Self-assembly and gelation properties of α-helix versus β-sheet forming peptides

Abstract

We have investigated the self-assembly and gelation properties of a set of four octa-peptides: AEAEAKAK, AEAKAEAK, FEFEFKFK and FEFKFEFK. The phenylalanine based peptides adopt β-sheet conformations in solution and the alanine based peptides form α-helices. No self-assembly in solution was observed for AEAKAEAK but AEAEAKAK was found to self-assemble forming thick, rigid fibres with a diameter of ∼6 nm. These fibres were composed of two fibrils aggregating side by side to form “pearl-necklace” morphologies. No gelation was observed for AEAEAKAK in the concentration range investigated (0 to 100 mg ml−1). In contrast, both phenylalanine based peptides were found to self-assemble in solution and to form hydrogels at an initial concentration of ∼8 mg ml−1. Similar morphologies were observed for both peptides corresponding to a relatively homogeneous dense network of semi-flexible fibres with a mesh size of ∼15 to 30 nm depending on the concentration. The fibre diameter was found to be ∼4 nm in good agreement with models found in the literature. TEM micrographs clearly showed that these fibres have a helicoidal or twisted structure. Comparison of TEM with AFM data highlighted the influence of substrate chemistry on the macromolecular assembly of small peptides. In contrast small angle neutron scattering (SANS) approaches, which allow for the probing of hydrogel morphology and structure without the need for sample preparation on solid substrates, provide vital data on hydrogel morphology in solution.