  Specific interactions between cations and proteins have a strong impact on
peptide and protein structure. We here shed light on the nature of the
underlying interactions, especially regarding the effects on the polyamide
backbone structure. To do so, we compare the conformational ensembles of model
peptides in isolation and in the presence of either Li+ or Na+ cations by
state-of-the-art density-functional theory (including van der Waals effects)
and gas-phase infrared spectroscopy. These monovalent cations have a drastic
effect on the local backbone conformation of turn-forming peptides, by
disruption of the H bonding networks and the resulting severe distortion of the
backbone conformations. In fact, Li+ and Na+ can even have different
conformational effects on the same peptide. We also assess the predictive power
of current approximate density functionals for peptide-cation systems and
compare to results from established protein force fields as well as to
high-level quantum chemistry (CCSD(T)).
