Self-assembly of synthetic and biological components in water using cucurbit[8]uril

Abstract

This thesis discusses progress made towards assembling molecular building blocks in the presence of our molecular host of choice, cucurbit[8]uril (CB[8]). Our studies on the self-assembly of synthetic and biological components in water bridge overlapping concepts and techniques drawn from the fields of synthetic organic chemistry, supramolecular self-assembly, and applied NMR techniques. Chapter 1 introduces the reader to chemical complexity, and how supramolecular chemists have advanced in their capability of assembling more complex molecular architectures. The discussion focusses particularly on self-assembly carried out in the aqueous phase, and how, like in biology, molecular design of the building blocks become critical in enabling non-covalent assembly to occur in this dynamic, and relatively competitive environment. The cucurbit[n]uril family of molecular hosts are then introduced with an overview of their modes of binding, and affinities towards typical guests. Finally, a practical introduction to NMR methods gaining prominence in supramolecular chemistry is presented. In particular, the use of diffusion NMR, a key tool for probing the solution dynamics of molecular assemblies, is highlighted. Chapter 2 details work carried out on the CB[8]-mediated self-assembly of supramolecular block copolymers from polymeric, and small molecule building blocks. Here, end group-functionalised polymer guests were shown to assemble with small molecule ditopic guests in the presence of CB[8] to form block copolymers. Copolymers of various molecular weights were assembled, and the supramolecular complexes were studied using solution viscometry and diffusion NMR. This study represented the first use of diffusion NMR for probing the assembly of polymeric guests with CB[8]. Chapter 3 describes the self-assembly of CB[8] with complementary ditopic guests. High molecular weight supramolecular polymers are known to form through the step-growth assembly of complementary ditopic building blocks. Here we sought to probe CB[8]’s ability to drive supramolecular polymerisation. Solution viscometry, ESI-MS, and diffusion NMR were used to investigate the self-assembly process, which indicated that cyclic oligomers had formed. The relatively low solubility of CB[8] in water was thought to be a major limitation to polymer formation in this instance. Important observations relating to the effect of salts on the solution viscosities and stabilities of the complexes, are also discussed. Chapter 4 places emphasis on the synthetic methods employed towards preparing multivalent guests for CB[8] binding studies. Our synthetic guests were based on watersoluble oligomers of ethylene glycol. A bidirectional elongation route is presented for accessing higher molecular weight, and monodisperse ethylene glycol oligomers (n = 12) in suitable purity. Chapter 5 describes the assembly of protein-polymer conjugates, and the versatility of diffusion NMR as a means to probe the assembly process. Here, end group-functionalised poly(ethylene glycol) guests were appended to bovine serum albumin (BSA) through a mixed chemical ligation-self assembly protocol. The NMR studies conducted are emphasised here, which served to complement other characterisation methods used that are reported elsewhere. Chapter 6 discusses ongoing work on lipid-based guests, and the resulting liposome assemblies formed. Head group-functionalised phospholipid guests, and cholesterol-based guests were synthesised. Phospholipid guests were obtained through an enzymatic route, a novelty in our group. Dye-encapsulated liposomes were then assembled, purified, and characterised by fluorescence microscopy. Finally, we sought to optimise lipid formulations to enhance liposome stability, towards conducting molecular recognition studies in the presence of CB[8]. Chapter 7 then closes the thesis with concluding remarks that summarise the described research, while highlighting points of note.