Gelation of Highly Cationic Alanine Based Peptide in Water in Absence of Charge Screening Anions

Abstract

Peptide aggregation and self-assembly is of interest due to the possible link to many neurodegenerative diseases, as well as possible biotechnological applications. Surprisingly, the alanine-based peptide, (AAKA)4, (“AK-16”) has been previously shown to aggregate and form a hydrogel in the presence of a sufficiently high concentration of anions, although it may seem intuitive that the positive lysine residues would prevent aggregation. Here, we report the delayed self-assembly of pre-aggregated AK-16 into a hydrogel in the absence of neutralizing anions. Self assembly kinetics was probed by both IR and vibrational circular dichroism. At low concentrations (<15mg/ml), the peptide initially forms β-sheet rich structures, which decays over a 5 day period, as evidenced by a decrease in the intensity of the amide I’ band at 1616cm-1. This decay likely indicates the formation of more disordered structures or amorphous aggregates. At higher concentrations, the β-sheet content remains stable over a much longer time scale, and a stable hydrogel eventually forms. The intrinsic intensity of the amide I’ band significantly increases upon gelation, revealing it as a spectroscopic marker for gelation. We attribute this to an increasing hydration of the peptide backbone. Moreover, the structural decay into amorphous aggregates as well as hydrogelation lead to rather substantial spectroscopic changes in the 1350 −1500 cm-1 region of the IR spectrum. AFM images reveal a complex nanoweb structure of the gelated peptide which is typical of noncovalently crosslinked fibrils. This cationic system lacks complimentary charges, a common feature of peptide hydrogels, and offers promise in regards to biotechnological applications.