Toward Cell Membrane Mimics: Creation and Study of Artificial Phosphatidylcholine and Functional Assembly

Phospholipids are amphiphillic molecules found in living tissues and the predominant components of cell membranes. Phospholipids self-assemble into bilayer structure, and self-closing of the bilayer structure forms a spherical membrane which names liposome. Liposomes composed of different phospholipids have been widely used as encapsulation materials in cosmetics, bio-imaging and drug-delivery. Due to cellular abundance, comparatively low reactivity and intrinsic ability in liposome-formation, phosphatidylcholine (PC) is a preferable phospholipid category for liposome-related application. Otherwise, in the pursuit of functional liposome vehicles, during which liposomes are engineered by introducing special structures or moieties into natural phospholipids to render the assemblies stimuli-responsive, lack of active group in PC structure, especial the inert choline head, retards functionalization of PC and development of functional PC-liposome. Moreover, for the structure-modified artificial phospholipids, alternation of structure may lead to other form of assemblies, rather than liposome. Thus, suitable molecule design and synthetic methods are important for creation of artificial PC capable of certain stimuli-responsiveness and liposome formation.

This thesis focuses on synthesis artificial phosphatidylcholines (APC) through a Copper-catalyzed Azide-Alkyne Cycloaddition (CuAAC) reaction between two precursors, an alkyne-lysolipid (AL) and an azide acyl precursor bearing functional structure of group. Presence of functional moiety in APC enables distinctive property, such as photo-responsiveness, crosslinkable bilayer structure, and the APC preserves the ability of liposome-formation as natural PCs. Thus, liposome composed of the APC has a high density of functionality in the bilayer membrane, and the property of the liposome entity can be controlled or finely tuned, exhibiting a series of smart behaviors.

Firstly, photocleavable liposomes are formed in situ through a CuAAC reaction between an AL and an o-nitrobenzyl-containing azide precursor. Photolysis of the o-nitrobenzyl structure changes the molecular structure of the photolabile PC, inducing phase transitions and permeability increases in the bilayer membrane, ultimately disrupting the liposome entity. Secondly, a spiropyran-containing PC and corresponding assembly were prepared. The assembly system composes of liposomes and fibers. Study of the self-assembly of SPTPCs and photo-induced liposome-fiber assembly-transition revealed that the presence of MC enabled additional inter-membrane interaction during self-assembly and that MC-stacking effect was the driving force for the assembly-transition. Thirdly, cerasomes are prepared from liposomes composed of triethoxysilane-containing PC through a combination of CuAAC and a sol-gel condensation. Presence of SiO₂ network in bilayer membrane of cerasome greatly improves the stability of the vesicular structure. Lastly, a thymine-containing PC (TTPC) is prepared in situ through CuAAC under aqueous conditions. The TTPCs assembles into fibers due to strong inter-molecular interaction from nucleobase moieties. Increase of TTPC concentration causes entanglement of fibers and results in a supramolecular hydrogel. Investigation of lyotropic mesophases in the gel shows presence of multiple phases including two liquid crystal phases (nematic and lamellar), which have a certain degree of structural order and are promising template to construct functional biomaterials.

Full pdf - https://www.colorado.edu/mse/node/517/attachment