Functionalised dipeptides as hydrogelators for energy transfer and as drug delivery vehicles

This thesis will cover aspects of functionalised dipeptide hydrogels and their application in energy transfer and as vehicles for drug delivery. In the first section, a large number of dipeptides conjugated to different aromatic groups were synthesised. We synthesised 35 dipeptides conjugated to different aromatic groups (naphthalene, anthracene, phenanthrol, anthraquinone, carbazole and pyrene). We synthesised a large number of dipeptides with different hydrophobicity and different aromatic groups in order to study their ability to form gels and study the mechanical properties of the gels. The second part of this thesis will investigate the formation of hydrogels based on dipeptides with different aromatic groups. The focus in this section was on the gelation as well as the effect of changing the solvent and changing the amino acids used. This section then explores the properties of the resulting hydrogels. A number of different dipeptides containing different amino acids were tested, some of which formed gels and others. The dipeptides also had different pKa values. This factor was shown to be important in driving the preferential selection of a certain amino acids. The thesis then describes energy transfer which can occur between two dipeptides (pyrene and anthracene dipeptides), or between a dipeptide and a dansyl derivative (phenanthrol and dansyl, or carbazole and dansyl). These results showed that energy transfers can occur in these specific hydrogels. In all other cases, no evidence for energy transfer was found. This may imply that the packing of the fibres is important for energy transfer and this should be the focus of future work. The final section of this thesis describes the controlled release of model dyes from these gels. We studied controlled release from FmocFF hydrogels and from one other functionalised dipeptide hydrogel at different gelator concentrations and at different pH. The release of the dye from the hydrogel can be controlled by different factors, including the pH, peptide concentration, the microstructure and the mesh size. Furthermore, choosing the right method to prepare hydrogel allows us to control the microstructure for hydrogel to be injectable. Therefore, by controlling these entire factors we can use these kinds of hydrogels for drug delivery applications

Using molecular rotors to probe gelation

A series of fluorescent probes, including a number of molecular rotors, have been used to follow the self-assembly of dipeptide-based low molecular weight gelators. We show that these probes can be used to gain an insight into the assembly process. Thioflavin T, a commonly used stain for β-sheets, appears to act as a molecular rotor in these gelling systems, with the fluorescence data closely matching that of other rotors. The molecular rotor was incorporated into an assay system with glucose oxidase to enable glucose-concentration specific gelation and hence generating a fluorescent output. Applying this system to urine from patients with various levels of glycosuria (a symptom of diabetes), it was found to provide excellent correlation with different clinical assessments of diabetes. This demonstrates a new concept in gelation-linked biosensing for a real clinical problem

Polymerization of low molecular weight hydrogelators to form electrochromic polymers

We present a method for the polymerization of low molecular weight hydrogelators to form polymers with unique structures. Carbazole-protected amino acids are shown to form hydrogels by self-assembly into fibrous structures. We show that is possible to directly electropolymerize the hydrogels. This results in the formation of microporous electrochromic polymers with distinctive structure. Polymers formed from the same gelator without the pregelation step show more compact structures. This method opens the possibility of creating polymers templated from pre-assembled gels that have the potential to be used in a wide range of applications

Probing gelation ability for a library of dipeptide gelators

Functionalised dipeptides are a class of interesting and useful low molecular weight hydrogelators. Here, we report a significantly expanded library of materials, including dipeptides conjugated to carbazole, phenanthracene, anthracene, pyrene and substituted naphthalenes. We assess the effect of using two different gelation methods; a pH-switch and a solvent switch on the gelation behaviour and properties of the dipeptides. Importantly, we investigate the relationship between the structure of these dipeptides and their ability to form gels. From an analysis of the gelation ability of all these dipeptides, it is clear that those containing a phenylalanine as either of the constituent amino acids are much more likely to lead to a gelator being formed as opposed to using non-aromatic amino acids only