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

Tuning the thermostability of GHG gels by salts at different positions on the Hofmeister scale

The influence of Hofmeister cations (NH4+, Na+, Mg2+) and anions (H2PO4-, CH3COO-, Cl-, NO3-) on the thermostability of a GHG hydrogel was investigated. The combined results of UV circular dichroism (UVCD) and Small Amplitude Oscillatory Shear Rheology experiments reveal that the addition of salt reduces the stability of the gel phase and the underlying fibrils. In line with the cationic Hofmeister hierarchy, the chaotropic Mg2+ ions caused the greatest thermal destabilization of the gel phase with the gel → sol transition temperature Tgs value lowered by 10 °C. In the absence of salt, the gel → sol transition probed by the storage modulus and microscopy is biphasic. In the presence of salt, it becomes monophasic. Contrary to expectations the presence of Hofmeister anions leads to a nearly identical reduction of the gel → sol transition temperatures. However, UVCD spectra suggest that they affect the ππ-stacking between imidazole groups to a different extent. We relate the absence of ion specificity regarding the solubility of fibrils (probed by UVCD) to the observed enthalpy-entropy compensation of the dissolution process. Our results combined show how CD spectroscopy and rheology combined yields a more nuanced picture of the processes underlying the gel → sol transition.

Determining the nanostructure and main axis of gly-his-gly fibrils using the amide I' bands in FTIR, VCD, and Raman spectra

The zwitterionic tripeptide glycyl-histidine-glycine (GHG) has been shown to self-assemble into visible crystalline fibrils that form a gel-supporting network with a very high storage modulus. Here we elaborate on the theory and experimental setup behind our novel approach employed to determining the main fibril axis for these gel-forming fibrils by simulating the amide I band profile for infrared absorption (IR), vibrational circular dichroism (VCD), and visible Raman scattering. We also highlight that combining these three vibrational spectroscopies can help in validating structures that are solved using powder x-ray diffraction analysis (PXRD). The PXRD analysis yielded a GHG fibril unit cell with P21 symmetry containing two peptide monomers and two water molecules. The monomers adopt a conformation reminiscent of the distorted polyproline II conformation obtained for tri-lysine in aqueous solution. Stabilization occurs primarily through peptide-peptide intermolecular hydrogen bond interactions, while the role of water in peptide hydration is minimal. The comparison of simulated and experimental amide I' band profiles suggests that the xz plane of the crystal unit cell is being predominantly probed in the experimental IR and VCD spectra, with the x axis of the unit cell pointing in the direction of the main fibril axis. The monomer peptide in the unit cell interacts with six adjacent peptides forming hydrophobic channels by edge-to-face and parallel-displaced ππstacking in the y direction. These cores are further stabilized by a plethora of intermolecular interactions in the x and z directions. Our result suggests that the hydrophobic xz-surfaces would be a good target for the adsorption of hydrophobic drugs.

Influence of central sidechain on self-assembly of glycine-x-glycine peptides

Low molecular weight gelators (LMWGs) are the subject of intense research for a range of biomedical and engineering applications. Peptides are a special class of LMWG, which offer infinite sequence possibilities and, therefore, engineered properties. This work examines the propensity of the GxG peptide family, where x denotes a guest residue, to self-assemble into fibril networks via changes in pH and ethanol concentration. These triggers for gelation are motivated by recent work on GHG and GAG, which unexpectedly self-assemble into centimeter long fibril networks with unique rheological properties. The propensity of GxG peptides to self-assemble, and the physical and chemical properties of the self-assembled structures are characterized by microscopy, spectroscopy, rheology, and X-ray diffraction. Interestingly, we show that the number, length, size, and morphology of the crystalline self-assembled aggregates depend significantly on the x-residue chemistry and the solution conditions, i.e. pH, temperature, peptide concentration, etc. The different x-residues allow us to probe the importance of different peptide interactions, e.g. π-π stacking, hydrogen bonding, and hydrophobicity, on the formation of fibrils. We conclude that fibril formation requires π-π stacking interactions in pure water, while hydrogen bonding can form fibrils in the presence of ethanol-water solutions. These results validate and support theoretical arguments on the propensity for self-assembly and leads to a better understanding of the relationship between peptide chemistry and fibril self-assembly. Overall, GxG peptides constitute a unique family of peptides, whose characterization will aid in advancing our understanding of self-assembly driving forces for fibril formation in peptide systems.

Forbidden Secondary Structures Found in Gel-Forming Fibrils of Glycylphenylalanylglycine

The zwitterionic l-tripeptide glycylphenylalanylglycine self-assembles into very long crystalline fibrils in an aqueous solution, which causes the formation of an exceptionally strong gel phase (G' ∼ 5 × 10⁶ Pa). The Rietveld refinement analysis of its powder X-ray diffraction (PXRD) pattern reveals a unit cell with four peptides forming a P2₁2₁2₁ space group and adopting an inverse polyproline II conformation, that is, a right-handed helical structure that occupies the "forbidden" region of the Ramachandran plot. This unusual structure is stabilized by a plethora of intermolecular interactions facilitated by the large number of different functional groups of the unblocked tripeptide. Comparisons of simulated and experimental Fourier transform infrared and vibrational circular dichroism (VCD) amide I' profiles corroborate the PXRD structure. Our experimental setup reduces the sample to a quasi-two-dimensional network of fibrils. We exploited the influence of this reduced dimensionality on the amide I VCD to identify the main fibril axis. We demonstrate that PXRD, vibrational spectroscopy, and amide I simulations provide a powerful toolset for secondary structure and fibril axis determination.