| Click reaction, firstly proposed by Sharpless et al., was defined as reaction with characteristics of modular, wide in scope, very high yield, without offensive byproducts, and stereospecific. From the perspectives of reaction setup and workup, click chemistry requires simple reaction conditions (the process is oxygen/water-inert), readily available starting materials and reagents, benign solvent (e.g., water), and simple product isolation. Due to its inherent advantages, click reaction has been widely used in biological imaging, material preparation, biomedicine, etc.Hydrogels, a class of "jelly-like" soft matters, contain lots of micropores filled with water. Currently hydrogels can be divided into two types:polymer hydrogels and supramolecular hydrogels. Polymer hydrogels, composed of polymers with covalent bonds, have been widely used in materials fabrication and long-term drug delivery given their excellent mechanic properties. But the main issues existing for polymer hydrogels are their bad biocompatibility because of the toxic reactants introduced in syntheses and difficulty in degradation. Supramolecular hydrogels, formed through noncovalent interactions (i.e., hydrogen bond,π-π interaction, electrostatic interaction, etc.), with weak mechanical strength, have been widely used in drug delivery, wound healing and tissue engineering because of their good biocompatibility.Hydrogelation process could be activated by changing the interaction between the hydrogelators and water (solvent) molecules by changing temperature (heat up-cool down), pH and solvent. Click reaction assisted in situ prepraration of hydrogelators is based on the same principle. Hydrophilic precursors can click to form amphiphilic hydrogelators triggered by additive, and hydrogelators self-assemble in water to form supramolecular hydrogels. Unlike traditional reactions, which usually require harsh conditions, nonaqueous solvents and produce byproducts, click reaction has properties of highly selectivity, no offensive byproducts, and could process in water, which provides perfect condition for prepraration of hydrogelators in situ.In Chapter II, click chemistry is applied in the development of oligomeric supramolecular hydrogels. Polymer hydrogels and small-molecule-based (SMB) supramolecular hydrogels have been widely explored. But oligomeric hydrogels have remained a challenge because of the synthetic difficulties of the oligomers and control of their amphiphilicities. Based on CBT-Cys click reaction (CBT= 2-cyano-6-aminobenzothiazole), we designed two small-molecular precursors Cys(SEt)-Lys-CBT and (Cys-Lys-CBT)2. Glutathione reduction of these two precursors yields the same gelator Cys-Lys-CBT, which condenses with each other to yield amphiphilic cyclic oligomers. The oligomers self-assemble into nanofibers and form oligomeric hydrogels at a suitable pH condition. Inverted tube test and fluorescence showed that the two hydrogels have similar critical gelation concentration and critical micelle concentration. Rheology study showed they have similar mechanic properties in the range of polymer hydrogels, which are stronger than those of most SMB supramolecular hydrogels. Besides, similar nanofibers can be observed by cryo-transmission electron microscopy (cryo-TEM). Chemical analyses by high performance liquid chromatography (HPLC) and matrix-assisted laser desorption/ionization-mass spectroscopy (MALDI-MS) indicated that the major condensation product in both two hydrogels is a cyclic dimer. Moreover, the second hydrogel contains more higher-order oligomers (e.g., trimer, tetramer, etc.). The proposed molecular arrangement of dimers conforms the observed nanofibers under cryo-TEM, which also echoes the conclusion that the oligomeric hydrogels are mainly composed of dimers. Considering thier biocompatibility, biodegradability as supramolecular hydrogels, and optimal mechanical strength similar to that of polymer hydrogels, we believe that our oligomeric hydrogels might be useful for long-term drug delivery in the future.In Chapter III, another click reaction is applied in hydrogelation triggered by methylglyoxal (MGO). Methyglyoxal (MGO), a reactive dicarbonyl small molecule, is tightly associated with diabetes, dysfunction of the proteins and inflammation. In cells, glyoxalase I (GLOI) catalyzes the formation of S-D-lactoylglutathione from MGO and glutathione (GSH), thus regulating the detoxification of MGO. As MGO can react with o-phenylenediamine (OPD) through click chemistry forming methylquinoxaline (MQ), hydrophlilic precursor OPD-Phe-Phe-OH was designed to react with MGO and yielded amphiphilic MQ-Phe-Phe-OH, which self-assembled into supramolecular hydrogel. Cryo-TEM images of the hydrogel suggested that there existed two orders of self-assembly to form the 32.8 nm-width-nanotubes in the hydrogel:the hydrogelator self-assemble into nanofibers, which then self-assemble to form nanotubes. The hypothesis was validated with the analyses of the fluorescence, transmittance, and circular dichroism data of the serial dilutions of the hydrogel. Interference tests indicated that hydrogelation of the precursor with MGO would not be affected by nitric oxide (NO), although NO can also react with OPD. Our results suggest that our precursor could be applied for specific hydrogelation with MGO, and potentially the visible detection and removal of MGO in vitro.In Chapter IV, click reaction is applied in an on-going project, the development of assembly/disassembly, which can be used for magnetic resonance imaging. Caspase-3 (Casp3), an important protease of caspase family, is tightly linked to cell apoptosis. Magnetic resonance (MR) imaging is a popular method in experimental molecular imaging and clinical radiology because of its 3D interrogation at cellular resolution. Gadolinium (Gd3+)-based contrast agents can reduce the spin-lattice relaxation times of nearby water to increase the signal from these protons, and make the effected voxel seem "brighter" in Ti-weighted image. We studied the click reaction between aldehyde and cysteine, and aldehyde can click react with 1,2-aminothiol of cysteine to form thiazolidine, which can be cleaved by methoxamine to form 1,2-aminothiol again. HPLC tracking of this click reaction indicated it is a second-order reaction with a rate constant of 4.58×10-2 M-1s-1. Based on these data, we rationally designed two probes Ac-Asp-Glu-Val-Asp-Cys(StBu)-HQC and Ac-Asp-Glu-Val-Asp-Cys(StBu)-Lys(DOTA-Gd)-HQC (HQC= 8-Hydroxy-2-quinolinecarboxaldehyde) which are forecasted to self-assemble upon reduction and Casp3 cleavage. Reversibly, the as-formed self-assembly can be cleaved by methoxamine, which will in turn lead to disassembly. As the second probe consists DOTA-Gd, a Gd (Ⅲ) chelate complex, it may assist signal change in Ti-weighted image during the process of self-assembly and disassembly. The project is under further exploration.The dissertation aims at using click reaction to prepare hydrogelators of supramolecular hydrogels in situ, which can lead specific hydrogelation triggered by additives. The research offers new directions for the efficient preparation of supramolecular hydrogels. |
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