Correlated ab initio calculations are used to compare the energetics when the CH and NH groups of the model dipeptide CHONHCH2CONH2 are each allowed to form a H-bond with the proton acceptor O of a peptide group. When the dipeptide is in its C7 conformation, the NH··O H-bond energy is found to be 7.4 kcal/mol, as compared to only 2.8 kcal/mol for the CH··O interaction. On the other hand, the situation reverses, and the CH··O H-bond becomes stronger than NH··O, when the dipeptide adopts a C5 structure. This reversal is important as C5 is nearly equal in stability to C7 for the dipeptide, and is representative of the commonly observed β-sheet structure in a protein. Immersing the dipeptide−peptide pair in a model solvent weakens both sorts of H-bonds, and in a fairly uniform manner. Consequently, the trends observed in the in vacuo situation retain their validity in either aqueous solution or the protein interior. Likewise, the desolvation penalty, suffered by removing a H-bonded complex from water and placing it in the less polar interior of a protein, is quite similar for the NH··O and CH··O bonds.