Self-assembly of giant peptide nanobelts

Abstract

Many alkylated peptide amphiphiles have been reported to self-assemble into cylindrical nanofibers with diameters on the order of a few nanometers and micrometer scale lengths; these nanostructures can be highly bioactive and are of great interest in many biomedical applications. We have discovered the sequences for these molecules that can eliminate all curvature from the nanostructures they form in water and generate completely flat nanobelts with giant dimensions relative to previously reported systems. The nanobelts have fairly monodisperse widths on the order of 150 nm and lengths of up to 0.1 mm. The sequences have an alternating sequence with hydrophobic and hydrophilic side chains and variations in monomer concentration generate a "broom" morphology with twisted ribbons that reveals the mechanism through which giant nanobelts form. Interestingly, a variation in pH generates reversibly periodic 2 nm grooves on the surfaces of the nanobelts. With proper functionalization, these nanostructures offer a novel architecture to present epitopes to cells for therapeutic applications.

Figure 1

Giant (ultralong and wide) nanobelts assembled from a peptide amphiphile containing four amino acids and an alkyl tail. a, chemical structure of the peptide amphiphile. b–d, AFM images of peptide nanobelts at different scanning sizes. The assembled nanobelts are the dominant structures in the assembly system (almost artifact free). e and f, AFM images of a single-layer and a double-layer nanobelt morphology. g, AFM amplitude image of f. h, CD spectrum of the peptide nanobelt solution at a concentration of 0.05 wt % proves the existence of β-sheet secondary structure in the supramolecular assemblies.

Figure 2

Cryo-TEM images of nanobelt morphology and small-angle neutron scattering profiles of nanobelt solutions. a, Cryo-TEM image shows that the nanobelts can flip, tilt and entangle with other nanobelts in solution. b, Cryo-TEM image reveals the mechanical flexibility of nanobelts. c, Schematic representation of the origin of contrast in cryo-TEM images. In the current system, the dominant contrast mechanism arises from the mass-thickness contrast associated with different tilt angles of the nanobelt morphology. When the nanobelt is tilted at a right angle, electrons travel the longest distance inside the nanobelt, and thus have the highest possibility to be scattered. d, SANS profiles at different nanobelt concentrations. The similar shape of three scattering profiles suggests a stable nanobelt morphology over the concentration range of 0.1 wt %, 0.5 wt % and 1.0 wt %.

Figure 3

Grooved nanobelt morphology produced at high pH. a, TEM image of nanobelts in a 0.1 wt % solution. b, Grooved nanobelts in a 0.1 wt % solution containing 2 mM NaOH. The parallel nanochannels can be clearly seen in the image. c, A closer view of the grooved nanobelts in b. d, Schematic representation of the molecular packing inside the nanobelts and the grooved nanobelts. All the TEM samples were negatively stained with 2% (w/v) uranyl acetate aqueous solution.

Figure 4

Twisted nanoribbons at 0.01 wt % aqueous solution and intermediate structures (broom morphology) of nanobelts transforming into twisted nanoribbons at 0.05 wt % solution. a–b, Narrower nanobelts and twisted nanoribbons are observed at a concentration of 0.01 wt %. The twist pitch increases with an increase of nanoribbon width. c–f, Twisted nanoribbons sprouting from one nanobelt end. d, A closer view of c. There is a gradual transition from flat nanobelt to twisted nanoribbons. The longer the distance from the wide nanobelt, the more likely the nanoribbons will twist in their natural states. g, Nanobelts split from both ends into narrower nanobelts. The split ribbons are too short to be twisted because they are too close to the wide nanobelt. g, Schematic representation of the morphological transitions with a change in concentration. Scale bars of Fig c–g: 100nm. All the TEM samples were negatively stained with 2% (w/v) uranyl acetate aqueous solution.

Figure 5

TEM images of twisted nanoribbons of C₁₆H₃₁O-VEVEGRGD at a 0.1 wt % concentration. a, Negatively stained TEM image. b, Cryo-TEM image. The nanoribbons exhibit a uniform width of approximately 50nm.