Investigating the Formation of a Repulsive Hydrogel of a Cationic 16mer Peptide at Low Ionic Strength in Water by Vibrational Spectroscopy and Rheology

The cationic peptide (AAKA)4 (AK16) exhibits a high propensity for aggregation into β-sheet-like structures in spite of the high positive charge of its protonated lysine side chains. Upon incubation into an aqueous solution, the peptide maintains a metastable β-sheet-like structure with fibrillar content, the apparent stability of which increases with peptide concentration. In the presence of a sufficiently high concentration of anions, the peptide spontaneously forms a hydrogel at millimolar concentrations. Interestingly, we find that even in the absence of gel-supporting anions, the peptide is capable of forming a hydrogel in the centimolar range. Rheological data reveal that the gel is a stable elastic solid. These data show that the peptide can overcome the repulsive interactions between the positively charged ammonium groups of the lysine residues. The addition of 1 M NaCl just accelerates this process. Atomic force microscopy images of the peptide gel reveal fibrils with thicknesses between 4 and 8 nm, which suggests that they contain multiple layers of sheets. We propose that long tapes of β-sheet are arranged in fibrils via stacking of alternating interfaces induced by hydrophobic interactions between alanine side chains and by the formation of a hydrogen bonded water network between hydrophilic sides of AK16 β-sheets, which leads to the observed immobilization of the solvent in the formed hydrogel. Water immobilization is proposed as the likely cause for a significant increase in the amide I′ oscillator strength of the formed β-sheet structures.