Determining How the Molecular Structure and Coassembly Interactions of Peptide Amphiphiles Influence the Energy Landscapes of Their Supramolecular Assembly

Peptide amphiphile (PA) molecules that incorporate a beta-sheet forming sequence supramolecularly assemble into micelles or one-dimensional fibers in solution. It has previously been discovered that changing the salt concentration of PAs alters their energy landscapes and results in supramolecular assemblies with changed structure and properties. Additionally, it is known that the molecular structure of the PA molecule affects the structure and properties of the supramolecular assemblies. This work investigates PAs with minor alterations to the beta-sheet forming sequence; first we looked into how these minor alterations affect the energy landscapes and then we looked into how coassembling these similar PAs at different proportions affects the energy landscapes.

Four PAs with minor differences in their beta-sheet forming sequence were characterized. The first PA consisted of a palmitoyl tail followed by three valines, three alanines, and three lysines in that order (V3). One edit had a missing valine (V2), one had an extra valine (V4), and the final edit had glycines instead of valines (G3). After characterizing the four PAs individually, V3 and V2 were coassembled at various proportions and characterized. The methods to probe the energetics of the various assembly states and the landscapes through which the states are connected have only been developed recently. The Nile Red assay was used to determine the transitions from monomers to micelles and from micelles to fibers. Circular Dichroism was used to observe transitions from beta sheets to random coil and vice versa. Dynamic Light Scattering was used to derive the energetic barrier between assemblies by fitting the rates of change of scattering counts at varying temperatures to an Arrhenius plot. Atomic Force Microscopy and cryo-Transmission Electron Microscopy were used to confirm morphology. Finally, computation simulations were compared with experimental results.

It was found that a missing valine resulted in a more dynamic assembly without sharp transitions in assembly. The beta-sheet strengths can be tuned by doping in small proportions of a stronger or weaker beta-sheet PA. We can now measure the transition beta-sheet strengths at which the supramolecular assembly changes. Interestingly, the coassembly of V3 and V2 resulted in thermodynamic and kinetically-trapped products that differed from the pure PAs. Finally, the results also suggested that small amounts of V3 seemed to stabilize the V2 fibers and that the V2 is likely nucleating off the V3 micelles or fibers. These findings will allow for more control in the structure and properties of self-assembling amphiphilic materials and could potentially lead to advancements in artificial extracellular matrix design.