Self-assembly of beta-sheets into nanostructures by poly(alanine) segments incorporated in multiblock copolymers inspired by spider silk.

Selective replacement of the amorphous peptide domain of a spider silk with poly(ethylene glycol) gave N. clavipes silk-inspired polymers having similar solid-state structures and very good mechanical properties. The tendency of poly(alanine) having appropriate chain length to form beta-sheets and the facility with which the beta-sheets self-assemble have been retained in the polymers. Solid-state (13)C NMR, solid-state FTIR, X-ray diffraction, and AFM studies showed that the polymers formed predominantly antiparallel beta-sheets that self-assembled into discrete nanostructures. The longer the peptide segment was, the greater was the tendency to self-assemble into antiparallel beta-sheet aggregates. AFM revealed that the morphology of the polymers was a microphase-separated architecture that contained irregularly shaped 100-200 nm poly(alanine) nanodomains interspersed within the PEG phase. The results suggest that the poly(alanine) domain influences the solid-state properties of spider silk through beta-sheet self-assembly into temporary cross-links. The results further demonstrate that by selectively replacing certain segments of a naturally occurring biopolymer with a judiciously selected nonnative segment while, at the same time, retaining other segments known to be critical for the essential properties of the native biopolymer, a synthetic polymer with similar properties and function can be obtained.