Anders Irbäck, Fredrik Sjunnesson and Stefan Wallin Hydrogen Bonds, Hydrophobicity Forces and the Character of the Collapse Transition Proceedings of the ISI Workshop "Protein Folding: Simple Models and Experiments", eds. P. Bruscolini et al. Journal of Biological Physics 27, 169-179 (2001) Abstract: 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. LU TP 00-34