  We study the thermodynamic behavior of a model protein with 54 amino acids
that is designed to form a three-helix bundle in its native state. The model
contains three types of amino acids and five to six atoms per amino acid, and
has the Ramachandran torsion angles as its only degrees of freedom. The force
field is based on hydrogen bonds and effective hydrophobicity forces. We study
how the character of the collapse transition depends on the strengths of these
forces. For a suitable choice of these two parameters, it is found that the
collapse transition is first-order-like and coincides with the folding
transition. Also shown is that the corresponding one- and two-helix segments
make less stable secondary structure than the three-helix sequence.
