An ab initio study of the peptide bond formation between alanine and glycine: electron correlation effects on the structure and binding energy

Ab initio methods are used to analyze the structure, energetics and binding energy of the four possible dipeptides that can be formed from alanine and glycine in gas phase. The structures of the peptides are optimized using Hartree-Fock, second-order Moller-Plesset perturbation theory and density functional methods (DFT). The effect of electron correlation is analyzed with special emphasis on the calculated binding energies. Single-point energy calculations are performed with CCSD(T) on MP2 geometries to get some additional information on the correlation effects. Electron correlation effects and zero-point vibrational energy corrections increase the binding energy. At the highest level, CCSD(T), we find that the binding energies for alanylalanine, alanylglycine, glycylalanine and glycylglycine are 4.86, 5.09, 5.61 and 5.89 kcal/mol, respectively. These numerical results suggest that glycine donates the OH group easier than alanine. A comparison between the Moller-Plesset and DFT in different basis sets is made and gives indication of the usefulness of these methods for bio-molecules and peptide formation. Two functionals, B3LYP and B3P86 with different basis sets differing by the systematic inclusion of diffuse and polarization functions, are used in the DFT method. The results obtained using both functionals with a basis that includes both diffuse and polarization functions are in reasonable agreement with the Moller-Plesset results. However, without including zero-point corrections, some DFT results lead to non-bonding of the peptide molecule. (C) 2002 Elsevier Science B.V. All rights reserved.