  A minimal off-lattice model for alpha-helical proteins is presented. It is
based on hydrophobicity forces and sequence independent local interactions. The
latter are chosen so as to favor the formation of alpha-helical structure. They
model chirality and alpha-helical hydrogen bonding. The global structures
resulting from the competition between these forces are studied by means of an
efficient Monte Carlo method. The model is tested on two sequences of length
N=21 and 33 which are intended to form 2- and 3-helix bundles, respectively.
The local structure of our model proteins is compared to that of real
alpha-helical proteins, and is found to be very similar. The two sequences
display the desired numbers of helices in the folded phase. Only a few
different relative orientations of the helices are thermodynamically allowed.
Our ability to investigate the thermodynamics relies heavily upon the
efficiency of the used algorithm, simulated tempering; in this Monte Carlo
approach, the temperature becomes a fluctuating variable, enabling the crossing
of free-energy barriers.
