Salt-specific stability and denaturation of a short salt-bridge-forming alpha-helix

The structure of a single alanine-based Ace-AEAAAKEAAAKA-Nme peptide in explicit aqueous electrolyte solutions (NaCl, KCl, NaI, and KF) at large salt concentrations (3-4 M) is investigated using approximately 1 mus molecular dynamics (MD) computer simulations. The peptide displays 71% alpha-helical structure without salt and destabilizes with the addition of NaCl in agreement with experiments of a somewhat longer version. It is mainly stabilized by direct and indirect (" i + 4")EK salt bridges between the Lys and Glu side chains and a concomitant backbone shielding mechanism. NaI is found to be a stronger denaturant than NaCl, while the potassium salts hardly show influence. Investigation of the molecular structures reveals that consistent with recent experiments Na (+) has a much stronger affinity to side chain carboxylates and backbone carbonyls than K (+), thereby weakening salt bridges and secondary structure hydrogen bonds. At the same time, the large I (-) has a considerable affinity to the nonpolar alanine in line with recent observations of a large propensity of I (-) to adsorb to simple hydrophobes, and thereby "assists" Na (+) in its destabilizing action. In the denatured states of the peptide, novel long-lived (10-20 ns) "loop" configurations are observed in which single Na (+) ions and water molecules are hydrogen-bonded to multiple backbone carbonyls. In an attempt to analyze the denaturation behavior within the preferential interaction formalism, we find indeed that for the strongest denaturant, NaI, the protein is least hydrated. Additionally, a possible indication for protein denaturation might be a preferential solvation of the peptide backbone by the destabilizing cosolute (sodium). The mechanisms found in this work may be of general importance to understand salt effects on protein secondary structure stability.