Energetic contribution of solvent-exposed ion pairs to alpha-helix structure

Understanding the role of amino acid side-chain interactions in forming secondary structure in proteins is useful for deciphering how proteins fold and for predicting folded structures of proteins from their sequence. Analysis of the secondary structure as a function of pH in two designed synthetic peptides with identical composition but different sequences, affords a quantitative estimate of the free energy contribution of a single ion pair to the stability of an isolated α-helix. One peptide contains repeated blocks of Glu₄Lys₄. The second has repeated blocks of Glu₂Lys₂. The former contains significant helical structure at neutral pH while the latter has none, based on ultraviolet light circular dichroism measurements and ¹H nuclear magnetic resonance spectroscopy. The difference is attributed to formation of helix-stabilizing salt-bridges between Glu^- and Lys⁺ spaced at i, i + 4 intervals in the former peptide. The free energy of formation of a single Glu^--Lys⁺ salt-bridge can be evaluated by using a statistical model of the helix-coil transition that explicitly includes salt-bridges: the result is -0.50(±0.05) kcal/mol at 4 °C and neutral pH in 10 mm salt, in agreement with a value derived for a single salt-bridge in a helix on the surface of a globular protein.