Ambidextrous helical nanotubes from self-assembly of designed helical hairpin motifs.

Tandem repeat proteins exhibit native designability and represent potentially useful scaffolds for the construction of synthetic biomimetic assemblies. We have designed 2 synthetic peptides, HEAT_R1 and LRV_M3Δ1, based on the consensus sequences of single repeats of thermophilic HEAT (PBS_HEAT) and Leucine-Rich Variant (LRV) structural motifs, respectively. Self-assembly of the peptides afforded high-aspect ratio helical nanotubes. Cryo-electron microscopy with direct electron detection was employed to analyze the structures of the solvated filaments. The 3D reconstructions from the cryo-EM maps led to atomic models for the HEAT_R1 and LRV_M3Δ1 filaments at resolutions of 6.0 and 4.4 Å, respectively. Surprisingly, despite sequence similarity at the lateral packing interface, HEAT_R1 and LRV_M3Δ1 filaments adopt the opposite helical hand and differ significantly in helical geometry, while retaining a local conformation similar to previously characterized repeat proteins of the same class. The differences in the 2 filaments could be rationalized on the basis of differences in cohesive interactions at the lateral and axial interfaces. These structural data reinforce previous observations regarding the structural plasticity of helical protein assemblies and the need for high-resolution structural analysis. Despite these observations, the native designability of tandem repeat proteins offers the opportunity to engineer novel helical nanotubes. Moreover, the resultant nanotubes have independently addressable and chemically distinguishable interior and exterior surfaces that would facilitate applications in selective recognition, transport, and release.

Self-Assembly of an α-Helical Peptide into a Crystalline Two-Dimensional Nanoporous Framework.

Sequence-specific peptides have been demonstrated to self-assemble into structurally defined nanoscale objects including nanofibers, nanotubes, and nanosheets. The latter structures display significant promise for the construction of hybrid materials for functional devices due to their extended planar geometry. Realization of this objective necessitates the ability to control the structural features of the resultant assemblies through the peptide sequence. The design of a amphiphilic peptide, 3FD-IL, is described that comprises two repeats of a canonical 18 amino acid sequence associated with straight α-helical structures. Peptide 3FD-IL displays 3-fold screw symmetry in a helical conformation and self-assembles into nanosheets based on hexagonal packing of helices. Biophysical evidence from TEM, cryo-TEM, SAXS, AFM, and STEM measurements on the 3FD-IL nanosheets support a structural model based on a honeycomb lattice, in which the length of the peptide determines the thickness of the nanosheet and the packing of helices defines the presence of nanoscale channels that permeate the sheet. The honeycomb structure can be rationalized on the basis of geometrical packing frustration in which the channels occupy defect sites that define a periodic superlattice. The resultant 2D materials may have potential as materials for nanoscale transport and controlled release applications.