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Permeation of Organometallic Compounds through Phospholipid Membranes

von Dr. Raycho Yonchev

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[1.] Ry/Fragment 024 01 - Diskussion
Zuletzt bearbeitet: 2016-02-03 11:33:44 Klgn
Anézo 2003, Fragment, Gesichtet, KomplettPlagiat, Ry, SMWFragment, Schutzlevel sysop

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[“Wobble” is a restricted motion,] since all possible orientations of the long axis are not allowed within the bilayers structure and large deviations from the bilayers normal are unlikely. Brownian translational diffusion or jump diffusion of the phospholipid molecules within the plane of the membrane is called lateral diffusion – the in-plane motion of the whole phospholipids consists of a local diffusion within their solvent cage and of long-range diffusional jumps between adjacent sites [19]. Transbilayer “flip-flop” processes, in which phospholipid molecules are translocated from one side of the bilayers to the other, are also of importance, although they occur more rarely. At last, patches of phospholipid molecules exhibit collective motions, leading to undulations of the bilayers surface and resulting in relatively slow fluctuations.

The anisotropic nature of the phospholipid bilayers implies strong anisotropic constraints upon the orientation, conformation and motion of the phospholipid molecules. The reorientational motions about different molecular axes occur thus on vastly different time scales [20]. For instance, rotation is much more facilitated around the long axis of the phospholipid molecule than around other axes. Another important point to mention is the different motional behavior of the headgroup compared with the hydrocarbon chains. A certain independence of motion is observed and some particular motions may occur on different time scales.

The most rapid of the motions described above is rotation about single bonds, characterized by correlation times inferior to 100 ns. Rotation about single bonds is usually slower in the headgroup region than in the hydrocarbon chains. Trans-gauche isomerizations in the fatty acid chains are very rapid with correlation times τJ from 1 to 10 ps in the liquid crystalline state. The values of τJ are relatively long near the glycerol backbone but become shorter towards the chain termini. The rotation of the terminal methyl groups in the chains is about one order of magnitude shorter than τJ. In the headgroups, the dynamics of the dihedrals is slower, with correlation times up to a few hundred of picoseconds. This can be explained by the strong interactions within and between the headgroups. The time scales become significantly longer when the whole phospholipid molecule is considered. Rotation of the molecule around its long axis involves a correlation time t of a few nanoseconds. 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.]


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.

"Wobble" is a restricted motion, since all possible orientations of the long axis are not allowed within the bilayer structure and large deviations from the bilayer normal are unlikely. Brownian translational diffusion or jump diffusion of the phospholipid molecules within the plane of the membrane is called lateral diffusion: the in-plane motion of the whole phospholipids consists of a local diffusion within their solvent cage and of long-range diffusional jumps between adjacent sites [28]. Transbilayer "flip-flop" processes, in which phospholipid molecules are translocated from one side of the bilayer to the other, are also of importance, although they occur more rarely. At last, patches of phospholipid molecules exhibit collective motions, leading to undulations of the bilayer surface and resulting in relatively slow fluctuations.

The anisotropic nature of the phospholipid bilayer implies strong anisotropic constraints upon the orientation, conformation, and motion of the phospholipid molecules. The reorientational motions about different molecular axes occur thus on vastly different time scales [29]. For instance, rotation is much more facilitated around the long axis of the phospholipid molecule than around other axes. Another important point to mention is the different motional behavior of the headgroup compared with the hydrocarbon chains. A certain independence of motion is observed and some particular motions may occur on different time scales.

The most rapid of the motions described above is rotation about single bonds, characterized by correlation times inferior to 100 ps. Rotation about single bonds is usually slower in the headgroup region than in the hydrocarbon chains. Trans-gauche isomerizations in

[page 38:]

the fatty acid chains are very rapid, with correlation times τJ from 1 to 10 ps in the liquid crystalline state. The values of τJ are relatively long near the glycerol backbone but become shorter towards the chain termini. The rotation of the terminal methyl groups in the chains is about one order of magnitude shorter than τJ. In the headgroups, the dynamics of the dihedrals is slower, with correlation times up to a few hundred of picoseconds. This can be explained by the strong interactions within and between the headgroups. The time scales become significantly longer when the whole phospholipid molecule is considered. Rotation of the molecule around its long axis involves a correlation time τ of a few nanoseconds. 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.


[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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