Self-assembly and gelation properties of alpha-helix versus beta-sheet forming peptides

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 beta-sheet conformations in solution and the alanine based peptides form alpha-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 similar to 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 similar to 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 similar to 15 to 30 nm depending on the concentration. The fibre diameter was found to be similar to 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.