Electrostatically Driven Transformation in Assembly of Charged Amphiphiles

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Self-assembly is an important process in biological system to build various bioactive structures from small amphiphilic molecules. The structural versatility of amphiphile self- assembly also provides a unique platform for the design of functional soft materials with controllable structural features. However, little is known about the correlation between external stimuli, intermolecular forces and the self-assembly structure, which is extremely significant for understanding the transformation of assembly structures in biological processes as well as the materials design. Intermolecular electrostatic interaction is one of the dominant forces for driving the self-assembly process. In this dissertation, different self-assembly system will be investigated and I will present how electrostatic interaction affects the self-assembly structures and properties. I will first demonstrate a modular series of C16Kn peptide amphiphiles where intermolecular electrostatic interaction and steric repulsion can be controlled by solution pH and size of the ionizble headgroup. A large diversity of self-assembly structures, ranging from spherical micelle to bilayer nanotube, can be observed through subtle changes in the electrostatic and steric repulsions. Moreover, C16K1 peptide bilayer membrane exhibits a structural transformation from planar bilayer ribbon to rolled-up cochleate structure when increasing the ionic strength, and the interlayer spacing in the cochleate structure is directly related to the solution Debye length. Theoretical studies on each step of the morphological transition successfully correlate the electrostatic interaction with the equilibrium structures of the membrane. Other than peptide amphiphile, thermodynamic phase behaviors of phospholipid bilayer membranes consisting of 3 binary lipid molecules are also studied. The gel-to-fluid phase transition temperatures of lipid bilayers with DMTAP (+1)/DMPC (0) and DMTAP (+1)/DMPS (-1) binary mixtures are measured to explore the contribution of intermolecular electrostatic attraction to stabilizing the gel-phase lipid bilayer membrane. In addition, 2-D lattices of lipid bilayers show “universal†thermal expansion behavior for all lipid binary mixtures, and the intermolecular spacing is also minimized as the intermolecular electrostatic attraction increases.

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  • 09/30/2019
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