Abstract
Super-resolution fluorescence microscopy, specifically stochastic reconstruction microscopy (STORM), and atomic force microscopy (AFM) were used to image the self-assembly processes of the peptide surfactant I3K. The peptide surfactants self-assembled into giant helical fibrils with diameters between 5 and 10 nm with significant helical twisting. The resolution of the STORM images was 30 nm, calculated using the Fourier ring correlation method. STORM compares favorably with AFM for the calculation of contour lengths (∼6 μm) and persistence lengths (10.1 ± 1.2 μm) due to its increased field of view (50 μm), and its ability to image bulk morphologies away from surfaces under ambient solution conditions. Two-color STORM experiments were performed to investigate the dynamic process of self-assembly after mixing of two separately labeled samples, and the results revealed the formation of long nanofibers via end-to-end connections of short ones. No evidence was found for significant monomer exchange between the samples, and the self-assembled structures were very stable and long-lived.
Highlights
IntroductionThe materials and structures created by the self-assembly of synthetic peptides have garnered significant research interest[1−4] due to their potential applications in drug delivery,[5] as antibacterial[6] and anticancer agents,[7] for the creation of synthetic extracellular matrices,[8] and to accelerate wound healing.[9,10] This is in part due to the peptides’ inherent biocompatibility[11] and the way self-assembly can be sensitively tuned by various environmental factors including temperature,[12] pH, salt,[14] and solvent.[2,15] Small amphiphilic peptides are of particular interest because of the reduced costs associated with the production of peptides that only contain a few residues
We demonstrated how stochastic reconstruction microscopy (STORM) can be used to study the structural evolution of I3K peptide fibrils during their dynamic process of self-assembly
The large field of view possible (∼50 μm) with STORM allows the quantification of the contour and persistence lengths of individual fibrils even when they are on the order of tens of micrometers
Summary
The materials and structures created by the self-assembly of synthetic peptides have garnered significant research interest[1−4] due to their potential applications in drug delivery,[5] as antibacterial[6] and anticancer agents,[7] for the creation of synthetic extracellular matrices,[8] and to accelerate wound healing.[9,10] This is in part due to the peptides’ inherent biocompatibility[11] and the way self-assembly can be sensitively tuned by various environmental factors including temperature,[12] pH, salt,[14] and solvent.[2,15] Small amphiphilic peptides are of particular interest because of the reduced costs associated with the production of peptides that only contain a few residues. The reduced complexity of small peptides offers many opportunities to model and study exactly how external factors may impact self-assembly. I3K self-assembles through a spontaneous multistep process involving the creation of peptide aggregates stabilized by hydrophobic interactions and antiparallel β sheets These aggregates grow to form ribbons, which curl under chiral forces to form nanotubes of approximately 10 nm in diameter and micrometers in length.[16] The peptide has been used as a model to study the role of salt[14] and hydrogen bonding[17] in self-assembly, and the self-assembled structures are sufficiently stable to act as templates for silica nanotube synthesis.[16,18]
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