Phase transitions in organic and inorganic materials are well-studied classical phenomena, where a change in the crystal space group symmetry induces a wide variation of physical properties, permitted by the crystalline symmetry in each phase. Here we observe a conformational induced transition in bioinspired peptide nanotubes (PNTs). We found that the PNTs change their original molecular assembly from a linear peptide conformation to a cyclic one, followed by a change of the nanocrystalline structure from a noncentrosymmetric hexagonal space group to a centrosymmetric orthorhombic space group. The observed transition is irreversible and induces a profound variation in the PNTs properties, from the microscopic to the macroscopic level. In this context, we follow the unique changes in the molecular, morphological, piezoelectric, second harmonic generation, and wettability properties of the PNTs.

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Many peptide nanostructures, self-assembled from chemically synthesized biomolecules, have drawn much attention in the fi eld of nanotechnology due to their physical, chemical, and biological properties, which make them promising candidates for applications in bionanomedicine, [ 1 ] bionanotechnology, [ 2,3 ] electronics, [ 4,5 ] optics, [ 6 ] energy storage, [ 7,8 ] etc. Some of these properties, such as ferro-and piezoelectricity observed in diphenylalanine nanotubes (FF-PNT) [ 9 ] are directly related to the nanocrystalline structural asymmetry of the elementary building blocks comprising these supramolecular materials. [ 6,10 ] One basic physical effect that depends on both the crystalline symmetry and the electronic properties of dielectric materials is second harmonic generation (SHG). SHG is observed only in crystals with no center of symmetry [ 11 ] and is related to ferroelectric phenomena together with linear electrooptical and piezoelectric effects. Ferroelectric effects have been observed in many biological materials such as plants, animals, and human tissues (amino acids, pineal gland of brain, skin, tendon, etc.). [ 12 ] Today, the SHG effect is also exploited in optical microscopy, especially in medical and biological research. [ 13 ] It allows the detection of two-photon emission from biomaterials and biopolymers [ 14 ] lacking a center of symmetry. The effect has been used with quantitative metrics for diagnosing a wide range of diseases. [ 15 ] Recently, second-order responses have also been found in bioinspired aromatic FF-PNT with hexagonal space group P6 1 using nonlinear optical microscopy. [ 16 ] Both the elementary crystalline symmetry and the electronic structure of bioinspired peptide nanostructures can be significantly changed by deep reconstruction process, such as phase transformation at a nanoscale level, which results in the disappearance of an SHG response. [ 17 ] Another method to modulate these fundamental properties is to use different solvents, [18][19][20][21] which strongly infl uence the self-assembly process and defi ne peptide nanostructures' morphologies. Modifi cation of the physical properties in peptide nanomaterials is a new way to fabricate basic nanoscale units for future bottom-up nanotechnologies. [ 6 ] Bioinspired peptide nanostructures, much like other organic nanostructures, [ 22,23 ] have ultra-small sizes and are easily produced by a rapid self-assembly fabrication process. All these properties make them favorable for implementation in diverse applications, and especially in biophotonics devices.In this work, we have studied the SHG effect in bioorganic peptide nanostructures of different morphologies and symmetries, such as nanotubes, nanofi bers, nanobelts, and nanospheres. These nanostructures were self-assembled in different solvents from peptide precursors with a variable number of A nonlinear optical effect of a second harmonic generation (SHG) was fi rst observed in quartz and then found in many inorganic materials that have an asymmetric crystalline s...