Tuning the Aggregation of GHG by Changing Sample Concentration and pH

Abstract

Self-assembly of peptides is an important issue explored in biomedical, biophysical, and biomaterial research. Small peptides have been heavily researched due to their inherent biodegradability. Low molecular weight di- and tripeptides with aromatic residues and terminal groups have been found to self-assemble into gels. However, despite their aromatic character, short histidine-containing peptides were predicted to exhibit a low aggregation propensity. Contrary to this expectation, we discovered that unblocked GHG forms a hydrogel upon deprotonation of the imidazole side chain. Concomitantly, its apparent pK decreases due to a gelation induced stabilization of its deprotonated state. The apparent pK decreases further with increasing concentration. The sub-millimeter fibril length differs significantly from those of other hydrogel-forming peptides, which are generally on the sub-micrometer scale. We utilized rheology, circular dichroism, and vibrational spectroscopy to study the gelation process and the thermal stability of the gel. Rheology measurements yielded G’ values on the order of 105 kPa. As we increase the pH past the initial point of gelation, the rate of formation and the final G’ value increase. The enhanced circular dichroism of the amide I’ band is diagnostic of long and stable fibrils. The circular dichroism signal indicates anisotropy of the sample. Corresponding IR band profiles indicate a complex mixture of β-sheet and amorphous aggregate structures. Our results show that histidine can be used as a switch for the formation of large supramolecular structures even in very short peptides and that the properties of the gel can be tuned by altering the concentration and pH of the sample.