Neutron scattering study of admixture influences on the lipid bilayer structure

This thesis is focused on the investigation of the phospholipid bilayer structure changes caused by the intercalation of amphiphilic molecules. Simple amphiphiles were chosen - primary aliphatic alcohols (CnOH, n = 10, 16) and a homologous series of N,N-dimethyl-N-alkylamine N-oxides (CnNO, n = 12, 14, 16, 18); n is the number of carbons in the alkyl chain. The small-angle neutron scattering (SANS) on unilamellar vesicles was used as the main experimental technique and the main bilayer structure parameters - interface area per lipid molecule A and hydrocarbon region thickness 2D_C - were obtained as functions of the amphiphile molar ratios in bilayers. It was found that in the case of CnNO in dioleoylphosphatidylcholine (DOPC) bilayers partial molecular areas of CnNO 'A_CnNO for n = 12, 14 and 16 are the same within their experiment uncertainties in the interval 0,364 - 0,369 nm², whereas the value of 'A_C18NO = 0,394 ± 0,004 nm² is significantly higher. Similar change in tendency was found in the slopes of dependencies 2D_C (r_CnNO ) . Therefore the observed effect is most probably caused by the different position of C18NO in the DOPC bilayer in comparison to the rest of the series. The SANS measurements on mixed bilayers consisting of palmitoyloleoylphosphatidylcholine (POPC), palmitoyloleoylphosphatidylethanolamine (POPE) and palmitoyloleoylphosphatidylserine (POPS) with intercalated C10OH and C16OH, respectively, showed that even without intercalated alcohols the bilayer structure depends on the structure of its hydrophilic regions. The intercalation of alcohol with a shorter alkyl chain than the lipid chains is influenced only slightly by the bilayer composition, but with increasing amount of POPE in the bilayer a lateral phase separation was observed. The intercalation of alcohol with an alkyl chain length comparable to the lipid chains is much stronger influenced by presence of POPE in the bilayer and the lateral separation is observed at lower molar fractions of POPE in the bilayer. The specular neutron reflectivity was used to investigate its possibilities to detect the internal structure of floating bilayers. A system of a floating deuterated d62-dipalmitoylphosphatidylcholine bilayer over a supported dibehenoylphosphatidylcholine bilayer was used for measurements. Using the contrast variation technique it was found that it is possible to describe the internal structure a floating bilayer on the level of functional groups only in its liquid-crystalline phase, but not in the gel phase. (Author)