Dipeptides

The simplest peptide building blocks for the construction of NTs are dipeptides from the diphenylalanine motif of the Alzheimer's β-amyloid peptide. When this peptide is dissolved at 100 mg/ml in 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) and then diluted down with water at a final concentration of ≤ 2 mg/ml, multiwall NTs are formed, with a typical diameter of 80–300 nm and micron length. A similar but more rigid diphenylglycine peptide analogue forms remarkably stable spheres 10–100 nm in diameter under the same solution conditions. Spherical particles also form instead of NTs when a thiol is introduced into the original diphenylalanine peptide. The occurance of either tubes or spheres provides an insight into the possible mechanism of their formation (fig 1a ), suggesting that the formation of either tubular or closed cages by fundamentally similar peptides is consistent with the closure of a two-dimensional layer, as described both for carbon and inorganic nanotubes and their corresponding buckminsterfullerene and fullerene-like structures.

Fig. 1. (a) Schematic of the formation of tubular (single or multiwalled), spherical, or fibrillar structures via dipeptide self-assembly. (b) Proposed model for the formation of aligned peptide nanotube arrays. (i) Scanning electron micrograph of the vertically aligned peptide nanotubes. (ii) Cold field-emission gun high-resolution scanning electron (CFEG-HRSEM) micrograph of the nanotube arrays. (iii) High-magnification micrograph of an individual nanotube obtained by CFEG-HRSEM.

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The NTs are stable under extreme conditions, e.g. in autoclave (121°C, 1.2 atm); circular dichroism spectra do not change from room temperature up to 90°C; dry tubes heated to 150°C are stable, while degradation occurs at 200°C. The NTs display remarkable chemical stability in a wide range of organic solvents and pH. Indentation atomic force microscopy experiments on the mechanical properties of dried NTs on mica give an estimated averaged point stiffness of 160 N/m and a high Young's modulus of 19 GPa (27 GPa in another study). This makes them amongst the stiffest known biological materials. For example, biological microtubules which provide a rigid cytoskeleton to the cell have a Young's modulus of 1 GPa. However, the stiffness of the dipeptide NTs is lower than that of carbon and inorganic nanotubes. It has been suggested that as well as intermolecular hydrogen bonding, the rigid aromatic side chains may also be responsible for stability and mechanical strength, as well as providing directionality for formation through specific π–π interactions. Biological systems are notorious for their instability and sensitivity to temperature and chemical treatments. The significant thermal and chemical stability of these NTs points to their possible use in micro- and nanoelectromechanics (MEMS and NEMS) and functional nanodevices.

Carbon nanotubes (CNTs) are extensively studied for sensing applications due to their mechanical stability, conductance and large surface area. A drawback of CNTs is the effect exposure to humidity, oxygen, N₂O, and NH₃ has on their electric properties. CNTs are also believed to pose problems in device fabrication due to lack of uniformity, hydrophobicity and thus, limited solubility, reproducibility of precise structural properties, cost, and limited opportunities for covalent modification. Therefore, the dipeptide NTs offer an attractive alternative for device fabrication. Several steps have been taken in this direction. In one example, peptide NTs have been deposited on graphite electrodes. Improved electrode sensitivity is observed, suggesting that this may be due to an increase in the functional electrode surface in the presence of NTs. In a related study, thiol-modified peptide NTs have been immobilized and dried on Au electrode surfaces and an enzyme coating was applied to them. The resulting electrodes show improved sensitivity and reproducibility for the detection of glucose and ethanol, short detection time, large current density, and comparatively high stability. The findings show that novel electrochemical biosensing platforms may be fabricated based on biocompatible NTs.

Cationic dipeptides NH₂–Phe–Phe–NH₂ self-assemble into NTs at neutral pH and rearrange into spherical structures (possibly vesicles) 100 nm in diameter, upon dilution below 8 mg/ml (fig 2). The tubes can be absorbed by cells through endocytosis upon spontaneous conversion into vesicles. This property has been used to deliver oligonucleotides into the interior of the cells as a proof of concept of the potential applications of the system in gene and drug delivery.

Fig. 2. Proposed model for the transition of DNA-loaded cationic dipeptide nanotubes (CDPNTs) into vesicles and their cellular uptake for oligonucleotide delivery.

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Dipeptide NTs have also been used to fabricate 20 nm Ag nanowires, effectively acting as a degradable casting mould. Ag ions are reduced to metallic Ag in the lumen of the tube, and the peptide template removed by enzyme degradation. This approach may have applications in molecular electronics as such small nanowires cannot be made by conventional lithography. In another related study, AgNO₃ is reduced in the hollow pores of the NTs, then thiol-containing linker peptides are bound on the outside of the nanotubes. These attached Au nanoparticles, which act as nucleation sites during the electroless deposition of a gold, cover the NTs. Thus the fabrication of metal-insulator-metal trilayer coaxial nanocables, which may give rise to unique electromagnetic properties, has been demonstrated.

In many applications, nanotubes cannot be applied as individual nanostructures, rather their macroscopic organization is necessary or preferred. Using a simple method, two-dimensional ordered films of NTs 1 μm in thickness have been created, comprising closely arranged spherulites of multiple NT bundles. This is achieved by dissolving the NTs in the carbon-nanotube-debundling solvent N-methyl-2-pyrrolidone (NMP), followed by deposition of the peptide solution on a surface (SiO₂, Au, Pd, alumina, mica, quartz, InP), heating at 60°C for solvent evaporation, and cooling to room temperature. Successful preparation of a Ag-embedded NT composite network has also been carried out using a similar approach. The authors intend to expand these experiments to form other hybrid films, thus offering opportunities for the construction of new complex nanostructures, for use in biosensors and biocoating materials.

Impressive macroscopic alignment (fig 1b ) has also been demonstrated by nanoforests of vertically aligned NTs, formed by unidirectional growth of dense arrays. This process involves application of the dipeptide dissolved in HFIP onto siliconized glass followed by solvent evaporation. It has been proposed that evaporation results in a super saturation state that facilitates the formation of numerous nucleation sites on the surface. This may then be followed by unidirectional growth of nanotubes as more peptides join the growing ends of the tubes. A method for horizontal alignment of NTs has also been demonstrated through non-covalent coating of the tubes with magnetite nanoparticles and the application of an external magnetic field. These results demonstrate the ability to form a two-dimensional dense array of these NTs with either vertical or horizontal patterns.