Effects of Hofmeister Ions on the α-Helical Structure of Proteins

The molecular conformation of proteins is sensitive to the nature of the aqueous environment. In particular, the presence of ions can stabilize or destabilize (denature) protein secondary structure. The underlying mechanisms of these actions are still not fully understood. Here, we combine circular dichroism (CD), single-molecule Forster resonance energy transfer, and atomistic computer simulations to elucidate salt-specific effects on the structure of three peptides with large alpha-helical propensity. CD indicates a complex ion-specific destabilization of the alpha-helix that can be rationalized by using a single salt-free computer simulation in combination with the recently introduced scheme of ion-partitioning between nonpolar and polar peptide surfaces. Simulations including salt provide a molecular underpinning of this partitioning concept. Furthermore, our single-molecule Forster resonance energy transfer measurements reveal highly compressed peptide conformations in molar concentrations of NaClO4 in contrast to strong swelling in the presence of GdmCl. The compacted states observed in the presence of NaClO4 originate from a tight ion-backbone network that leads to a highly heterogeneous secondary structure distribution and an overall lower alpha-helical content that would be estimated from CD. Thus, NaClO4 denatures by inducing a molten globule-like structure that seems completely off-pathway between a fully folded helix and a coil state.