Helix-coil transitions of amino-acid homo-oligomers in aqueous solution studied by multicanonical simulations

Helix-coil transitions of homo-oligomers in aqueous solution are studied by multicanonical Monte Carlo simulations. The solvation effects are represented by the sum of the terms that are proportional to the solvent-accessible surface area of the atomic groups. Homo-oligomers of length 10 are considered for three characteristic amino acids, alanine, valine, and glycine, which are helix former, helix indifferent, and helix breaker, respectively. We calculated as a function of temperature the distributions of the backbone dihedral angles, the average values of total energy, and its component terms of the homo-oligomers. It is shown that for homo-alanine, the helix-coil transition exists and that the transition temperature in water is considerably lower than in gas phase, which implies that the effects of solvation tend to reduce helical content. Moreover, the helix propagation parameter s and nucleation parameter sigma of the Zimm-Bragg model were calculated. The s values that were obtained from the simulations in aqueous solution are in remarkable agreement with the experimental results. (C) 2000 American Institute of Physics. [S0021-9606(00)51723-1].