Supporting Material Mechanical anisotropy of ankyrin repeats

When a transient pair of antiparallel helices is formed (Fig. 5A, middle panel), 11 contacts are formed, and also 11 contacts are formed when a transient loop-hairpin structure is created. Both processes shorten the polypeptide chain by 2 nm and the structures are inherently unstable and can easily open due to thermal fluctuations. If H2 of R16 had refolded to the stack, it would have formed only 12 contacts (marked by a yellow/blue arrow in Fig. 5A), but would have shortened the chain by 3 nm. This number of contacts is comparable to the number of possible native contacts formed in a pair of antiparallel helices, or a loop-hairpin structure elsewhere along the chain. The refolding at the stack interface competes with refolding elsewhere when the polypeptide chain relaxes, but neither is stable enough to resist the thermal fluctuations. During that time window between 973 and 1010 ns, the transient structures formed one by one, prohibiting the H2 of R16 from refolding because the transient structures required less contraction length than R16-H2 (2 nm vs. 3 nm). Between 1010 and 1012 ns, two transient structures in R13-R14 opened simultaneously, providing an opportunity for the entire R16 (H1, H2 and the loop-hairpin) to refold in a single step forming a very large number of contacts.