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Permeation of Organometallic Compounds through Phospholipid Membranes

von Dr. Raycho Yonchev

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[1.] Ry/Fragment 020 01 - Diskussion
Zuletzt bearbeitet: 2016-01-29 04:59:45 Klgn
Anézo 2003, Fragment, Gesichtet, KomplettPlagiat, Ry, SMWFragment, Schutzlevel sysop

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[Within a particular class of phospholipids, the length of the hydrocarbon chains has a direct influence on the] transition temperature, e.g. from the gel state Lβ to the liquid crystalline state Lα. Increasing the chain length has the effect of increasing the chain-chain interactions, stabilizing the phospholipid bilayers in the gel state and thus increasing the chain-melting temperature. The Lα-HII transition temperature falls also steeply with increasing the chain length. The hydrophobicity of longer chains is indeed more pronounced, which leads to favor the formation of inversed non-lamellar phases. The presence of double bonds in the phospholipid hydrocarbon chains has a more powerful effect on the transition temperature than the chain length. Chain unsaturation drastically lowers the gel-fluid transition temperature (typically by approximately 60oC for a cis-unsaturated bond) as well as any transitions to inverse non-lamellar phases. A cis-double bond generates the formation of a permanent kink in the hydrocarbon chains, which inhibits a tight packing of the chains. Unsaturation destabilizes the bilayers structure, especially in the gel state, and the phase transition occurs therefore at a lower temperature. The effect on the phospholipid phase behavior depends strongly upon the position of the double bond along the chain, the maximal effect occurring when the double bond is located close to the middle of the chain [15]. Monosaturated [sic] fatty acids commonly found in phospholipids from cell membranes exhibit a double bond between carbons 9 and 10 – the double bond is thus located precisely where the maximal effect on the phase behavior can be achieved. For diacylphospholipids, increasing asymmetry between the lengths of the two chains also tends to lower the chain-melting temperature [16]. The chemical structure of the phospholipid headgroups plays also a major role in determining the phase transition behavior. The effective polarity of the headgroups, defined by their intrinsic hydrophilicity but also depending on their accessibility to water or their possibility for hydrogen bonding, constitutes a crucial factor. Minor modifications, such as the replacement of a single proton by e [sic] methyl group, can profoundly alter the phase behavior. Striking differences are for instance observed between the polymorphism of PE and PC. The transition temperature from the gel to liquid crystalline states is about 20oC higher in PE than in PC. For the former phospholipid species, intermolecular hydrogen bonds between the amino group of one headgroup and the phosphate moiety of the neighboring headgroup are possible. Such hydrogen bonds intensify headgroup [interactions but weaken the interactions between headgroups and water.]

15. Barton, P. G.; Gunstone, F. D. J. Biol. Chem. 1975, 250, 4470.

16. Cevc, G. Chem. Phys. Lipids 1991, 57, 293.

Within a particular class of phospholipids, the length of the hydrocarbon chains has a direct influence on the transition temperature, e.g. from the gel state Lβ to the liquid crystalline state Lα. Increasing the chain length has the effect of increasing the chain-chain interactions, stabilizing the phospholipid bilayer in the gel state and thus increasing the chain-melting temperature. The Lα-HII transition temperature falls also steeply with increasing the chain length. The hydrophobicity of longer chains is indeed more pronounced, which tends to favor the formation of inversed non-lamellar phases. The presence of double bonds in the phospholipid hydrocarbon chains has a more powerful effect on the transition temperature than the chain length. Chain unsaturation dras-

[page 34:]

tically lowers the gel-fluid transition temperature (typically by approximately 60oC for a cis-unsaturated bond) as well as any transitions to inverse non-lamellar phases. A cis-double bond generates the formation of a permanent kink in the hydrocarbon chains, which inhibits a tight packing of the chains. Unsaturation destabilizes the bilayer structure, especially in the gel state, and the phase transition occurs therefore at a lower temperature. The effect on the phospholipid phase behavior depends strongly upon the position of the double bond along the chain, the maximal effect occurring when the double bond is located close to the middle of the chain [24]. Monounsaturated fatty acids commonly found in phospholipids from cell membranes exhibit a double bond between carbons 9 and 10: the double bond is thus located precisely where the maximal effect on the phase behavior can be achieved. For diacylphospholipids, increasing asymmetry between the lengths of the two chains also tends to lower the chain-melting temperature [25]. The chemical structure of the phospholipid headgroups plays also a major role in determining the phase transition behavior. The effective polarity of the headgroups, defined by their intrinsic hydrophilicity but also depending on their accessibility to water or their possibility for hydrogen bonding, constitutes a crucial factor. Minor modifications, such as the replacement of a single proton by a methyl group, can profoundly alter the phase behavior. Striking differences are for instance observed between the polymorphism of PE and PC. The transition temperature from the gel to liquid crystalline states is about 20oC higher in PE than in PC. For the former phospholipid species, intermolecular hydrogen bonds between the amino group of one headgroup and the phosphate moiety of the neighboring headgroup are possible. Such hydrogen bonds intensify headgroup interactions but weaken the interactions between headgroups and water.


[24] P. G. Barton and F. D. Gunstone. J. Biol. Chem., 250:4470–4476, 1975.

[25] G. Cevc. Chem. Phys. Lipids, 57:293–307, 1991.

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