Capping alanine with a formyl and an amino group raises the C α -H BDE by around 30 kJ/mol compared to the free amino acid by lowering the captodative stabilization by the electron donating and withdrawing groups. This strong dependency on capping indicates a significant influence of the protein backbone on BDEs. Calculations at the G4(MP2)-6X level of theory further show that, although the C α -H BDE of alanine as a free amino acid is lower than that of glycine as a free amino acid, already when considering the next atoms along the peptide backbone the relative order is reversed. The difference between the two BDEs becomes larger if one includes the next two atoms. This is in agreement with previous calculations at the G3(MP2)-RAD level of theory, . The trend might be due to increased steric interactions for the C α alanine dipeptide radical that outcompete the inductive stabilization (a trend that appears reversed for leucine, see Table ). Therefore, both composite methods come to the conclusion that the C α -H BDE of glycine in a protein should be lower than that of alanine. At the BMK/6-31+G(2df,p) level of theory, this is reproduced for the acetyl and N-methyl capping groups. Hence, all X-H BDEs in this work were computed using these capping groups. The resulting peptide models are shown in Figure . At the B3LYP/6-31G(d) level of theory, the BDE of the alanine dipeptide remains higher than that of the glycine dipeptide even if acetyl and N-methyl capping groups are used, although the exact values are closer to the G4(MP2)-6X results than at the BMK/6-31+G(2df,p) level of theory. Consequently, if one were to use B3LYP, at least the next three atoms along the protein backbone would have to be included.