Date Published: May 11, 2017
Publisher: John Wiley and Sons Inc.
Author(s): Barak Gilboa, Clément Lafargue, Amir Handelman, Linda J. W. Shimon, Gil Rosenman, Joseph Zyss, Tal Ellenbogen.
Short peptides made from repeating units of phenylalanine self‐assemble into a remarkable variety of micro‐ and nanostructures including tubes, tapes, spheres, and fibrils. These bio‐organic structures are found to possess striking mechanical, electrical, and optical properties, which are rarely seen in organic materials, and are therefore shown useful for diverse applications including regenerative medicine, targeted drug delivery, and biocompatible fluorescent probes. Consequently, finding new optical properties in these materials can significantly advance their practical use, for example, by allowing new ways to visualize, manipulate, and utilize them in new, in vivo, sensing applications. Here, by leveraging a unique electro‐optic phase microscopy technique, combined with traditional structural analysis, it is measured in di‐ and triphenylalanine peptide structures a surprisingly large electro‐optic response of the same order as the best performing inorganic crystals. In addition, spontaneous domain formation is observed in triphenylalanine tapes, and the origin of their electro‐optic activity is unveiled to be related to a porous triclinic structure, with extensive antiparallel beta‐sheet arrangement. The strong electro‐optic response of these porous peptide structures with the capability of hosting guest molecules opens the door to create new biocompatible, environmental friendly functional materials for electro‐optic applications, including biomedical imaging, sensing, and optical manipulation.
Self‐assembled peptide structures have been researched extensively in recent years due to their potential in numerous important biocompatible applications.1, 2, 3, 4, 5 A privileged position among self‐assembling peptides is reserved for short phenylalanine repeats, owing to the diversity of self‐assembled structures that they display including tubes, tapes, spheres, and fibrils.6, 7, 8, 9, 10 This structural diversity has found potential applications in different fields including scaffolds for regenerative medicine, light harvesting materials, fluorescent probes, and mechanically tunable hydrogels, to name but a few.11, 12, 13, 14, 15 The interest in these ultrashort peptides originated from the extensive research on amyloid fibrils, which are self‐assembled fibrillar structures formed by misfolding peptides or proteins, that were shown to be involved in several degenerative diseases including Alzheimer’s, Parkinson’s, type II diabetes and more.16, 17, 18, 19, 20, 21 In particular, in 2002, Gazit showed that the dipeptide diphenylalanine (FF) is the core recognition motif of the beta amyloid polypeptide implicated in Alzheimer’s disease.22 FF was found to self‐assemble into nanoscaled tubes in water with remarkable physical properties such as large mechanical stiffness, super hydrophobicity, supercapacitance, and strong piezoelectric response.23, 24, 25, 26, 27 The latter property is associated with a lack of inversion symmetry of the material’s structure, which is also associated with quadratic optical nonlinearity, enabling optical frequency conversion. This was very recently confirmed by Handelman et al. that demonstrated efficient second harmonic generation (SHG) from FF tubes and triphenylalanine (FFF) nanotapes.9
We examined two types of polypeptide structures, FFF‐tapes and FF‐tubes (Figure1). The FFF‐tapes grow into elongated structures with rectangular cross sections. Their thicknesses range from a few hundreds of nanometers and up to ≈1.5 µm. Their widths usually range between 3 and 6 µm and they are up to hundreds of micrometers in length (Figure 1c). The FF‐tubes that we grew (see the Experimental Section) self‐assemble in water into tubes with hexagonal cross sections (Figure 1d–f). Their typical wall thickness is of a few hundreds of nanometers, diameters of 1–2 µm typically, and lengths that can extend up to millimeters (see also Figure S1, Supporting Information).
We have observed for the first time strong linear electro‐optic response in bio‐organic phenylalanine homopeptides microstructures. The electro‐optic coefficients for both assemblies have demonstrated very large values similar to that of commercially available inorganic electro‐optic crystals and in FF‐tubes reached ≈32 pm V−1 for r33 in line with the best ferroelectric crystals. XRD studies of FFF‐tapes have shown a porous structure of antiparallel β‐sheets capable of hosting guest entities such as water and ions as well as larger molecules. By measuring electro‐optic activity, we have revealed in FFF‐tapes a complex state of polarization and a pronounced structure for domains. The efficient optical nonlinearity of these structures, their porosity, biocompatibility and domain formation can lead to the establishment of new functional optical materials by the incorporation of guest molecules, novel chemical, and biological optical sensors, and new types of self‐assembled bioorganic electro‐optic modulators.
Peptide Self‐Assembly: FF and FFF were purchased in lyophilized form from Bachem (Switzerland). The peptides were dissolved in 1,1,1,3,3,3‐hexafluoro‐2‐propanol at concentrations of 100 and 50 mg mL−1, respectively. The solutions were diluted in water to a final peptide concentration of 1.5 mg mL−1 each. For FF, a drop of 10 µL of fresh FF solution in water was left to dry on an electrode patterned glass coverslip to form tubes. FFF solution was first aged for 3 d until crystal aggregates were visible. A drop from the aged solution was gently placed on an electrode coated glass to prevent breakage due to the high aspect ratio of the tapes, while removing excess solution in order to minimize spontaneous self‐assembly on the slide. Both the FF and FFF solutions were optimized to produce the largest and best quality structures by controlling the concentrations, ratio of solvents, and growth temperature.
The authors declare no conflict of interest.