Metal Clips That Induce Unstructured Pentapeptides To Be α-Helical In Water

Short peptides corresponding to protein helices do not form thermodynamically stable helical structures in water, a solvent that strongly competes for hydrogen-bonding amides of the peptide backbone. Metalloproteins often feature metal ions coordinated to amino acids within hydrogen-bonded helical regions of protein structure, so there is a prospect of metals stabilizing or inducing helical structures in short peptides. However, this has only previously been observed in nonaqueous solvents or under strongly helix-favoring conditions in water. Here cis-[Ru(NH₃)₄(solvent)₂]²⁺ and [Pd(en)(solvent)₂]²⁺ are compared in water for their capacity as metal clips to induce α-helicity in completely unstructured peptides as short as five residues, Ac-HARAH-NH₂ and Ac-MARAM-NH₂. More α-helicity was observed for the latter pentapeptide and, when chelated to ruthenium, it showed the greatest α-helicity yet reported for a short metallopeptide in water (∼80%). Helicity was clearly induced rather than stabilized, and the two methionines were 10¹³-fold more effective than two histidines in stabilizing the lower oxidation state Ru(II) over Ru(III). The study identifies key factors that influence stability of an α-helical turn in water, suggests metal ions as tools for peptide folding, and raises an intriguing possibility of transiently coordinated metal ions playing important roles in native folding of polypeptides in water.