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Permeation of Organometallic Compounds through Phospholipid Membranes

von Dr. Raycho Yonchev

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[1.] Ry/Fragment 029 01 - Diskussion
Zuletzt bearbeitet: 2016-01-20 06:01:36 Klgn
Anézo 2003, Fragment, Gesichtet, KomplettPlagiat, Ry, SMWFragment, Schutzlevel sysop

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[D is the] diffusion coefficient of the solute in the membrane and represents the speed with which the solute can move through the membrane. D depends on the size and shape of the solute but also on the viscosity and density of the membrane. h is the thickness of the membrane. The partition coefficient is the most important determinant of the permeation rate. Hydrophobic substances are viewed as being able to permeate the lipid bilayers of a membrane more easily than hydrophilic ones, owing in part to their ability to dissolve in and subsequently diffuse through the hydrophobic core of the bilayers. The generally high hydration degree of a polar species also constitutes a barrier to its transport across the lipid bilayers. Usually, this hydration shell must be stripped away before the solute can enter the membrane bilayers, which is often accompanied by a high energy of dehydration. Smaller solutes diffuse more rapidly than larger ones, so that better correlations between permeability and hydrophobicity are obtained when applying a correction for the size of the solute. The structural properties of the membrane as well as its dynamic state are also an important component of diffusion process. Anything enhancing the occurrence of defects in the bilayers, for instance an increase in unsaturation in the lipid chains or packing defects at lipid-protein interfaces due to transient mismatches between the rough protein surface and the lipid bilayers will enhance passive diffusion. Transit of the hydrophobic interior of the lipid bilayers is not the only barrier to passive diffusion through a membrane. Before encountering the membrane interior, a solute must get through the interfacial region, which exhibits properties very different from those of the bulk solution. Study of the passive diffusion of a solute across the lipoidal matrix implies therefore a good knowledge of both solute and membrane properties.

One of the great barriers to passage of polar and/or ionic substances across a membrane is their incompatibility with the hydrophobic core of the lipid bilayers. Pores and channels, formed by proteins, provide a water-filled pathway through the membrane and are capable of supporting great fluxes of polar and ionic solutes. This form of transport implies passive diffusion of the solute in an aqueous medium and is also driven by an electrochemical gradient between two sides of the membrane. The term of pore and channel are generally used interchangeably, but pore is used most frequently to describe structures that discriminate between solutes primarily on the basis of size, allowing the [passage of molecules that are sufficiently small to fit.]

D is the diffusion coefficient of the solute in the membrane and represents the speed with which the solute can move through the membrane. D depends on the size and shape of the solute but also on the viscosity and density of the membrane. h is the thickness of the membrane. The partition coefficient is the most important determinant of the permeation rate (see Section 1.3.2.2, page 46). Hydrophobic substances are viewed as being able to permeate the lipid bilayer of a membrane more easily than hydrophilic ones, owing in part to their ability to dissolve in and subsequently diffuse through the hydrophobic core of the bilayer. The generally high hydration degree of a polar species also constitutes a barrier to its transport across the lipid bilayer. Usually, this hydration shell must be stripped away before the solute can enter the membrane bilayer, which is often accompanied by a high energy of dehydration. Smaller solutes diffuse more rapidly than larger ones, so that better correlations between permeability and hydrophobicity are obtained when applying a correction for the size of the solute. The

structural properties of the membrane as well as its dynamic state are also an important component of diffusion processes. Anything enhancing the occurrence of defects in the bilayer, for instance an increase in unsaturation in the lipid chains or packing defects at lipid-protein interfaces due to transient mismatches between the rough protein surface and the lipid bilayer, will enhance passive diffusion. Transit of the hydrophobic interior of the lipid bilayer is not the only barrier to passive diffusion through a membrane. Before encountering the membrane interior, a solute must get through the interfacial region which exhibits properties very different from those of the bulk solution. Study of the passive diffusion of a solute

[page 43:]

across the lipoidal matrix implies therefore a good knowledge of both solute and membrane properties.

Passive diffusion through pores and channels One of the great barriers to passage of polar and/or ionic substances across a membrane is their incompatibility with the hydrophobic core of the lipid bilayer. Pores and channels, formed by proteins, provide a water-filled pathway through the membrane and are capable of supporting great fluxes of polar and ionic solutes. This form of transport implies passive diffusion of the solute in an aqueous medium and is also driven by an electrochemical gradient between the two sides of the membrane. The terms of pore and channel are generally used interchangeably, but pore is used most frequently to describe structures that discriminate between solutes primarily on the basis of size, allowing the passage of molecules that are sufficiently small to fit.

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(SleepyHollow02), Klgn


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