Early-time, beta-hairpin peptide self-assembly and hydrogelation: Structure, kinetics, and shear-recovery

Recently, there has been growing interest in the supramolecular self-assembly and hydrogelation of peptides for potential biomaterials applications. However, there has been limited work on the physicochemical characterization of these systems that will be crucial for the development of these materials for future applications. The main objective of this dissertation was to provide a solid understanding of the self-assembly kinetics, hydrogelation pathways and the physical origins of the shear-recovery behavior of a self-assembled, peptidic hydrogel system.;The MAX1 peptide, (VK)4-VDPPT-(KV)4-NH 2 is unfolded, and completely soluble in acidic to neutral aqueous solution. Increasing the pH or ionic strength of the solution triggers intramolecular peptide folding into beta-hairpins and concomitant intermolecular self-assembly into bilayered nanofibrils. Combined static and dynamic light scattering experiments revealed a direct transition from the initial, monomeric state to self-assembled nanofibrils, without an intermediate self-assembly step. The energy barrier associated with MAX1 self-assembly suggested that the self-assembly process involved intramolecular peptide folding events and cluster reorganization to facilitate intermolecular self-assembly. The assembly kinetics could be modeled using Smoluchowski's equation which indicated an essentially diffusion-limited assembly process. The analysis of early-stage dependence of apparent mass on size and the concentration dependence of assembly kinetics gave different fractal dimension values: The former indicated that the nanofibrils behaved locally as rigid rods, while the latter analysis suggested an increase in the apparent fractal dimension at larger length scales.;Cryogenic transmission electron microscopy indicated that the increase in the apparent fractal dimension could be attributed to the formation of branched clusters of well-defined (uniform, 3 nm cross section), semi-flexible, beta-sheet-rich nanofibrils. Dangling fibrils extended from one growing cluster to another and lead to early, intercluster communication in solution. At the apparent percolation threshold, the dynamic shear modulus measured by oscillatory rheology (G'(o),G"(o) infinityo n) and the field-intensity autocorrelation function measured by dynamic light scattering (g1 (tau) infinity tau --beta) showed power-law behavior with comparable critical dynamic exponents (n ≈ 0.47 and beta' ≈ 0.45). Finite interpenetration of percolating clusters with smaller clusters, along with permanent intercluster entanglements, increased the rigidity of the beta-sheet hydrogel network.;The network shear-thinned when high amplitude strain was applied, while the rigidity recovered immediately after removal of stress. Hydrogels were PEGylated immediately after shear-thinning to investigate the effects of interfibrillar interactions on the recovery behavior. The PEGylation prevented reformation of interfibrillar crosslinks and lead to self-avoiding, branched nanofibril clusters. Consequently, the PEGylated network did not recover after shear-thinning, implying a strong correlation between the lifetime of the interfibrillar interactions between branched, nanofibril clusters and the shear-recovery behavior. Considering the ethylene oxide chain length necessary for sufficient screening of interfibrillar crosslinks, it was estimated that these interactions operated at length scales comparable to the fibril cross-section. On the other hand, the recovery behavior could not be explained by reformation of defect-induced branches after shear-thinning. This was a significant finding for the potential applications of these materials for in vivo tissue regeneration applications.