Synthesisand Characteristics Research Of Low Molecular Gelators Based On Cyclo(L-Lys-L-Lys)

Low-molecular-weight gels are generally constructed by means of supramolecular interactions of low-molecular-weight gelators?LMWGs?self-assembling into 3D network structure to trap a vast amount of solvent molecules.Compared to macromolecular gels,the low-molecular-weight gels are more sensitive to the environmental stimuli,and therefore,they have recently attracted growing attention in the area of solar cells,chemical sensors,three-dimensional cell culture and tissue repairing materials,recovery of leaking petroleum and organic dye in waste water,etc.Cyclic dipeptides?DKPs?or 2,5-diketopiperazine deri-vatives,which are widespread in nature with a variety of structures and pharmacological activities,favor the one-dimensional self-assembly in solvents and water due to the unique DKP ring structure.So DKP structure can be used as an ideal“synthon”for the synthesis of LMWGs.As a result,a series of LMWGs based on the lysine cyclic dipeptide were designed and synthesized,their gelation properties and the driving forces during the self-assembling process were investigated,and the corresponding models of aggregates were also proposedin this dissertation.Firstly,Fmoc and Boc disubstituted cyclo?L-Lys-L-Lys?s were synthesized using Boc-Lys?Cbz?-OH as starting material to prepare lysine cyclic dipeptide followed by the reaction with Fmoc-OSu and?Boc?₂O,respectively.Using test tube inversion method,two derivatives were found to form stable organogels in common alcoholic,methylbenzene and chlorinated solvents.The TEM and SEM observations of their xerogels showed that these organogelators were self-assembled into 3D network structures with the morphologies of nanofibres,nanoribbons or nanosheets.The thinner the fiber diameter was,the more transparent the organogel became.The rheological measurements revealed that the storage modulus of two kinds of gels was higher than the loss modulus,and the complex viscosity was reduced linearly with the increase of scanning frequency.FT-IR,fluorescence spectroscopy and ¹H NMR were used to explore the driving force during the self-assemble process of the gelators with different protecting groups.The results showed?-?stacking generally acts as a driving force for the self-assembly process in alcoholic solvents when there is a Fmoc in the gelator,wherehydrogen bonds playa mainrole in driving the self-assembly process of gelators in Boc disubstituted cyclo?L-Lys-L-Lys?.Secondly,Fmoc and Boc mono-substituted cyclo?L-Lys-L-Lys?s were synthesized to improve the hydrophilicity,which enabled to form stable thermo-reversible organogels in common alcoholic,substituted benzene and chlorinated solvents.As a pre-hydrogelator,the Fmoc mono-substituted cyclo?L-Lys-L-Lys??FL-NH₂?can form partial hydrogel,and the?-?stacking interactions appear the driving force of the self-assembly in organic solvents.Meanwhile,the Boc mono-substituted cyclo?L-Lys-L-Lys??BL-NH₂?is self-assembled into nanotubes by stacking of multiple bilayer membranes.As these nanotubes self-assembled possess a well-defined hollow cylindrical morphology,they showed the great potential in the fields of microencapsulation and controlled release of protein and polypeptide drugs.Finally,cyclic anhydrides were used to modify FL-NH₂ via the acrylation of side?-amino to change NH₂ into-COOH,and a study on the influence of the aliphatic chain length of cyclic anhydrides on the gelation performance was carried out.In particular,the glutaric anhydride acrylated derivative?FL-Ga?exhibited the better hydrogel ability than succinic anhydride and adipic anhydride,and the hydrogen bond and?-?stacking intera-ctions play a cooperative role during the self-assembly of nanofibers.Based on XRD patterns of the xerogel prepared from the hydrogel of FL-Ga and Rietveld method,and a possible molecular packingmodel of single layer membranewas proposed.