Relationship between molecular structure, gelation behaviour and gel properties of Fmoc-dipeptides

We have investigated the influence of molecular structure on the gelation behaviour of a range of Fmoc-dipeptides. Gelation is triggered using glucono-delta-lactone, which controllably lowers the pH, allowing the gel properties to be followed with time. This was observed in terms of the development of the gel strength using rheology. In general, we find that the ability to form a stable gel is determined by the overall hydrophobicity of the Fmoc-dipeptide. For the Fmoc-dipeptides investigated at concentrations of 14.62 mM, gels that undergo syneresis are formed for those that are less hydrophobic (log P < 2.8). At intermediate hydrophobicities (2.8 < log P < 5.5), self-supporting gels are formed at pH 4. However, the different dipeptides assemble at different rates, possibly due to differences in pK(a) (arising from the different hydrophobicities of the dipeptides) and form gels of a variety of stiffnesses. Two Fmoc-dipeptides (FmocLG and FmocFG) were studied in more detail. The dependence of the gel modulus on concentration was investigated and found to approximate a 1.4 power law, which is unusually low for such materials. The behaviour of the gels when subjected to large deformation compression testing was also unusual. At sufficiently high compression rates, the gel response is brittle in nature, showing an initial elastic region with a Young's modulus consistent with the measured shear moduli and failure occurring at 3-5% strain. However, at lower compression rates, water is expressed from the gel, resulting in its irreversible compaction. Such behaviour is consistent with the idea that the gels are composed of networks of rigid rod-like structures, which, if deformed beyond their elastic limit will not recover their original form.