Despite the key role played by deamidation reactions in biochemical phenomena such as aging processes, knowledge of factors determining peptide backbone N-H acidities is scarce. We report a theoretical study on this topic by means of quantum-chemical calculations. Gas-phase acidities and pK a's in water have been estimated. The results agree reasonably well with available experimental data. Further analysis suggests that the secondary peptide structure, in addition to hydration effects, is the main factor determining pKa. In particular, we predict N-H protons to be more acidic in β-turns than in α-helices, a finding that may have broad biological implications. © 2013 Elsevier B.V. All rights reserved.