Conformational properties of neutral and charged alanine and glycine   chains
  We have investigated the potential energy surface for neutral and charged alanine and glycine chains consisting of up to 6 amino acids. For these molecules we have calculated potential energy surfaces as a function of the Ramachandran angles phi and psi. Our calculations are performed within it ab initio theoretical framework based on the density functional theory and also within semi-empirical model approaches. We have demonstrated that the excessive positive charge of the system influences strongly its geometrical and conformational properties. With increasing of the excessive charge amino acid chains become unstable and decay into two or more fragments. We have analysed how the secondary structure of polypeptide chains influences the formation of the potential energy landscapes. We have calculated the energy barriers for transitions between different molecular conformations and determined the ones being energetically the most favourable. 
Alexander V. Yakubovitch, Ilia A. Solov'yov, Andrey V. Solov'yov and   Walter Greiner