Figure 8

The unfolding events (peaks) are matched between F-D curves if they correspond to the same helices unfolding. Left : Helices E and D unfold in a single step. The polypeptide chain extending between the AFM cantilever tip and surface exhibits a length of 148 aa (tip-sample separation of ~ 53 nm). Middle : Helices E and D unfold in a two-step process. First, helix E unfolds with the polypeptide chain lengthened to 105 aa (TSS of ~ 38 nm). Second, helix D unfolds with the polypeptide chain lengthened to 148 aa (TSS of ~ 53 nm). Right : Similar to middle, except first helix E unfolds partly, with the polypeptide chain lengthened to 94 aa (TSS of ~ 34 nm) [5]. Matched unfolding events (peaks) are within a window of 5 amino acids (~ 2 nm) from each other, as indicated by the tip-sample separation at the end of the peak (1aa ≈ 0.36 nm). An entire F-D curve is shifted by a terminal length of at most 30 amino acids, which results in the most matches; the terminal length represents the location of cantilever attachment to the protein.