Influence of peptide chirality on their protein-triggered supramolecular hydrogelation

Many articles describe the use of enzymes to induce the formation of a supramolecular hydrogel. These enzymes catalyze the transformation of water-soluble precursors, often short peptides, into hydrogelators. The use of non-enzymatic proteins to induce or stabilize peptide self-assembly is a rarely reported phenomenon, which raises fundamental questions: how can a protein induce peptide self-assembly? How is the peptide recognized and how does it, or the peptide assembly, interact with the protein? The heptapeptide Fmoc-GFFYE-NH-(CH₂)₂-s-s-(CH₂)₂-NH-CO-(CH₂)₂-CO-EE-OH, called L-1 (L = natural chiral amino acids), is a water-soluble compound leading to an increasingly viscous solution over time due to the formation of nanofibers, but does not result in hydrogel (at least not within 3 months). When bovine serum albumin (BSA) is added to a freshly prepared solution of L-1, a hydrogel is obtained in less than 10 min. The variation in the L-1/BSA ratio has an impact on the gelation rate and the mechanical properties of the resulting hydrogel. Thus, the protein appears to act as (i) a catalyst and (ii) a cross-linking point. Strikingly, if the enantiomer D-1 (D = unnatural chiral amino acids) is used instead of L-1, the mixture with BSA remains liquid and non-viscous. Similar behavior is also observed for other proteins. Spectroscopic analyses (CD, fluorescence) and electronic microscopy images confirm that the L-1 peptide self-assembles in nanofibers of 10 nm diameter through β-sheet organization, which is not the case for the peptide D-1. A molecular dynamics study shows that BSA is capable of interacting with both enantiomer peptides L-1 and D-1. However, interaction with L-1 tends to unfold the peptide backbone, making the interaction with the protein more stable and promoting the assembly of L-1 peptides. Conversely, the interaction between BSA and D-1 is more dynamic and appears to be less spatially localized on the BSA. Furthermore, in this interaction, the D-1 peptide keeps its globular conformation. These results highlight the impact of a short peptide's chirality on protein-triggered supramolecular hydrogelation.