Host-guest chemistry between cucurbituril and neutral and cationic guests
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This thesis describes the host-guest chemistry between cucurbituril (CB) and various series of guests, including neutral polar organic solvents, bis(pyridinium)alkane dications, local anaesthetics, acetylcholine analogues, as well as succinylcholine and decamethonium analogues, in aqueous solution. A focus of this thesis is the effects of varying the chemical structures within different series of guests upon the nature of the host-guest chemistry, such as the relative position and orientation of the guest relative to the CB cavity, and the strengths of the binding affinities. The binding affinities of polar organic solvents with CB depend upon the hydrophobic effect and dipole-quadrupole interactions. The polar guests align themselves so that their dipole moment is perpendicular to the quadrupole moment of CB. The binding strengths of acetone and acetophenone to CB decrease in the presence of alkali metals. Discrete 1:1 and 2:1 host-guest complexes are formed between CB and a series of bis(pyridinium)alkane guests. In most cases the CB initially occupies the aliphatic linker when the 1:1 complex is formed and migrates to the terminal regions as the second CB is added. When bulky, hydrophobic tert-butyl substituents are present, however, the CB occupies the terminal pyridinium region and not the central linker. Supramolecular complexes between CB and a series of local anaesthetics have binding affinities 2-3 orders of magnitude greater than reported values with beta-cyclodextrin. The first pKa values of the guests increase by 0.5-1.9 units upon complexation. The binding positions of the guests within CB differ in neutral and acidic media, with the systems thus behaving as pH-activated switches. With supramolecular complexes between CB and various cationic cholines and their phosphonium analogues, the CB cavity is occupied by the charge-diffuse cationic region. The binding affinities and positions vary depending on the nature of the onium group as well as the substituents within the guest molecule. Host-guest complexes between CB and dicationic acetylcholinesterase inhibitors have very strong 1:1 binding affinities, with 2:1 binding being significantly weaker. These binding affinities are related to the nature of the cationic onium groups, and the length and hydrophobicity of the connecting linkers.
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