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 Gdm(2)SO(4) 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.