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Two hypothetical folding pathways show the formation and subsequent transformation of secondary structure. (a) A preformed β-hairpin coming spatially close to a preformed α-helix. To form a three-stranded sheet, the α-helix needs to transform into a strand. (b) A β-strand distant in sequence coming spatially close to two helices packing against each other. The β-strand transforms into an α-helix to form a three-helix bundle.
