Abstract
We examine the self-assembly of a peptide A6H comprising a hexa-alanine sequence A6 with a histidine (H) “head group”, which chelates Zn2+ cations. We study the self-assembly of A6H and binding of Zn2+ ions in ZnCl2 solutions, under acidic and neutral conditions. A6H self-assembles into nanotapes held together by a β-sheet structure in acidic aqueous solutions. By dissolving A6H in acidic ZnCl2 solutions, the carbonyl oxygen atoms in A6H chelate the Zn2+ ions and allow for β-sheet formation at lower concentrations, consequently reducing the onset concentration for nanotape formation. A6H mixed with water or ZnCl2 solutions under neutral conditions produces short sheets or pseudocrystalline tapes, respectively. The imidazole ring of A6H chelates Zn2+ ions in neutral solutions. The internal structure of nanosheets and pseudocrystalline sheets in neutral solutions is similar to the internal structure of A6H nanotapes in acidic solutions. Our results show that it is possible to induce dramatic changes in the self-assembly and chelation sites of A6H by changing the pH of the solution. However, it is likely that the amphiphilic nature of A6H determines the internal structure of the self-assembled aggregates independent from changes in chelation.
Highlights
The group of peptides classified as surfactant-like peptides (SLPs)[1−3] can be designed to mimic the properties of a surfactant molecule
Films dried from low concentration A6RGD solutions allowed human cornea stromal fibroblasts to attach and significantly enhaced cell proliferation, while films dried from concentrated A6RGD solutions were toxic to hCSFs
The sample precipitation might be the signature of pseudocrystallites formation due to charge neutralization. We studied these samples at room temperature using techniques that allow for fast specimen preparation and data acquisition (FTIR, Small-Angle X-ray Scattering (SAXS)) or that are not affected by the formation of pseudocrystallites in the sample (TEM, cryo-Transmission Electron Microscopy (TEM) and XRD)
Summary
The group of peptides classified as surfactant-like peptides (SLPs)[1−3] can be designed to mimic the properties of a surfactant molecule. In this work we studied samples of A6H dissolved in water or in ZnCl2 solutions. As detailed in the Results, both A6H and A6H/ZnCl2 solutions have a pH below 7 throughout the range of concentrations studied in this work. After the sample was injected in the capillary and reached the X-ray beam, the flow was stopped during the SAXS data acquisition. Thereafter, 3 μL of sample solution was applied on the grid and it was blotted one time for 1 s and vitrified in a 1/1 mixture of liquid ethane and propane at temperature of −180 °C. Grids with vitrified sample solutions were maintained at liquid nitrogen temperature and cryo transferred in to the microscope. X-ray diffraction was performed on stalks prepared from A6H solutions. The XRD data was collected using a Saturn 992 CCD camera
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