Direct Observation of Early-Time Hydrogelation in beta-Hairpin Peptide Self-Assembly

Triggered hydrogelation of MAX1 peptide, (VK)(4)-V(D)PPT-(KV)(4)-NH(2), proceeds through peptide intramolecular folding into beta-hairpins and concomitant self-assembly into branched clusters of well-defined (uniform, 3 nm cross section), semiflexible, beta-sheet-rich nanofibrils. Cryogenic transmission electron microscopy indicates that dangling fibrils extend 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'(omega), G''(omega) proportional, variant omega(n)) and the field-intensity autocorrelation function measured by dynamic light scattering (g(1)(tau) proportional, variant tau(-beta')) show power-law behavior with comparable critical dynamic exponents (n approximately 0.47 and beta' approximately 0.45). Finite interpenetration of percolating clusters with smaller clusters, along with permanent intercluster entanglements, increase the network rigidity. The self-assembly of MAX1 peptide was compared and contrasted with the assembly of other biopolymeric networks in literature.