Concentration Dependence of a Hydrogel Phase Formed by the Deprotonation of the Imidazole Side Chain of Glycylhistidylglycine.

Upon deprotonation of its imidazole group at ∼pH 6, the unblocked tripeptide glycylhistidylglycine (GHG) self-assembles into very long crystalline fibrils on a 10-1000 µm scale which are capable of forming a volume spanning network, that is, hydrogel. The critical peptide concentration for self-assembly at a pH of 6 lies between 50 and 60 mM. The fraction of peptides that self-assemble into fibrils depends on the concentration of deprotonated GHG. While IR spectra seem to indicate the formation of fibrils with standard amyloid fibril ß-sheet structures, vibrational circular dichroism spectra show a strongly enhanced amide I' signal, suggesting that the formed fibrils exhibit significant chirality. The fibril chirality appears to be a function of peptide concentration. Rheological measurements reveal that the rate of gelation is concentration-dependent and that there is an optimum gel strength at intermediate peptide concentrations of ca. 175 mM. This paper outlines the unique properties of the GHG gel phase which is underlain by a surprisingly dense fibril network with an exceptionally strong modulus that make them potential additives for biomedical applications.

Formation of peptide-based oligomers in dimethylsulfoxide: identifying the precursor of fibril formation.

The well-studied dipeptide fluorenylmethyloxycarbonyl-di-phenylalanine (FmocFF) forms a rigid hydrogel upon dissolving in dimethylsulfoxide (DMSO) and dilution in H2O. Here, we explored the pre-aggregation of the peptide in pure DMSO by vibrational spectroscopies, X-ray powder diffraction and dynamic light scattering. Our results show an equilibrium between a dominant population of amorphous oligomers (on a length scale of 2 nm) and a small number of protofibrils/fibrils (on a length scale of 30 nm in the centimolar and of 200 nm in the sub-molar region). To probe the mechanism underlying the formation of these protofilaments, we measured the 1H-NMR, IR and visible Raman spectra of DMSO containing different FmocFF concentrations, ranging between 10 and 300 mM. Our data reveal that interpeptide hydrogen bonding leads to the self-assembly of FmocFF in the centimolar region, while π-π stacking between Fmoc-groups is observed above 100 mM. The high 3J(HNHCα) coupling constant of the N-terminal amide proton indicates that the Fmoc end-cap of the peptide locks the N-terminal residue into a conformational ensemble centered at a φ-value of ca. -120°, which corresponds to a parallel ß-sheet type conformation. The 3J(HNHCα) coupling constant of the C-terminal residue is indicative of a polyproline II (pPII)/ßt mixture. Our results suggest that the gelation of FmocFF caused by the addition of a small amount of water to DMSO mixtures is facilitated by the formation of disordered protofibrils in pure DMSO.

The tripeptide GHG as an unexpected hydrogelator triggered by imidazole deprotonation.

The tripeptide glycyl-histidyl-glycine (GHG) self-assembles into long, crystalline fibrils forming a strong hydrogel (G'∼ 50 kPa) above a critical concentration of 40 mM upon the deprotonation of its imidazole group. Spectroscopic data reveal a mixture of helically twisted ß-sheets and monomers to coexist in the gel phase.