Binding Of Small Guest Molecules In Cucurbiturils

Host-guest chemistry, the fundamental of supramolecular chemistry, involves the binding of a substrate molecule (guest) by a receptor molecule (host) through non-covalent forces (electrostatic interactions, hydrophobic interactions etc.) instead of covalent bonds to hold molecules together. The guest can be an organic or inorganic cation or anion, an ion pair, or a neutral organic molecule, whereas the host is usually a large molecule or aggregate such as an enzyme or synthetic macrocyclic compound possessing a size-selective central hole or cavity. Examples of the most studied host molecules are cyclodextrins, calixarenes, crown ethers, and in this thesis, cucurbiturils.Cucurbit[n]uril is a group of pumkin-shaped macrocycles consists of n glycoluril units that are linked by 2n methylene bridges. The significant increase of interest in the cucurbit[n]uril family benefited much from the successful preparation of four new homologues, CB[5], CB[7], CB[8], and CB[10]. In the recent decades, the big family was expanded with a variety of derivatives which represented marked differences in water solubility, guest selectivity and binding affinity. The research on cucurbiturils ranges over various aspects, e.g. molecular devices, self-assemblies, gas capture etc.Chapter 1 of this thesis is a brief review of the cucurbituril-based host-guest chemistry, including the traditional cucurbituril family and it derivatives. It begins with the introduction of the non-covalent interactions presented in the host-guest system and then continued with the strategies for measuring the thermodynamic properties. Afterwards, the binding behaviors of cucurbiturils towards cationic guests and dyes (in particular fluorescent dyes) and the related applications have been summarized, while basically focused on self-assemblies and tandem assays.In chapter 2, cucurbituril homologues (CB[n], n=5,6,7,8,10) are synthesized, purified via the routine method and the products are characterized with NMR and SCD. Internal standard method of NMR and UV titrations have been established to obtain the content of the corresponding cucurbiturl in the product.Chapter 3 has studied the binding affinity of various small gas molecules (involve the noble gases, the hydrocarbon gases and some diatomic gases) included in the CB[5] cavity. Binding constants of those gas molecules with CB[5] that obtained from indirect method are showing a preferential binding towards the spherical molecules with appropriate PC (?55%). Regardless the size effect, London dispersion interaction and hydrophobic interaction were revealed to be the crucial interactions within the gas-CB[5] complex.In chapter 4 we have investigated the salt effect on the formation of gas-CB[5] complex. More than ten kinds of salts composed of various cations and ions were selected for the test. It was beyond our expectation that nearly all the salts have positive influence on the capture of methane, with rubidium cation the best promoter among all. The only exception was sodium chloride, who had contrastively lowered the binding affinity. There was no obvious salt effect to the binding of ethane, except for the ammonium salt and the potassium salt.Chapter 5 has taken methyl orange and HBPB-8 as examples for the study of cucurbiturils on selecting the potential binding sites. Encapsulation of methyl orange in cucurbituril could promote the transformation to its quinoid structure through a host-assisted protonation mechanism. The depth of MO inside the cavity was determined by the electrostatic attraction or repulsion at the portal. HBPB-8, a gemini-type surfactant, owned alkyl chains of different length, the selective binding on these moieties has revealed the geometrical effect of the host-guest interactions.