THE EFFECTS OF BASIS-SET AND BLOCKING GROUPS ON THE CONFORMATIONAL ENERGIES OF GLYCYL AND ALANYL DIPEPTIDES - A HARTREE-FOCK AND MP2 STUDY

We present the results of high level ab initio molecular orbital calcu lations on glycyl and alanyl dipeptides. The results of calculations o n the low energy conformers of the methyl-blocked analogs at the MP2/T ZVP//HF/6-31G* level of theory have previously been reported by two o f the authors. In this paper, we examine the effect of carrying out th e geometry optimizations of the three methyl-blocked glycyl dipeptide conformers using a larger TZVP basis set followed by an MP2 single-poi nt calculation with that basis set. The resulting geometries and energ ies were essentially the same as those obtained from optimization with the smaller 6-31G* basis set followed by an MP2 single-point calcula tion with the TZVP basis set. This study then provides additional supp ort for the use of the smaller 6-31G* quantum mechanical basis set fo r the geometry optimizations, since it yields essentially the same str uctures as the larger TZVP basis set at a reduced computational cost. We also carried out MP2/TZVP//HF/6-31G* calculations on the hydrogen- blocked analogs of both dipeptides, so that we might make a more direc t comparison with energies previously reported by Head-Gordon and co-w orkers. The two earlier studies yielded fairly similar results for the two dipeptides, with the exception of glycyl dipeptide where there wa s an -0.9 kcal mol-l difference in the relative energies determined fo r the C5 conformation. The calculations reported here reveal that this difference was caused by the choice of different blocking groups (hyd rogen vs. methyl), rather than the choice of quantum mechanical basis set. (C) 1997 Elsevier Science B.V.