Different members of a simple three-helix bundle protein family have very different folding rate constants and fold by different mechanisms.

The 15th, 16th, and 17th repeats of chicken brain alpha-spectrin (R15, R16, and R17, respectively) are very similar in terms of structure and stability. However, R15 folds and unfolds 3 orders of magnitude faster than R16 and R17. This is unexpected.