Post-assembly α-helix to β-sheet structural transformation within SAF-p1/p2a peptide nanofibers.

We report an unanticipated helix-to-sheet structural transformation within an assembly of SAF-p1 and SAF-p2a designer peptides. Solid-state NMR spectroscopic data support the assembled structure that was targeted by rational peptide design: an -helical coiled-coil co-assembly of both peptides. Subsequent to assembly, however, the system converts to a -sheet structure that continues to exhibit nearest-neighbor interactions between the two peptide components. The structural transition occurs at pH 7.4 and exhibits strongly temperature-dependent kinetics between room temperature (weeks) and 40 degrees C (minutes). We further observed evidence of reversibility on the timescale of months at 4 degrees C. The structural conversion from the anticipated structure to an unexpected structure highlights an important aspect to the challenge of designing peptide assemblies. Furthermore, the conformational switching mechanism mediated by a prerequisite -helical nanostructure represents a previously unknown route for -sheet designer peptide assembly.