Electronic structure analysis of glycine oligopeptides and glycine-tryptophan oligopeptides

Using the density functional theory (DFT), we have studied the energy gap, charge distribution, density of states and chemical activity of glycine (Gn) oligopeptides and glycine-tryptophan (GWn) oligopeptides. The results show that: (1) with the increasing of Gn residues, the chemical activity of Gn oligopeptides focuses on the terminal amino and carboxyl groups, which may be the main cause of self-assembly behaviors in Gn oligopeptide chains; (2) the chemical reaction activity has size effect. The size effect disappears when the residue number exceeds 7. The Gn oligopeptides with 7 residues is the shortest chain which has the same reaction activity as that of longer size peptide; (3) the activity of GWn oligopeptides presents size effect and odd-even effect. However, the size effect and odd-even effect both vanish when the chain of GWn oligopeptides is longer than 12 residues. (4) It is difficult in self-assembly for GWn oligopeptide chains, because its activity mainly focuses on the indole ring and the Gn residues at the end of oligopeptides. (5) The big side groups result in the very near energy level of LUMO and LUMO+1 of GWn oligopeptide chains. It shows that the electron-accepting ability of oligopeptide chainsis composed of two orbitals addition. The results in the paper may help us understand the changes of physical and chemical properties of peptide synthesis process.