Abstract
The bola-amphiphilic arginine-capped peptide RFL4RF self-assembles into nanotubes in aqueous solution. The nanostructure and rheology are probed by in situ simultaneous rheology/small-angle scattering experiments including rheo-SAXS, rheo-SANS, and rheo-GISANS (SAXS: small-angle X-ray scattering, SANS: small-angle neutron scattering, GISANS: grazing incidence small-angle neutron scattering). Nematic alignment of peptide nanotubes under shear is observed at sufficiently high shear rates under steady shear in either Couette or cone-and-plate geometry. The extent of alignment increases with shear rate. A shear plateau is observed in a flow curve measured in the Couette geometry, indicating the presence of shear banding above the shear rate at which significant orientation is observed (0.1-1 s-1). The orientation under shear is transient and is lost as soon as shear is stopped. GISANS shows that alignment at the surface of a cone-and-plate cell develops at sufficiently high shear rates, very similar to that observed in the bulk using the Couette geometry. A small isotope effect (comparing H2O/D2O solvents) is noted in the CD spectra indicating increased interpeptide hydrogen bonding in D2O, although this does not influence nanotube self-assembly. These results provide new insights into the controlled alignment of peptide nanotubes for future applications.
Highlights
We have studied the shear-induced orientation of peptide nanotubes in the nematic phase
We report the first observation of arginine-functionalized peptide nanotubes by self-assembly of a bola-amphiphilic peptide
The H/D isotopic substitution in the solvent influences the Circular Dichroism (CD) spectra and increased peptide β-sheet formation is observed in D2O, which is explained on the basis of to an increase in peptide−peptide H-bonding in preference to peptide-solvent H-bonding
Summary
Received: September 23, 2016 Revised: November 30, 2016 Published: December 1, 2016. Article side of a central hydrophobic sequence. We investigated in detail the self-assembly and secondary structure of the peptide bola-amphiphile RFL4FR.[13] This peptide comprises a hydrophobic tetra-leucine core to which terminal FR motifs are attached. These incorporate a cationic arginine residue to enable solubility in aqueous solutions and a phenylalanine residue to favor π−π stacking interactions. The alignment of RFL4FR nanotubes under controlled steady shear conditions is investigated via a combination of grazing-incidence small-angle neutron scattering (GISANS) with simultaneous rheology and bulk rheo-SANS The CD signal from the background was subtracted from the CD data of the sample solutions
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