Comparisons of the Structures Mechanics and Thermodynamic Stabilities for Biomimetic Ion Pair Amphiphile and Phosphatidylcholine Vesicles

  • 田 慶安

Student thesis: Master's Thesis


Ion pair amphiphile (IPA) a molecular complex composed of two oppositely charged surfactants is a biomimetic amphiphile that has been proposed as a low-cost substitute for phospholipid IPA can self-assemble into catanionic vesicles that have potentials in various fields such as cosmetic gene therapy and drug delivery etc Modulating the stability of IPA vesicle is critical for its application development Yet there have been limited studies on the comparison between the phospholipid bilayers and the corresponding IPA membranes for the biomimetic characterizations Here we combined the atomistic and coarse-grained molecular dynamic simulations to investigate the intra- and inter-vesicular stabilities of vesicles composed of alkyltrimethylammonium-alkylsulfate (CnTMA+-CnS-) IPA complex and phosphatidylcholine (DCnPC) Due to the difference in the hydrophilic groups between PC and IPA IPA bilayers exhibit denser molecular packing and higher alkyl chain ordering than the corresponding PC bilayers as revealed by the radial distribution functions (RDF) molecular area deuterium order parameter (SCD) and gauche conformation analyses Meanwhile IPA bilayers possess higher mechanical moduli including the area compressibility modulus (KA) the molecular tilt modulus (χ) and the effective bending rigidity (KCeff) than PC bilayers At the water-bilayer interface IPA bilayers form fewer hydrogen bonds (H-bonds) with water and possess lower electrostatic potential than the corresponding PC bilayers owing to more hydrophilic functional groups within a PC molecule For the PC bilayers the motion of hydrophobic alkyl chains closely correlate with the hydrophilic head groups motion and longer alkyl chains prolong the water residence time (τ) at the bilayer surface Such phenomena are less pronounced for an IPA complex due to the hydrophilic group variations Furthermore we calculated the desorption free energy from the bilayer to bulk solution via the free energy perturbation (FEP) for IPA systems Our results showed that IPA with longer alkyl chain enhances the thermodynamic stability of vesicular bilayers; yet compared with PC membranes IPA bilayers are still less thermodynamically stable To probe inter-vesicular stability of IPA vesicles we measured the membrane fusion free energy using the coarse-grained SDK model and found that IPA vesicles have lower free energy barriers for the stalk formation and the fusion pore opening compared with PC liposome Through the comparison between PC and biomimetic IPA vesicles at the molecular level the combined results can provide insights for future designs of biomimetic membrane systems
Date of Award2018 Feb 7
Original languageEnglish
SupervisorChi-cheng Chiu (Supervisor)

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