Tuning Supramolecular Rigidity of Peptide Fibers through Molecular Structure

Abstract

We synthesized a series of peptide amphiphiles (PAs) with systematically modified amino acid sequences to control the mechanical properties of the nanofiber gels they form by self-assembly. By manipulating the number and position of valines and alanines in the peptide sequence we found that valines increase the stiffness of the gel while additional alanines decrease the mechanical properties. Vitreous ice cryo-transmission electron microscopy shows that all PA molecules investigated here form nanofibers 8–10 nm in diameter and several micrometers in length. We found through Fourier transform IR experiments a strong correlation between gel stiffness and hydrogen bond alignment along the long axis of the fiber. Molecules that form supramolecular structures with the highest mechanical stiffness were found by circular dichroism to self-assemble into β-sheets with the least amount of twisting and disorder, a result which is consistent with IR experiments. Molecular control of mechanical stiffness in three-dimensional artificial peptide amphiphile matrices offers a chemical strategy to control biological phenomena such as stem cell differentiation and cell morphology.

Tunable Pentapeptide Self‐Assembled β‐Sheet Hydrogels

“…Comparing IDIDI (1 wt %) and D2I3 (2 wt %) hydrogels, the G′ value increased by two orders of magnitude from 2 to 200 kPa, respectively. These stiffness values are in the region of many soft tissues and compare well to previously published peptide hydrogel systems, including aromatic peptides4, 17 and peptide–amphiphile hydrogels 27, 28. The ability to tune the G′ value across a large range holds great promise for applications in tissue engineering, given that the behavior of cells has been found to be heavily influenced by the mechanical properties of their surrounding environment 29, 30…”

“…However, at 2 wt % the D2I3 sequence was significantly stiffer and had the greatest red‐shift in the CD spectra at this concentration (Figure 3 A,B). Previous studies have suggested that a red‐shift in the CD spectra of β‐sheets is representative of more twisted and distorted arrangements 28, 36, 37. The degree of twisting of β‐sheets is centered around the middle of the sequence 34.…”

“…The CD signatures of β‐sheets are known to have greater variability than those of other peptide secondary structures 28. β‐Sheets have both significant intermolecular and intrastrand hydrogen bonding 35.…”

“…β‐Sheets have both significant intermolecular and intrastrand hydrogen bonding 35. Furthermore, peptides can form antiparallel, parallel, or mixed β‐sheets, which will influence both the strands in the assemblies as well as the networks they form 28. We analyzed the relative red‐shifts in the CD minima of the different hydrogels.…”

“…The FTIR spectra for the D2I3 and DI3D sequences had two minor peaks at 1655 and 1675 cm −1 (see Figures S6 B and S7 B). It has been shown that twisted β‐sheets (both in parallel and antiparallel conformations) can display an amide I splitting with a peak between 1680 and 1690 cm −1 as well as a peak at 1650 cm −1 28, 34. Although the D2I3 and DI3D sequences in this study cannot be explicitly defined as being in an antiparallel or a parallel orientation, these observations are in agreement with the red‐shifted CD spectra found, which suggests that both the D2I3 and DI3D hydrogels contain more twisted β‐sheets.…”