Abstract
We report here on a peptide hydrogel system, which in contrast to most other such systems, is made up of relatively short fibrillar aggregates, discussing resemblance with colloidal rods. The synthetic model peptides A8K and A10K, where A denotes alanine and K lysine, self-assemble in aqueous solutions into ribbon-like aggregates having an average length 〈L〉 on the order of 100 nm and with a diameter d≈ 6 nm. The aggregates can be seen as weakly charged rigid rods and they undergo an isotropic to nematic phase transition at higher concentrations. Translational motion perpendicular to the rod axis gets strongly hindered when the concentration is increased above the overlap concentration. Similarly, the rotational motion is hindered, leading to very long stress relaxation times. The peptide self-assembly is driven by hydrophobic interactions and due to a net peptide charge the system is colloidally stable. However, at the same time short range, presumably hydrophobic, attractive interactions appear to affect the rheology of the system. Upon screening the long range electrostatic repulsion, with the addition of salt, the hydrophobic attraction becomes more dominant and we observe a transition from a repulsive glassy state to an attractive gel-state of the rod-like peptide aggregates.
Highlights
Fiber dispersions are interesting due to the strong rheological effects that can be observed already at very low concentrations.[1]
Samples were prepared by mixing the amorphous powder in either water or D2O, the latter chosen for the possibility to perform nuclear magnetic resonance (NMR) measurements
Self-assembled peptide hydrogels are interesting biomaterials with a range of potential applications.[7,8,9,10]. Such systems consist of micrometer long fibrils forming a transient network.[3,6,15]
Summary
Fiber dispersions are interesting due to the strong rheological effects that can be observed already at very low concentrations.[1] gelation or glass formation can be obtained at a solid content of only a fraction of percent.[2,3,4,5,6] Materials of such low molecular weight content, where the continuous solvent is water, are often referred to as hydrogels. A specific class of anisotropic colloidal particles forming hydrogels are selfassembled peptide fibers, where peptides are the individual building blocks which spontaneously assemble into long onedimensional fiber-like structures. These fibers may further interact with one another to form higher level assemblies or networks with varying mechanical properties. The biocompatibility and often high tunability make these fibrous networks of great interest for biomedical applications, such as drug delivery, implantable materials or as scaffolds for tissue regeneration.[7,8,9,10]
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