Linking micellar phases to peptide supramolecular hydrogels

Abstract

Supramolecular peptide solutions and hydrogels are pathway-dependent multi-scale structured materials. This Thesis investigates three major aspects that impact on the self-assembly pathway. Firstly, the importance of the kinetics is investigated in the dipeptide gelators self-assembly pathway. Second, a proposed gelator-solvent phase diagram suggested a worm-like phase and an entangled worm-like micellar phase for N-protected dipeptide gelators. Thirdly, some links between hydrogel network structure and gelator solution phase properties were identified over multiple length scales for a particular class of peptide-based low molecular weight gelators (LMWG). These links allows some predictions to be made on the mechanical properties of peptide hydrogels triggered by salts based on the solution phase properties. A new method based on carbon dioxide acidification of specific gelator solutions formed unusual membrane hydrogels. This unusual heterogeneous hydrogel formation occured when the gelator’s apparent pKa was a pH unit close to the final pH and the starting gelator solution did not have a high viscous solution at the high pH (typically above pH 10.5). This membrane hydrogel phase had similar viscoelastic properties to the intermediary transition state previously found with pH-switch methods in bulk hydrogel formation (from high to low pH). The carbon dioxide method was also capable of forming bulk hydrogels for gelators with apparent pKa significantly above the final pH. This method was thoroughly investigated with 6-bromo-2-naphthalene-alanine-valine (BrNapAV). This research also focused on the first detailed phase diagram of an individual gelator solution phase, in this case of 2-naphthalene-diphenylalanine (2NapFF), an N-protected dipeptides over three orders of magnitude in concentration and between temperatures of 15 °C and 45 °C. The solution phase of 2NapFF was found to go through a range of micellar transformations with an increase in concentration from free-surfactant, spherical aggregate phase, worm-like micellar phase and packed worm-like micellar phase. The critical micellar concentrations (cmc’s), at which phase transitions occur, and the minimum gelator concentrations (mgc) with calcium nitrate salt solutions were found for 2NapFF. The common trends in the 2NapFF solution phase were extended to a library of 17 gelators. It was found that the 2NapFF peptide hydrogel phase is structurally connected to the corresponding solution phase. This allows prediction of the final properties of the Calcium-hydrogels (Ca-hydrogels) from the starting conditions of the corresponding peptide surfactant solutions, based on consideration of the solution phase diagram and self-assembly process. These results showed that the 2NapFF solutions could form Ca-hydrogels in a concentration from 0.02 wt% to 1.0 wt%, corresponding to three orders of magnitude in complex modulus. It was also found that the presence of worm like micelles in the solution phase was linked to mechanically stronger Ca-hydrogels. The gelation by addition of the calcium salt shifted the worm-like micellar concentration region and changed the microstructure to increase packing. The concentration was found to correlate with the mechanical properties with an exponential function with a 1.99 coefficient, typical for cross-linked networks and biopolymer gels. Finally, four types of microscopy techniques were used to conduct a structural analysis on multiple length scales with: optical microscopy, scanning electron microscopy, confocal microscopy and atomic force microscopy. A new open-source fibre tracking software was used on microscopy images and the structural parameters obtained were characterised by: fibre and worm diameter, bundle diameter, persistence length, contour length, nematic order, and type of fibre. These results suggest that microscopy interpretation of hierarchical structured materials has to be done for a specific length scale image, only relate to the features of length scale covered from the size that image to the resolution of the image. The Ca-hydrogel nanofibres in between a concentration of 0.01 and 1.0 wt% had the main nanofibre width of 20.5 ± 4.3 nm measured by SEM. There were also detectable fibres with an extended width from tenths of nanometres to few micrometres. Laser Scanning Confocal Microscopy (LSCM) measurements allowed a microstructural snapshot of the Ca-hydrogels. Additionally, LSCM identified that in solution phase no correlation is observable between the microstructure (persistence length of the fibre bundles) and the complex modulus G*, while for the Ca-hydrogel phase, the persistence length of the nanofibre bundles increases with the increase G*. The worm-like structures were found to be highly oriented in the solution phase across concentrations from 0.1 wt% to 1.0 wt%. In the Ca-hydrogel phase, the degree or oriented structures increased from 0.05 wt% to 1.0 wt%.