Soft metal ions, Cd(II) and Hg(II), induce triple-stranded α-helical assembly and folding of a de novo designed peptide in their trigonal geometries

We previously reported the de novo design of an amphiphilic peptide (YGG(IEKKIEA)₄) that forms a native-like, parallel triple-stranded coiled coil. Starting from this peptide, we sought to regulate the assembly of the peptide by a metal ion. The replacement of the Ile18 and Ile22 residues with Ala and Cys residues, respectively, in the hydrophobic positions disrupted of the triple-stranded α-helix structure. The addition of Cd(II), however, resulted in the reconstitution of the triple-stranded α-helix bundle, as revealed by circular dichroism (CD) spectroscopy and sedimentation equilibrium analysis. By titration with metal ions and monitoring the change in the intensity of the CD spectra at 222 nm, the dissociation constant K(d) was determined to be 1.5 ± 0.8 μM for Cd(II). The triple-stranded complex formed by the ¹¹³Cd(II) ion showed a single ¹¹³Cd NMR resonance at 572 ppm whose chemical shift was not affected by the presence of Cl^- ions. The ¹¹³Cd NMR resonance was connected with the βH protons of the cysteine residue by ¹H-¹¹³Cd heteronuclear multiple quantum correlation spectroscopy. These NMR results indicate that the three cysteine residues are coordinated to the cadmium ion in a trigonal-planar complex. Hg(II) also induced the assembly of the peptide into a triple-stranded α-helical bundle below the Hg(II)/peptide ratio of 1/3. With excess Hg(II), however, the α- helicity of the peptide was decreased, with the change of the Hg(II) coordination state from three to two. Combining this construct with other functional domains should facilitate the production of artificial proteins with functions controlled by metal ions.