The effect of solvent choice on the gelation and final hydrogel properties of Fmoc–diphenylalanine

Gels can be formed by dissolving Fmoc–diphenylalanine (Fmoc–PhePhe or FmocFF) in an organic solvent and adding water. We show here that the choice and amount of organic solvent allows the rheological properties of the gel to be tuned. The differences in properties arise from the microstructure of the fibre network formed. The organic solvent can then be removed post-gelation, without significant changes in the rheological properties. Gels formed using acetone are meta-stable and crystals of FmocFF suitable for X-ray diffraction can be collected from this gel

Linking micellar structures to hydrogelation for salt-triggered dipeptide gelators

Some functionalised dipeptides can form hydrogels when salts are added to solutions at high pH. We have used surface tension, conductivity, rheology, optical, confocal and scanning electron microscopy, 1H NMR and UV-Vis spectroscopy measurements to characterise fully the phase behaviour of solutions of one specific gelator, 2NapFF, at 25 °C at pH 10.5. We show that this specific naphthalene–dipeptide undergoes structural transformations as the concentration is increased, initially forming spherical micelles, then worm-like micelles, followed by association of these worm-like micelles. On addition of a calcium salt, gels are generally formed as long as worm-like micelles are initially present in solution, although there are structural re-organisations that occur at lower concentrations, allowing gelation at lower than expected concentration. Using IR and SANS, we show the differences between the structures present in the solution and hydrogel phases

The importance of the self-assembly process to control mechanical properties of low molecular weight hydrogels

Hydrogels can be formed by the self-assembly of certain small molecules in water. Self-assembly occurs via non-covalent interactions. The self-assembly leads to the formation of fibrous structures which form the matrix of the gel. The mechanical properties of the gels arise from the properties of the fibres themselves (thickness, persistence length etc.), the number and type of cross-links and also how the fibres are distributed in space (the microstructure). We discuss here the effect of assembling the molecules under different conditions, i.e. the self-assembly process. There is sufficient literature showing that how the molecules are assembled can have a significant effect on the properties of the resulting gels

Drop-casting hydrogels at a liquid interface: the case of hydrophobic dipeptides

Hydrophobic dipeptide molecules have been induced to self-assemble into thin interfacial films at the air–water interface via drop-casting. The mechanism involves fiberlike strands, which exist in the high-pH spreading solvent, becoming intertwined at the surface of a low-pH subphase. Atomic force microscopy (AFM) reveals that the strands are ∼40 nm wide and ∼20 nm high and are woven together to form layers that can be up to ∼800 nm thick. The use of Thioflavin T (ThT) fluorescence suggests that the dipeptides are ordered in a β-sheet configuration irrespective of whether they form an interfacial film, while Fourier transform infrared spectroscopy (FTIR) shows the protonation effect for those which do form an interfacial film. The entanglement between protonated strands results in the formation of an elastic sheet. The interfacial films buckled under compression in a Langmuir trough and have the ability to convey long-term stability to large air bubbles