For the chain lengths where helices are stable, cooperativity increases with the number of residues, and stability is highest for 18-24 residues. The heat capacity curves (on a perresidue basis) and corresponding native contacts curves are plotted in Figure . The medoid structures taken from the cluster with the lowest intra-cluster RMSD, with subsequent energy minimization, are also shown for each chain length considered. For the case of 6 residues, there are too few native contacts along the backbone to stabilize the helix, and no peak in heat capacity is observed over the temperature range used. For 12 through 30 residues, the expected helix structure emerges. For 12 residues, the heat capacity peak and transition region in the native contacts curve are very broad, indicating weak cooperativity and/or stability. Increasing the number of residues from 18 to 24 to 30, the width of the heat capacity curve decreases, the melting point decreases, and the height of the heat capacity curve increases. This corresponds to increasing cooperativity and decreasing stability. Upon increasing chain length to 36 residues, additional structure emerges-a strand of ∼10 residues in an extended conformation forms adjacent to the helix containing the remaining ∼26 residues. Despite using the helical backbone native contacts scheme for the 36 residue system, the melting point and characteristic width of the native contacts curve parallels that of the heat capacity curve. However, the native contact fraction in the folded state is artificially low, since the strand-helix contacts were not considered.