A Minimal Off-Lattice Model for Alpha-helical Proteins
  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. 
Frank Potthast