Norman, LH; (2015) Experimental studies of phospholipid self assembly in hydrocarbon liquids: reverse vesicles as possible compartmentalisation strategies for hydrocarbon-base life on Titan. Doctoral thesis , UCL (University College London).

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Abstract

The goal of this thesis was to determine the nature of phosphatidylcholine self assembly within hydrocarbon liquids. This study was enacted in order to investigate the potential use of amphiphiles in compartmentalisation strategies for putative organisms inhabiting the hydrocarbon lakes of Titan (the largest moon of Saturn). The backbone of terrestrial cell membranes are vesicular structures composed of a phospholipid bilayer, with the hydrophilic head groups arranged around the periphery, and simple lipid vesicles are thought to be akin to the first terrestrial protocells. It may be possible that reverse vesicles, surrounding a nonpolar core and composed of a bilayer with the hydrophilic head groups arranged internally, may be ideal model cell membranes for putative hydrocarbon-based biota inhabiting Titan‟s hydrocarbon lakes. Compounds that are shown to form reverse vesicles in conditions comparable to those of Titan„s lakes could be potential „biomarkers‟ and searched for in future missions to Titan. In order to discover whether certain phosphatidylcholines can exhibit vesicular behaviour within hydrocarbon liquids, and to analyse their structure, I have carried out experimental studies using environmental conditions that are increasing comparable to those found on the surface of Titan. Studies of macroscopic and microscopic phase behaviours were used to determine the presence of self assembled particles, including reverse vesicles. These studies included the use of microscopy, confocal laser scanning fluorescence microscopy, transmission electron microscopy (TEM), dynamic light scattering (DLS), small-angle neutron scattering (SANS) and small-angle x-ray scattering (SAXS). The systems studied included: the hydrocarbon solvents cyclohexane, octane, heptane, hexane, pentane and butane; various amphiphilic ratios of PC4:0, PC18:2 and lysoPC18 at dilute, ≤30 mM, concentrations; inclusion or exclusion of NaCl as a stabiliser; methanol or direct preparation methods; variation of sonication intensities and times; and a variety of temperatures. Results of this research demonstrate that unilamellar, multilamellar and multi-chambered reverse vesicles can form in a wide range of phosphatidylcholine-hydrocarbon systems. Small concentrations of NaCl and lyso-phosphatidylcholines were found to facilitate reverse vesicle formation. A reduction in temperature (down to the freezing point of the solvent) did not change the structural phase behaviour of most systems, but often increased the size of reverse vesicles. Decreasing the molecular weight of the alkanes was found to effect which amphiphilic ratios formed reverse vesicles; pentane solvent molecules were particularly effective at bilayer penetration. These results support the feasibility of further cryogenic self assembly experiments as analogues to the hydrocarbon environments found on Titan.

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