Accessing Highly Tunable Nanostructured Hydrogels in a Short Ionic Complementary Peptide Sequence via pH Trigger.

Creating diverse nanostructures from a single gelator through modulating the self-assembly pathway has been gaining much attention in recent years. To this direction, we are exploring the effect of modulation of pH as a potential self-assembly pathway in governing the physicochemical properties of the final gel phase material. In this context, we used a classical nongelator with the ionic complementary sequence FEFK, which was rationally conjugated to an aromatic group naphthoxyacetic acid (Nap) at the N-terminal end to tune its gelation behavior. Interestingly, the presence of oppositely charged amino acids in the peptide amphiphile resulted in pH-responsive behavior, leading to the formation of hydrogels over a wide pH range (2.0-12.0); however, their structures differ significantly at the nanoscale. Thus, by simply manipulating the overall charge over the exposed surface of the peptide amphiphiles as a function of pH, we were able to access diverse self-assembled nanostructures within a single gelator domain. The charged state of the gelator at the extreme pH (2.0, 12.0) led to a thinner fiber formation, in contrast to the thicker fibers observed near the physiological pH owing to charge neutralization, thus promoting the lateral association. Such variation in molecular packing was found to be further reflected in the variable mechanical strengths of the peptide hydrogels obtained at different pH values. Moreover, the gelation of the peptide at physiological pH offers an additional advantage to explore this hydrogel as a cell culture scaffold. We anticipate that our study on controlling the self-assembly pathway of the ionic complementary peptide amphiphile can be an elegant approach to access diverse self-assembled materials, which can expand the zone of its applicability as a stimuli-responsive biomaterial.