AbstractZSPA–1 is an engineered protein that binds to its parent, the three‐helix‐bundle Z domain of staphylococcal protein A. Uncomplexed ZSPA–1 shows a reduced helix content and a melting behavior that is less cooperative, compared with the wild‐type Z domain. Here we show that the difference in folding behavior between these two sequences can be partly understood in terms of an off‐lattice model with 5–6 atoms per amino acid and a minimalistic potential, in which folding is driven by backbone hydrogen bonding and effective hydrophobic attraction. Proteins 2004;54:000–000. © 2003 Wiley‐Liss, Inc.