fluidity influences largely the ability of a solute to pass through the membrane. All factors increasing membrane fluidity may also enhance membrane permeability. The formation of structural discontinuities in the bilayer, even only transiently, increases membrane permeability, especially in the case of small permeants, which can jump from one discontinuity to another. Such discontinuities are for instance observed in the presence of unsaturated lipid chains, which generate packing defects in the bilayer interior. Phase separation phenomena within the membrane can also increase permeation processes. Under certain conditions, indeed, the bilayer organization can be interrupted by non-bilayer phases as well as by bilayer phases of different compositions. The regions of mismatch between these coexisting phases can be viewed as fractures within the membrane. Such topological discontinuities may promote solute permeation through the membrane.

During the diffusion process, the molecules may exhibit a well-defined orientation with respect to the bilayer normal and adopt specific conformations, different from those generally found in an aqueous medium. This can be related to the highly structured environment of the bilayer and to its difference in polarity compared with water. 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.]

[page 48]

The absorption of compounds by passive diffusion can be reasonably well predicted on the basis of their physicochemical properties. The presence of polar and charged functional groups, low octanol/water partitioning, a substantial number of hydrogen-bonding sites, high molecular weight, and large polar surface area are generally associated with poor membrane permeability. In many cases, however, the consideration of drug properties alone proves to be not sufficient for a proper evaluation of permeation data: permeability coefficients may be overestimated or, on the contrary, underestimated. Another important factor to take into account is, indeed, the nature and the permeability of the membranes that have to be crossed by the drug.

[...]

Independently on the type of membranes, membrane fluidity is generally correlated with membrane permeability and the degree of fluidity influences largely the ability of a solute to pass through the membrane. All factors increasing membrane fluidity (see Section 1.2.4.3, page 39) may also enhance membrane permeability. The formation of structural discontinuities in the bilayer, even only transiently, increases membrane permeability, especially in the case of small permeants which can jump from one discontinuity to another. Such disconti-

[page 49]

nuities are for instance observed in the presence of unsaturated lipid chains which generate packing defects in the bilayer interior. Phase separation phenomena within the membrane can also increase permeation processes. Under certain conditions, indeed, the bilayer organization can be interrupted by non-bilayer phases as well as by bilayer phases of different compositions. The regions of mismatch between these coexisting phases can be viewed as fractures within the membrane. Such topological discontinuities may promote solute permeation through the membrane. [...]

During the diffusion process, drug molecules may exhibit a well-defined orientation with respect to the bilayer normal and adopt specific conformations, different from those generally found in an aqueous medium. This can be related to the highly structured environment of the bilayer and to its difference in polarity compared with water. 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.

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