The diphenylalanine peptide, the core recognition motif of the beta-amyloid polypeptide, efficiently self-assembles into discrete, well-ordered nanotubes. Here, we describe the notable thermal and chemical stability of these tubular structures both in aqueous solution and under dry conditions. Scanning and transmission electron microscopy (SEM and TEM) as well as atomic force microscopy (AFM) revealed the stability of the nanotubes in aqueous solution at temperatures above the boiling point of water upon autoclave treatment. The nanotubes preserved their secondary structure at temperatures up to 90 degrees C, as shown by circular dichroism (CD) spectra. Cold field emission gun (CFEG) high-resolution scanning electron microscope (HRSEM) and thermogravimetric analysis (TGA) of the peptide nanotubes after dry heat revealed durability at higher temperature. It was shown that the thermal stability of diphenylalanine peptide nanotubes is significantly higher than that of a nonassembling dipeptide, dialanine. In addition to thermal stability, the peptide nanotubes were chemically stable in organic solvents such as ethanol, methanol, 2-propanol, acetone, and acetonitrile, as shown by SEM analysis. Moreover, the acetone environment enabled AFM imaging of the nanotubes in solution. The significant thermal and chemical stability of the peptide nanotubes demonstrated here points toward their possible use in conventional microelectronic and microelectromechanics processes and fabrication into functional nanotechnological devices.

The core recognition motif of the amyloidogenic beta-amyloid polypeptide is a dipeptide of phenylalanine. This dipeptide readily self-assembles to form discrete, hollow nanotubes with high persistence lengths. The simplicity of the nanotube formation, combined with ideal physical properties, make these nanotubes highly desirable for a range of applications in bionanotechnology. To fully realize the potential of such structures, it is first necessary to gain a comprehensive understanding of their chemical and physical properties. Previously, the thermal stability of these nanotubes has been investigated by electron microscopy. Here, we further our understanding of the structural stability of the nanotubes upon dry-heating using the atomic force microscope (AFM), and for the first time identify their degradation product utilizing time-of-flight secondary-ion mass spectrometry. We show that the nanotubes are stable at temperatures up to 100 degrees C, but on heating to higher temperatures begin to lose their structural integrity with an apparent collapse in tubular structure. With further increases in temperature up to and above 150 degrees C, there is a degradation of the structure of the nanotubes through the release of phenylalanine building blocks. The breakdown of structure is observed in samples that are either imaged at elevated temperatures or imaged following cooling, suggesting that once phenylalanine is lost from the nanotubes they are susceptible to mechanical deformation by the imaging AFM probe. This temperature-induced plasticity may provide novel properties for these peptide nanotubes, including possible applications as scaffolds and drug delivery devices.

Amyloid fibrils are highly ordered supramolecular biological assemblies that are formed by the self-association of misfolded proteins. These nanoscale structures are a common pathological feature associated with a variety of degenerative diseases, including Alzheimer's disease, type-2 diabetes mellitus, and transmissible spongiform encephalopathies (such as Scrapie, bovine spongiform encephalopathy (BSE), and Creutzfeldt-Jakob disease (CJD)). Investigations into the shortest sequence capable of fibril formation have revealed that the diphenylalanine (FF) motif of the Alzheimer's disease ß-amyloid polypeptide self-assembles into well-ordered, discrete, hollow tubular structures. [1,2] The simplicity of the conditions required to spontaneously form FF tubes and their uniform morphology makes them attractive building blocks for a range of innovative nanotechnological applications: FF tubes are being investigated for their potential in the microelectronics industry, and have previously been used as a scaffold for metal nanowire formation [1] and modification of electrochemical biosensors. [3,4] Modifed FF tubes were demonstrated to form various nanofibrillar and nanocrystalline assemblies, [5] as well as rigid hydrogels.[6] Furthermore, the rigidity, high persistence length, strength, [7] and potential of FF tubes to be biologically or chemically functionalized are well-suited to a range of applications. One key factor in developing novel uses for these tubes is the ability to exploit and control their alignment. Previous studies have reported that a magnetic field may be used as a tool to align ß-amyloid fibrils in preparation for X-ray fiber diffraction. [8][9][10][11][12] In this study, we investigate the magnetic alignment of preprepared FF tubes suspended in a 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP)-water solution. Droplets of the solution were allowed to dry in magnetic fields of up to 12 T and the residue was imaged using atomic force microscopy (AFM). Our experiment differs from that reported in a recently published paper in which FF tubes coated with ferrofluid were shown to align in a magnetic field; [13] in this study, we demonstrate that FF tubes will align in a magnetic field without any additional treatment. We attribute the alignment to the effect of the magnetic torque associated with the diamagnetic anisotropy of the aromatic rings of phenylalanine. [14] These aromatic moieties play a key role in the fibril self-assembly process and drive the well-ordered stacking of aromatic rings by p-p interactions. [15,16] The ordered orientation of the rings gives the major contribution to the net diamagnetic anisotropy in this structure. This contrasts with other studies of magnetic alignment of proteins, in which the net anisotropy has been attributed primarily to ordered arrangements of peptide bonds. From our observations of the alignment direction in the magnetic field, we derive a necessary constraint on the orientation of the aromatic rings within the tube and compare this with a model structure. The mag...

Self-assembling aromatic dipeptides are among the smallest known biological materials which readily form ordered nanostructures. The simplicity of nanotube formation makes them highly desirable for a range of bionanotechnology applications. Here, we investigate the application of the atomic force microscope as a thermomechanical lithographic tool for the machining of nanotubes formed by two self-assembling aromatic peptides; diphenylalanine and dinapthylalanine. Trenches and indentations of varying depth and width were patterned into the peptide tubes with nanometer precision highlighting the ability to thermally machine and manipulate these robust and versatile nanotubes.