The secondary structure of peptides and proteins is crucial for their functions in cells. One of the most abundant and most studied structure is the α-helix. Many factors are playing key roles in the folding of a peptide: its primary sequence, intramolecular interactions, or the environment. However, the effect of the sequence length on the intact helical folding has only been investigated by means of simulations. These computations have suggested a critical length of about 20 amino acids for α-helices, longer sequences adopting self-folded conformations of multiple helices.<br/><br/>Here we address this critical length question experimentally by single-molecule force spectroscopy using an atomic force microscope (AFM). We designed a long homopolypeptide (PBLG) and modified it for AFM interfacing. Individual peptides were stretched mechanically by the AFM tip and the helix-coil transition was observed as a force plateau. We evidenced different plateau lengths, corresponding to the unfolding of multiple helices in series. These experimental results are in complete agreement with previous simulations suggesting a critical length of 20 amino acids for an intact α-helix.<br/>Furthermore, we performed pulling-relaxing experiments on single molecules and demonstrated the high stability and fast refolding of PBLG α-helices in a helicogenic solvent.

Critical length of α-helices measured experimentally by single-molecule unfolding experiments

ACS Spring Meeting 2021

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