Complex ion effects on polypeptide conformational stability: Chloride and sulfate salts of guanidinium and tetrapropylammonium

The effects of chloride and sulfate salts of tetrapropylammonium (TPA+) and guanidinium (Gdm+) on the conformational stabilities of tryptophan zipper (trpzip) and α-helical (alahel) peptides were measured by circular dichroism spectroscopy. Like Gdm+, TPA+ interacts with the planar tryptophan indole group, perturbing the conformational stability of trpzip peptides. TPA+ effects are largely unaffected by sulfate, indicating an absence of the heteroion pairing that is observed in concentrated Gdm2SO4 solutions. TPA+ stabilizes helical conformations in alahel peptides, indicating exclusion from the peptide bond. The observations are broadly consistent with predictions of molecular dynamics simulations [Mason, P. E.; et al. J. Phys. Chem. B2009, 113, 3227–3234], indicating that the effects of complex ions on proteins are increasingly predictable in terms of ion hydration, complementary interactions with specific protein groups, and ion-pairing contributions.