Effects of Hofmeister ions on the α-helical structure of proteins

Abstract

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 Förster resonance energy transfer, and atomistic computer simulations to elucidate salt-specific effects on the structure of three peptides with large α-helical propensity. CD indicates a complex ion-specific destabilization of the α-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 Förster resonance energy transfer measurements reveal highly compressed peptide conformations in molar concentrations of NaClO(4) in contrast to strong swelling in the presence of GdmCl. The compacted states observed in the presence of NaClO(4) originate from a tight ion-backbone network that leads to a highly heterogeneous secondary structure distribution and an overall lower α-helical content that would be estimated from CD. Thus, NaClO(4) denatures by inducing a molten globule-like structure that seems completely off-pathway between a fully folded helix and a coil state.

Figure 1

Salt-specific effects on α-helical secondary structure measured by CD. The thermal melting curves and effect of salt concentration at 273 K on the α-helicity of (A and B) EK, (C and D) (AK)₆, and (E and F) (AE)₆ peptides in 3.9 M concentration are plotted for various salts. Helicity was calculated from the CD signal at 222 nm (see Methods in the Supporting Material).

Figure 2

Salt-specific structures studied by MD simulations for T = 300 K. (A) Normalized density distribution g(r) of water (O) and cations (all from chloride salts) around the amide oxygen of the EK-peptide. Inset: g(r) for water and cations around the ALA methyl group. (B) Normalized density distribution g(r) of water (O) and anions around the amide nitrogen of the EK-peptide. Inset left: g(r) for water and anions around the ALA methyl group. (C) Ion coordination in a 0.5 nm shell around the backbone plotted versus average helicity h. Coordination increases upon unfolding due to direct binding to the backbone. (D) Nonpolar and polar ASA of the EK and (AK)₆ peptides plotted versus helicity h. Both nonpolar and polar ASA linearly increase upon unfolding.

Figure 3

Representative MD simulations snapshots of Gdm⁺ and ClO₄⁻ binding to the EK and (AK)₆ peptides. (A) A Gdm⁺-ion (spheres) hydrogen bonded to the backbone (stick structure) amide and embedded by a nonpolar lysine and alanine methyl groups of the EK peptide. (B) Gdm⁺ binding to (AK)₆ in the coil state. (C) A ClO₄⁻ -ion hydrogen bonded to the backbone amide and embedded by lysine side chains in the (AK)₆ peptide. (D) ClO₄⁻ and Na⁺ displaying concerted binding to the backbone in a compact state of the EK-peptide. Snapshots in A and B were chosen using the kinetic criterion such that ion binding was longer than 0.1 ns. In D the ionic complex was bound for >1 ns.

Figure 4

Ion-specific effect on the CD spectra and donor-acceptor fluorophore separation R_DA of (AK)₁₄. (A) Measured R_DA (symbols) in the absence of salt at pH 7 (black), with KCl (yellow upper triangles), GdmCl (red squares), NaClO₄ (green circles), and no salt pH 11 (light blue line). Standard deviations from several measurements are smaller than the size of the symbol (0.05 nm). Estimated range of R_DA from MD-derived structures at 3.5M salt is represented as shaded areas depicting the upper and lower limits. (B) The salt-dependent helicity (derived from the signal at 222 nm) of (AK)₁₄ in the absence of salt at pH 7 (black), with KCl (yellow), GdmCl (red), NaClO₄ (green), and no salt pH 11 (light blue). The average value is a dotted line while the thickness of the shaded area represents the upper and lower boundaries. (C) CD spectra of (AK)₁₄ in the absence of salt at pH 7 (black), with KCl (yellow), GdmCl (red), NaClO₄ (green), and no salt pH 11 (light blue).