Chemically Modified Linear Peptides

Nonbiological modifications of linear peptides can help tip the balance towards the formation of hollow nanotubes as opposed to solid nanofibrils. Three notable examples illustrate this point. The first example comes from the 20–29 segment of amylin protein which undergoes amyloid β-sheet fibrillization. Several derivatives of this segment have been produced with a modified backbone to prevent intermolecular hydrogen bonding and thus act as β-sheet breakers, and abolish fibril formation. Unexpectedly, two of these derivatives form helical ribbons and NTs 200–300 nm in diameter and several microns in length. The increased hydrophobicity of these derivatives, as opposed to intermolecular backbone hydrogen bonding, is believed to be the driving force for self-assembly in this case.

In the second example, studies focused on a model peptide show the formation of solid β-sheet fibrils. Variant biotinylated peptides designed with different linkers between biotin and the peptide termini are shown by fourier transform infrared (FTIR) to adopt β-sheet structure and to form homogeneous tubes in water with an external diameter of 60 nm and an internal diameter of 30 nm. The antibiotin antibody effectively binds to biotin groups on the NTs. The authors propose using these tubes as scaffolds for proteins. In a recent study, short peptide derivatives (N-(fluorenyl-9-methoxycarbonyl) commonly known as Fmoc, Fmoc-Leu₂ and Fmoc-Leu₃, have also been shown to form gels in water consisting of nanotubes with external and internal diameters of 15.2–19.5 nm and 4–6 nm, respectively. Interestingly, they were twisted into two-dimensional β-sheets by enzymatically triggered self-assembly.

A class of well-studied NT-forming molecules containing both peptidic and non-peptidic segments are the bola-amphiphilic peptides (fig 5a), which over the course of several days in water, form NTs (fig 5b) with a wide range of diameters from 20 nm to 1 μm. In order to control their diameter, NTs have been assembled within polycarbonate membranes. Depending on pore size, the membrane can act as a template to produce NTs with a controlled width from 50 nm to 1 μm. This opens up the opportunity to apply NTs as templates for nanowires. Nanowires are essential building blocks for electronics and sensors; production of nanowires of uniform diameter is crucial for these applications as their electric and magnetic properties are sensitive to size. In one study, NTs have been immobilized on self-assembling monolayer (SAM)-functionalized Au substrates via hydrogen bonding and then metallized by nickel. This approach may be useful for nanoelecronic fabrication since the NTs can be coated with various metals to form metallic nanowires.

Fig. 5. (a) Chemical structure of the bola-amphiphilic peptide monomer, bis-(N-R-amido-glycylglycine)-1,7-heptane dicarboxylate. (b) Proposed model for the formation of peptide nanotubes. (c) Schematic illustrating the formation of lipase-loaded peptide nanotubes and their enzymatic application.

NTs have also been functionalized with proteins, nanocrystals, and metalloporphyrin coatings via hydrogen bonding. The application of these coated tubes to nanoscale, highly sensitive chemical sensors, electronics or photonics may be possible. In another study, a model lipase enzyme is encapsulated in NTs by incubation for a week (fig 5c). The catalytic activity of the nanotube-bound lipase increases by 33% as compared with free-standing lipases at room temperature. At elevated temperatures of 65°C, the lipase activity inside the NTs is 70% higher than free standing lipases. These amazing results are believed to stem from the fact that the bound lipase activity is most likely induced by the conformational change of lipases to an enzymatically active structure during adsorption.

This class of NTs can also be organized macroscopically, e.g. Ni⁺² ions employed as a bridge between free peptide amines result in bundles (diameter 100 μm and length 3 mm) of NTs. Addition of EDTA to the suspension causes disassembly of the NT bundles. Bundles will have greater mechanical stability than individual ones, which may be necessary for applications. They are also easier to handle than individual NTs. Another important property for application development is the ability to immobilize NTs at a specific location on a substrate. Accurate localization of NTs can be demonstrated by assembling antibody-functionalized NTs at specific locations on substrates (fig 6) where their complementary antigen proteins are patterned.

Fig. 6. Atomic force microscopy of antihuman-IgG-coated bolaamphiphilic peptide nanotubes attached to trenches filled with human IgG (a) before incubation with antibody-coated nanotubes – 0%, (b) when the concentration of human IgG is 70% with 30% of BSA spacer on the trench. (c) When the concentration of human IgG in the trench is reduced by 50%, antibody-nanotube attachment improves dramatically. (d) The trend continues even at a reduced concentration of 10% human IgG. (e) Maximum nanotube attachment is achieved when the concentration of human IgG in the trench is 7%, and (f) at human IgG concentrations < bars="500">