Influence of metal salts on the hydrogelation properties of ultrashort aliphatic peptides

Abstract

Over the last decade, a variety of peptides have been designed and studied for their ability to form hydrogels for potential use as functional biomaterials. Peptide hydrogels have been employed as tissue-engineering scaffolds, drug delivery systems, and platforms for biosensors. We have recently added to the existing peptide systems a new class of ultrashort self-assembling peptides containing 3–7 aliphatic amino acids, which are able to easily form solid hydrogels in plain water. These hydrogels can entrap up to 99.9% of water in scaffolds that are made of long helical fibers. Although the aliphatic peptides do not need ionic conditions to form hydrogels, we were interested to investigate if physiological metal salts interfere with hydrogelation. This is particularly important for future in vivo application in biomedicine and biotechnology. Thus, we have studied the effect of multivalent metal salts on the hydrogelation properties of two members of this peptide class, the hexapeptide Ac-LIVAGD and tripeptide Ac-IVD. The majority of the metal salts did not disrupt the hydrogelation process, whereas the trivalent salts could initiate precipitation of the peptides. The characteristic fibrillar network of the hydrogels was still observable by field emission scanning electron microscopy in the presence of moderate and high concentrations of metal salts. Rheological studies of the hexapeptide hydrogel in the presence of monovalent metal salts demonstrated the ability of the salts to reduce the viscoelasticity of the hydrogel. Rheology, together with Fourier transform infrared spectroscopy, confirmed that the ionic strength of the peptide solution affects the bonding interaction between peptide fibers/sheets and the water molecules during hydrogelation. Overall, our results demonstrate that metal salts in general do not significantly affect the self-assembly process of two aliphatic peptides, but exert an influence on their viscoelastic properties, presumably via disruption of the ordered water layer around the peptide fibers. Given that common physiological metal salts have minimal effect on the integrity/morphology of the studied hydrogels, but only slightly influence the mechanical properties, biomedical applications seem to be feasible.