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Permeation of Organometallic Compounds through Phospholipid Membranes

von Dr. Raycho Yonchev

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[1.] Ry/Fragment 025 01 - Diskussion
Zuletzt bearbeitet: 2016-02-04 05:57:17 Klgn
Anézo 2003, Fragment, Gesichtet, KomplettPlagiat, Ry, SMWFragment, Schutzlevel sysop

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[The wobbling motion, however,] happens on a larger time scale and is characterized by a correlation time t of the order of tens of nanoseconds or more. The wobbling motions in a cone are indeed only possible within a restricted angular range determined by the neighboring molecules. Translational motion of the phospholipid molecules in the plane occurs with a lateral diffusion coefficient of the order of 10-7 cm2/s [19]. The transmembrane movement of phospholipids or “flip-flop” is very slow and takes place on a typical time scale of minutes to hours or even days [20]. The dynamic properties of a phospholipid membrane are therefore characterized by a very wide range of motional modes, which makes their study particularly challenging.

Membrane fluidity. Now that the motional properties of the phospholipid components in a membrane have been examined, the concept of “fluidity” in membrane biology can be redefined and its involvement in many biological functions can be better understood.

The term “fluidity” is imprecise and elusive in its physical meaning, especially when applied in the context of membrane structure. For an isotropic liquid such as water, fluidity represents the inverse of viscosity, which is a well-defined and easily measured bulk thermodynamic property. Viscosity is essentially a measure of the frictional resistance encountered when adjacent layers of fluid are moving with different velocities [9]. However, biological membranes as well as pure phospholipid bilayers are highly anisotropic and the above definition of fluidity cannot be applied directly. The membrane components are principally confined to two dimensions, i.e. the surface of the membrane, with only a limited third dimension available, being the bilayers normal. The phospholipid behavior is not only anisotropic but also strongly dependent on the location considered along the bilayers normal. If the hydrophobic membrane interior, characterized by highly disordered chains experiencing considerable freedom, can be modeled by any isotropic hydrocarbon phase, the glycerol region is much more ordered and static, and the interfacial region, composed of the headgroups facing the water phase, exhibits specific order and dynamic properties. As applied to membrane, the term “fluidity” has to be considered both as a dynamic property related to the motion of individual components and as a static feature related to the arrangement or order of the molecules in the membrane. Generally, fluidity is measured by observing the motion of [fluorescent probes incorporated into membrane.]


9. Gennis, R. B. Biomembranes: Molecular Structure and Function; Springer-Verlag: Berlin, 1989.

19. Sackmann, E. Handbook of Biological Physics - Structure and Dynamics of Membranes: From Cells to Vesicles; Elsevier Science: Amsterdam, 1995; Vol. 1A.

20. Blume, A. Dynamic Properties. In Phospholipids Handbook; Cevc, G., Ed.; Marcel Dekker: New York, 1993.

The wobbling motion, however, happens on a larger time scale and is characterized by a correlation time τ of the order of tens of nanoseconds or more. The wobbling motions in a cone are indeed only possible within a restricted angular range determined by the neighboring molecules. Translational motion of the phospholipid molecules in the plane occurs with a lateral diffusion coefficient of the order of 10-7 cm2/s [28]. The transmembrane movement of phospholipids or "flip-flop" is very slow and takes place on a typical time scale of minutes to hours or even days [29]. The dynamic properties of a phospholipid membrane are

[page 39:]

therefore characterized by a very wide range of motional modes, which makes their study particularly challenging.

1.2.4.3 Membrane fluidity

Now that the motional properties of the phospholipid components in a membrane have been examined, the concept of "fluidity" in membrane biology can be redefined and its involvement in many biological functions can be better understood.

The term "fluidity" is imprecise and elusive in its physical meaning, especially when applied in the context of membrane structure. For an isotropic liquid such as water, fluidity represents the inverse of viscosity, which is a well-defined and easily measured bulk thermodynamic property. Viscosity is essentially a measure of the frictional resistance encountered when adjacent layers of fluid are moving with different velocities [19]. However, biological membranes as well as pure phospholipid bilayers are highly anisotropic and the above definition of fluidity cannot be applied directly. The membrane components are principally confined to two dimensions, i.e. the surface of the membrane, with only a limited third dimension available, being the bilayer normal. The phospholipid behavior is not only anisotropic but also strongly dependent on the location considered along the bilayer normal. If the hydrophobic membrane interior, characterized by highly disordered chains experiencing considerable freedom, can be modeled by any isotropic hydrocarbon phase, the glycerol region is much more ordered and static, and the interfacial region, composed of the headgroups facing the water phase, exhibits specific order and dynamic properties. As applied to membrane, the term "fluidity" has to be considered both as a dynamic property related to the motion of the individual components and as a static feature related to the arrangement or order of the molecules in the membrane. Generally, fluidity is measured by observing the motion of fluorescent probes incorporated into the membrane.


[19] R. B. Gennis. Biomembranes: Molecular Structure and Function. Springer-Verlag, C. R. Cantor (Ed.), Berlin, 1989.

[28] E. Sackmann. Physical Basis of Self-Organization and Function of Membranes. In: Handbook of Biological Physics - Structure and Dynamics of Membranes: From Cells to Vesicles, volume 1A. Elsevier Science, R. Lipowsky and E. Sackmann (Eds.), Amsterdam, 1995.

[29] A. Blume. Dynamic Properties. In: Phospholipids Handbook. G. Cevc (Ed.), Marcel Dekker, New York, 1993.

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