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A study of n-alcohol alkyl chain length dependent structural perturbations of unilamellar 1,2-Dimyristoyl-sn-Glycero-3-Phosphocholine (DMPC) vesicles
Speciale, 3. modul, 2007, id:241
Vejleder:Dorthe Posselt (fysik) og Peter Westh (kemi)
Findes på RUb:Ja
Abstract
English abstract
In this report the n-alcohol induced structural changes af unilamellar 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) vesicles in the liquid-crystalline (formel) phase are investigated by molecular dynamics simulations (MD) simulations, densitometry and small angle neutron scattering (SANS). There are used three different n-alcohols, 1-hexanol, 1-octanol and 1-dodecanol, ehich have a different alkyl chain length. 1-Hexanol has an alkyl chain length which is almost half as long as that of the DMPC alkyl chains while 1-dodecanol has nearly the same alkyl chain length as the DMPC alkyl chains. The aim is to achieve more information about the structural changes induced by these three n-alcohols and investigate it at different temperatures. There are made MD simulations of a DMPC bilayer with 1-octanol and a DMPC bilayer with 1-dodecanol. Unilamellar DMPC vesicles with 1-hexanol, 1-octanol and 1-dodecanol are investigated with densitometry and SANS in a temperature range going from 30 degrees C to 50 degrees C. The MD simulations show that both 1-octanol and 1-dodenaol are positioned in the bilayer with the hydroxy group in the bilayer headgroup region and with the alkyl chain positioned in the region of the DMPC alkyl chains. Dodecanol is found to increase the order of the DMPC alkyl chains and thereby increase the thickness of the bilayer. This effect is also seen by the relatively good packing properties of 1-dodecanol, which is accompanied by a decrease in molecular volume and a decrease in area per alkyl chain. Octanol does not show as good packing properties as 1-dodecanol, however it increases the order of the neighboring alkyl chains and decreases the area per alkyl chain. The densitometric measurements show that the packing properties increase in the order; 1-hexanol, 1-octanol and 1-dodecanol, at 30 degrees C. The changes in molar volume by transferring an n-alchol from the bulk phase to the bilayer is found to be 5.2 ml/mole for 1-hexanol, -4.3 ml/mole for 1-octanol and - 16.2 ml/mole for 1- dodecanol at 30 degrees C. At a higher temperature the tendency is different and the change in molar volume is found to be - 9.6 ml/mole for 1-hexanol, - 3.2 ml/mole for 1-octanol and -9.3 ml/mole for 1-dodecanol at 45 degrees C. The SANS measurement show that 1-hexanol decrease the bilayer thickness with (formel). There is found an unusual large polydispersity of the vesicles studied with SANS and the radius found for the vesicles, is too low. However systematic changes in the area per alkyl chain, which is calculated from the radius obtained for the unilamellar DMPC vesicles, are found. It is found that all the n-alcohols decrese the area per alkyl chain. Results obtained at 30 degrees C show that (formel). The difference in packing properties between 1-hexanol, 1-octanol and 1-dodecanol decrese with increasing temperature. However the lateral packing properties increase approximately in the order 1-hexanol, 1-octanol and 1-dodecanol independently of the temperature. This difference in the temperature dependence between the spatial and the lateral packing properties must then be explained by changes in the packing properties along the bilayer normal, which are affected by the changes in fluidity of the alkyl chains and thereby the temperature. It seems that an increase in the fluidity cancels the bad packing properties of 1-hexanol along the bilayer normal and decreases the good packing properties of 1-dodecanol alon the bilayer normal.

Database opslag den: 23-09-2017