Metal Clips That Induce Unstructured Pentapeptides To Be r-Helical In Water Michelle T. Ma, † Huy N. Hoang, ‡ Conor C. G. Scully, ‡ Trevor G. Appleton, † and David P. Fairlie* ,‡ School of Chemistry and Molecular Biosciences, and Institute for Molecular Bioscience, The UniVersity of Queensland, Brisbane Qld 4072, Australia Received January 4, 2009; E-mail: d.fairlie@imb.uq.edu.au Abstract: 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(NH 3 ) 4 (solvent) 2 ] 2+ and [Pd(en)(solvent) 2 ] 2+ are compared in water for their capacity as metal clips to induce R-helicity in completely unstructured peptides as short as five residues, Ac-HARAH-NH 2 and Ac-MARAM-NH 2 . More R-helicity was observed for the latter pentapeptide and, when chelated to ruthenium, it showed the greatest R-helicity yet reported for a short metallopeptide in water (∼80%). Helicity was clearly induced rather than stabilized, and the two methionines were 10 13 -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 R-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. Introduction Over 30% of amino acids in proteins exist in R-helical structures. 1 In metalloproteins, transition metal ions are often bound to R-helical protein segments. 2 When the helix is buried in the hydrophobic interior of a protein, metal-helix interactions can be important in stabilizing protein tertiary structure. When the helix is exposed on a protein surface, metal-helix interac- tions can shape either a catalytic site, a ligand binding cleft, or peptide domains that interact with macromolecules. 2,3 Despite extensive studies of metalloproteins and model metallopeptides, the capacity of metal ions to initiate or stabilize helical structures for short peptides in water remains obscure. 4,5 An intriguing question is whether a preformed peptide helix is stabilized by capture of a metal ion (Scheme 1, path a) or whether peptide helicity is induced following metal capture (Scheme 1, path b). A significant problem in studying helices is that short synthetic peptides corresponding to helical segments (4-15 amino acids) of proteins are not thermodynamically stable helices in water, away from hydrophobic protein environments. 6 † School of Chemistry and Molecular Biosciences. ‡ Institute for Molecular Bioscience. (1) (a) Barlow, D. J.; Thornton, J. M. J. Mol 1988, 201, 601. (b) Fairlie, D. P.; West, M. L.; Wong, A. K. Curr. Med. Chem. 1998, 5, 29–62. (c) Andrews, M. J.; Tabor, A. B. Tetrahedron 1999, 55, 11711–43. (d) Fletcher, S.; Hamilton, A. D. Curr. Opin. Chem. Biol. 2005, 9, 632–8. (2) (a) Metalloproteins in BioactiVe Molecules; Otsuka, S.; Yamanaka, T., Eds.; Elsevier: Tokyo, 1988; Vol. 8. (b) Xue, Y.; Okvist, M.; Hansson, O.; Young, S. Protein Sci. 1998, 7, 2099. (c) Holland, D. R.; Hausrath, A. C.; Juers, D.; Matthews, B. W. Protein Sci. 1995, 4, 1955. (3) (a) Elrod-Erickson, M.; Rould, M. A.; Nekludova, L.; Pabo, C. O. Structure 1996, 4, 1171. (b) Cowan, J. A. J. Inorg. Biochem. 1993, 49, 171–175. (4) (a) Ghadiri, M. R.; Fernholz, A. K. J. Am. Chem. Soc. 1990, 112, 9633–5. (b) Ghadiri, M. R.; Choi, C. J. Am. Chem. Soc. 1990, 112, 1630–2. (5) (a) Ruan, F.; Chen, Y.; Hopkins, P. B. J. Am. Chem. Soc. 1990, 112, 9403–4. (b) Tofteng, A. P.; Hansen, T. H.; Brask, J.; Nielsen, J.; Thulstrup, P. W.; Jensen, K. J. Org. Biomol. Chem. 2007, 2225–2233. (c) Kohn, W. D.; Kay, C. M.; Sykes, B. D.; Hodges, R. S. J. Am. Chem. Soc. 1998, 120, 1124–32. (d) Kohtani, M.; Kinnear, B. S.; Jarrold, M. F. J. Am. Chem. Soc. 2000, 122, 12377. (e) Kise, K. J., Jr.; Bowler, B. E. Biochemistry 2002, 41, 15826–37. (f) Nicoll, A. J.; Miller, D. J.; Fuetterer, K.; Ravelli, R.; Allemann, R. K. J. Am. Chem. Soc. 2006, 128, 9187–93. (6) (a) Zimm, B. H.; Bragg, J. K. J. Chem. Phys. 1959, 31, 526–35. (b) Scholtz, J. M.; Baldwin, R. L. Annu. ReV. Biophys. Biomol. 1992, 21, 95–118. Scheme 1 (a) Helix stabilization by metal (M) capture after helix formation versus (B) Helix induction after metal capture. Published on Web 03/05/2009 10.1021/ja900047w CCC: $40.75  2009 American Chemical Society J. AM. CHEM. SOC. 2009, 131, 4505–4512 9 4505