An ab initio exploratory study on the conformational features of the dipeptide MeCO-Ala-Ala-NH-Me in its four different configurations: determination of the behaviour of d-enantiomer amino acids within a peptide chain

Ab initio conformational studies at the RHF/3-21G level of theory were carried out for the dipeptide MeCO-Ala-Ala-NH-Me in its four different configurations (MeCO-l-Ala-l-Ala-NH-Me, MeCO-d-Ala-d-Ala-NH-Me, MeCO-l-Ala-d-Ala-NH-Me and MeCO-d-Ala-l-Ala-NH-Me). From this method the conformations for these dipeptides were found to be βlγl, βlγd, βlγd and βlγl, respectively. Patterns were investigated on Ramachandran maps to identify annihilated critical points among the four dipeptides to determine whether the d-enantiomer was in fact the ‘mirror image’ of the l-enantiomer dipeptide in addition to determining the role of the d-isomer in the peptide chain. The differences in energies for each conformation were also compared between the dipeptides.