Dynamic self-assembly of surfactant-like peptides A6K and A9K

Abstract

Dynamic processes of molecular self-assembly from two surfactant-like peptides A6K and A9K have been investigated by combining AFM and TEM measurements with SANS measurements. Aggregated peptide stacks were formed during the first hour of solution preparation, followed by their assembly into short nanofibrillar segments in the subsequent few hours to 24 hour period. The alignment of short nanofibers into mature long ones then occurred, with final lengths extended to several microns but with diameters remaining fixed at 5–8 nm. Even after a week, gaps or joints still remained in the mature nanofibers, reminiscent of an imperfect self-healing under the experimental conditions. The final A6K nanofibers looked like strings of sausages. In contrast, A9K self-assembled into smaller nanorods with diameters of around 3–4 nm and lengths mostly within about 100 nm. The entire self-assembling process was completed within the first hour and there were few further morphological variations afterwards. The main feature of formation of A6K nanofibers and A9K nanorods was directly confirmed by SANS, evident from the diameters and lengths consistent with AFM and TEM. However, the SANS measurements were rather insensitive to the dynamic changes of nano-fibrillar lengths over the full length scale and were even less sensitive to possible changes in the proportion of peptide stacks to fibrillar fragments present. Thus, SANS turned out to be less effective for following the time dependent aggregation in these situations.