For the BDEs given in Table , peptides were modeled in a fully extended trans geometry (ω ≈ 180 • ), except for proline and hydroxyproline, where both cis and trans conformers were modeled by setting ω, φ , ψ, and ω ′ going from the N to the C terminus to approximately 0 • , -75 • , 160 • , 180 • , and 180 • , -75 • , 150 • , 180 • , respectively. Where applicable, amino acids were considered in their charged and neutral forms. For histidine, the neutral N 1 -H and N 3 -H tautomers were considered. In the gas phase, the N 3 -H tautomer is more stable by 4.9 kJ/mol. For arginine, in agreement with previous investigations, out of the five possible neutral tautomers, the one in which the two terminal N atoms are doubly protonated was found to be most stable, and so this tautomer was considered.