Salt-Specific Stability of Short and Charged Alanine-Based α-Helices

The alpha-helical stability of the short and net-charged (+/- 6e) alanine-based peptides (AE)(6) and (AK)(6) ill similar or equal to 2.5 M electrolyte solution (NaCl, KCl, NaI, and KI) is investigated by 1 mu s long all-atom computer simulations. While the nonspecific screening of electrostatic repulsion between the charged side chains stabilizes the compact helical configuration, a competing destabilization effect is induced by considerable binding of sodium (Na+) to the backbone carbonyl groups which is much weaker for potassium (K+). If Cl- is exchanged by the large anion I-, cation binding and thus helix destabilization is increased due to osmotic effects and the binding affinity of I- to the hydrophobic alanine side chains, thereby dragging cations to the peptide. While the I- propensity to alanine is enhanced for the positively net-charged (AK)(6), the helix destabilization effect by NaI for this peptide, however, is much weaker when compared to (AE)(6) due to the (electrostatically induced) depletion of Na+ at the peptide backbone; this, and the fact that KI is found to be a weak destabilizer too, demonstrates that I- alone is not responsible for denaturation but assists and amplifies cationic action. Our study exemplifies the Molecular and highly synergetic mechanisms behind specific ion-induced (de)stabilization of protein secondary structures and its sensitive dependence on local peptide net charge and sign of charge.