Versatile ruthenium complexes based on 2,2′-bipyridine modified peptoids

Abstract

Helical peptoids bearing 2,2′-bipyridine form ruthenium complexes via intermolecular binding to linear peptoid strands or intramolecular binding to a cyclic scaffold. Ru(ii) binding promoted changes in the conformational order of the peptoids, and chiral induction from the peptoids to their metal center was observed.

---

One of the most extensively used ligands in coordination chemistry is 2,2′-bipyridine (bipy),¹ a bidentate chelator capable of forming complexes with various metal ions including Ru²⁺.² In the last two decades, bipy derivatives were also incorporated within peptides³ and peptide mimics,^4,5 and their metal-binding properties were described. The combination between bipy and peptidomimetic scaffolds allowed coordination of versatile metal cations to peptides^3b and high binding affinities for metal ions⁶ targeting various biological and chemical applications.^6–10 Thus, the introduction of bipy to peptidomimetics holds promise as a platform for the construction of unique metallofoldamers.¹¹ Peptoids,¹²N-substituted glycine oligomers, are a class of foldamers¹³ that can adopt helical secondary structures with a helical pitch of three residues per turn via the incorporation of bulky chiral side chains within the oligomer sequence.¹⁴ Peptoids can be efficiently synthesized from primary amines on a solid support via the two-step “submonomer” approach,¹⁵ resulting in highly versatile scaffolds. This synthesis should enable not only the facile incorporation of metal-binding ligands but also a simple tuning of the ligand(s) position, quantity and identity, towards the creation of various metallopeptoids.¹⁶ Herein we explore the incorporation of bipy within the peptoid sequence and study three bipy modified helical peptoids – a linear pentamer bearing one bipy ligand (L1B), a linear hexamer with two bipy ligands (L2B) and a cyclic hexamer having three bipy ligands (C3B) – for the creation of versatile peptoid–ruthenium architectures as the intermolecular complexes (L1B)₃Ru and (L2B)₃Ru₂, and the intramolecular complex (C3B)Ru (Fig. 1).

Fig. 1 Chemical structures of peptoid oligomers bearing 2,2′-bipyridine units and their anticipated ruthenium complexes.

Initial attempts to incorporate 6-methylamine-2,2′-bipyridine¹⁷ within several peptoid sequences were not successful, probably due to extensive side reactions on the nitrogen atoms.¹⁸ We therefore decided to synthesize the bipy amine derivative 2-(2,2′-bipyridin-6-yloxy) ethylamine according to a previously published procedure for the synthesis of 2-(2,2′:6′,2′′-terpyridin-4′-yloxy) ethylamine.¹⁹ The compatibility of this bipy derivative (Nbp) with the “submonomer” method was tested by its incorporation at the third position of a peptoid tetramer containing methyl benzyl synthons at the other positions. HPLC analysis at 214 nm of the crude peptoid after each step – Nbp displacement, subsequent acylation and the successive methyl benzylamine displacement (Fig. S16, ESI†), as well as ESI-MS analysis of the crude peptoid tetramer, confirmed the successful incorporation of this bipy derivative within the sequence.

Peptoid oligomers L1B and L2B were designed such that each contains four bulky chiral side chains, which are sufficient for attaining helicity in oligomers of this length.^14d In L2B, the ligands were placed at positions i and i + 3, which match the pitch of the helix, in order to orient these groups in proximity on the same face of the backbone and facilitated the creation of the intermolecular complex depicted in Fig. 1.²⁰ Peptoid C3B was designed as a hexamer with three bipy ligands in alternating positions on the sequence such that they will all face the same side of the macrocycle plain and facilitate an intramolecular binding.²¹ The linear peptoids L1B, L2B and L3B, as well as the dimer Di-L1B (see the ESI†), incorporating Nbp, (S)-(−)-1-phenylethylamine (Nspe) and chloropropylamine (Npl, only L3B) as synthons were synthesized on the solid support employing the “submonomer” protocol. The peptoid C3B was prepared by a microwave-assisted cyclization of peptoid L3B following a method that we have recently published.²² All the peptoids were analysed and purified by HPLC, and their sequences were confirmed by ESI-MS. The Ru²⁺ complexes (L1B)₃Ru, (L2B)₃Ru₂, (C3B)Ru and (Di-L1B)₃Ru were synthesized using a modification of a previously reported protocol,²³ by adding 1 equiv. of RuCl₃ hydrate to 3 equiv. of L1B or Di-L1B, 1.5 equiv. of L2B and 1 equiv. of C3B, respectively in ethanol under reflux conditions. After a few hours of stirring, the pale yellow colour of the peptoid solution turned blue in the case of L1B and Di-L1B, and red-brown in the case of L2B and C3B. The final colour of all the complexes was red-orange. These complexes were purified and analysed by HPLC, and their identity was confirmed by detailed mass spectrometry studies including computed and experimental isotopic envelope analysis (see the ESI†). These studies show that all the Cl⁻ ligands were replaced by the bipy-peptoids, supporting the suggested binding mode of the complexes presented in Fig. 1.

Metal free peptoids L1B, L2B and C3B exhibit absorption bands near λ = 302, 299 and 298 nm, respectively, in acetonitrile, corresponding to the π–π* transition of bipy. The complexes (L1B)₃Ru, (L2B)₃Ru₂ and (C3B)Ru reveal shifts in these absorption bands with values of λ = 303, 305 and 305 nm, respectively, and additional bands near λ = 463, 461 and 464 nm, respectively, which correspond to the metal-to-ligand charge transfer (MLCT) bands of the ruthenium polypyridine complexes (Fig. 2A, D and G).

Fig. 2 UV-Vis and CD spectra measured at rt. in acetonitrile: (A, D and G) UV-Vis spectra of L1B (50 μM), L2B (50 μM), C3B (17 μM) and their Ru²⁺ complexes (17 μM each) respectively. (B, E and H) CD spectra of L1B, L2B, C3B (100 μM each) and their Ru²⁺ complexes (30 or 100 μM each), respectively. (C, F and I) CD spectra of (L1B)₃Ru and (L2B)₃Ru₂, and (C3B)Ru (200 μM each), respectively.

Circular dichroism (CD) measurements revealed some changes upon the formation of the complexes (L1B)₃Ru and (L2B)₃Ru₂, and significant changes upon (C3B)Ru formation. Solutions of all three complexes showed an increase, relative to the metal-free peptoids, in the ellipticity near 200 nm while only the complex (C3B)Ru exhibited a major decrease, relative to the metal-free peptoid, in the magnitude of the CD signal near 218 nm (Fig. 2B, E and H, black and red lines).

It has been previously demonstrated that Nspe peptoids adopt right-handed helices,^14b that have characteristic CD spectra with bands near 190 and 200 nm, the latter has been associated with the trans-amide bond conformation, and 218 nm, which is associated with the cis-amide bond conformation.¹⁴ Both peptoids L1B and C3B exhibit CD spectra with an intense band at 218 nm (Fig. 2B and H, red lines), indicating that helices with only cis-amide bonds are the major population in solution. Upon formation of the complexes (L1B)₃Ru and (C3B)Ru an increase in the band near 200 nm was obtained (Fig. 2B and H, red lines), indicating that the helices with only cis-amide bonds are no longer the dominant population in solution. For (L1B)₃Ru, this increase implies a decrease in the overall conformational order of the peptoid upon ruthenium binding.²⁴ In the case of (C3B)Ru, there is also a decrease in the band near 218 nm, suggesting that the conformational order is almost diminished due to a significant alteration of the helical backbone after ruthenium complexation. The CD spectrum of L2B exhibits similar intensities of both bands near 200 and 218 nm (Fig. 2E, black line), reflecting the conformational heterogeneity of this peptoid. A solution of its ruthenium complex exhibited increases, relative to the metal-free peptoid, in the magnitude of both of these CD signals (Fig. 2E, red line), suggesting an increase in its overall conformational order as the magnitude of the signal in this case, reflecting the degree of helicity.^8d,e,16b

Ruthenium binding to all three peptoids also produced new bands in the region between 260 and 360 nm corresponding to the π–π* transition of bipy. These bands, however, were very weak and did not show the expected Cotton effects. In order to probe this point we repeated the CD measurements using higher solution concentrations (200 μM instead of 30 or 100 μM) of (L1B)₃Ru, (L2B)₃Ru₂, and (C3B)Ru and indeed obtained stronger CD signals that showed clear Cotton effects with minima at λ_max = 305, 316 and 314 nm, respectively (Fig. 2C, F and I, red lines). These signals reflect the induction of chirality from the peptoid scaffold to the metal center,^16b and imply at more preferable stereochemistry of the Δ isomer over the Λ isomer.²⁵ The bipy π–π* CD signal of (C3B)Ru exhibits an exciton couplet,²⁶ with a minimum at 314 nm and a maximum at 289 nm, crossing ε = 0 near 296 nm. In contrast, (L2B)₃Ru₂ exhibits a weaker exciton couplet, which is completely absent in the case of (L1B)₃Ru. This can be attributed to reduced conformational constraints in the intermolecular complexes, which can diminish the effect of the dipole–dipole interactions responsible for an exciton couplet. In order to further explore the preference of stereochemistry, we prepared three more peptoids using Nrpe monomers instead of Nspe monomers, namely R-L1B, R-L2B and R-C3B, and their corresponding Ru²⁺ complexes (R-L1B)₃Ru, (R-L2B)₃Ru₂ and (R-C3B)Ru. These complexes were purified and analysed by HPLC, characterized by UV and their identity was confirmed by detailed MS studies, exhibiting the same absorbance bands and masses as their corresponding Nspe containing Ru–peptoids (see the ESI†). CD measurements using 200 μM solutions of these complexes displayed the exact opposite spectra and Cotton effect relative to the Nspe containing peptoids–Ru complexes (Fig. 2C, F and I, blue lines). These results demonstrate that the stereochemistry of the Ru²⁺ center can indeed be controlled and dictated by the chirality of the peptoid scaffold.

The redox properties of the new ruthenium complexes were realized from cyclic voltammetry. (L1B)₃Ru, (L2B)₃Ru₂, (C3B)Ru and (Di-L1B)₃Ru exhibit a reversible oxidation process of the metal centered Ru^2+/3+ couple at E^ox_p of 0.94 V, 0.88 V, 0.90 V and 0.88 V vs. Ag/AgNO₃ respectively. These oxidation potentials imply that all the bipy ligands from the peptoids are coordinating Ru²⁺ and there are no Cl⁻ ions bound as ligands.²⁷ These oxidation potentials are negatively shifted compared to that of (bp)₃Ru, which is centered around 1.02 V (Fig. 3A, green line), probably due to the electron withdrawing nature of the amides. To probe this point, we have synthesized 2-(2,2′-bipyridin-6-yloxy) ethyl(dimethyl)amine (bp-DM), which contains the methoxy group but lacks the amide. Its Ru²⁺ complex (bp-DM)₃Ru was prepared, purified and characterized by UV and MS spectroscopies, and its CV showed an oxidation potential at E^ox_p of 1.22 V, which expresses a positive shift compared to Ru(bipy)₃, probably due to the donating character of the methoxy group. Notably, similar intensities of the current flows are obtained in identical concentrations of the complexes (L1B)₃Ru and (C3B)Ru (0.5 mM) and in as twice as less concentrated solution of (L2B)₃Ru₂ (0.25 mM). We therefore propose that the two Ru centers in (L2B)₃Ru₂ are oxidized simultaneously, thus giving rise to only one oxidation peak.

Fig. 3 Comparison between the cyclic voltammograms of (A) 0.5 mM intermolecular Ru complexes and (B) 0.5 mM (L1B)₃Ru and (C3B)Ru and 0.25 mM (L2B)₃Ru₂ that were recorded in acetonitrile at rt. using a glassy carbon working electrode, platinum counter electrode, and Ag/AgNO₃ reference electrode. Scan rate: 100 mV s⁻¹.

Finally, we note that L2B can bind ruthenium in an intramolecular fashion leading to a different (L2B)₃Ru₂ complex in which two intramolecular (L2B)₂Ru complexes bind an additional L2B peptoid (Scheme 1). The weak exciton couplet exhibited by (L2B)₃Ru₂ and the similarity in the oxidation potentials of (L2B)₃Ru₂ and (C3B)Ru that demonstrate their similar central structure,^8c support the existence of such a complex. We can therefore assume that both the intermolecular (L2B)₃Ru₂ and the intramolecular/intermolecular (L2B)₃Ru₂ are present in solution.

Scheme 1

To conclude, this work describes the facile incorporation of bipy into the peptoid sequence for the design and synthesis of versatile bipy modified peptoid–ruthenium complexes. These include a helical triplex via intermolecular metal binding and a unique cyclometallopeptoid complex via intramolecular metal ligation of three pendant groups. Our results demonstrate that the dual effect of ruthenium binding; the chirality of the peptoid establishes an asymmetric environment about the metal center while metal complexation causes modifications in the conformational order of the peptoid backbone. Specifically, CD analysis indicates that the stereochemistry of the Ru²⁺ center is influenced by the chirality of the peptoid scaffold. We are currently studying other bipy-modified metallopeptoids and exploring the photophysical properties of peptoid–ruthenium complexes towards photocatalysis.

Notes and references

1. C. Kaes, A. Katz and M. W. Hosseini, Chem. Rev., 2000, 100, 3553–3590.
2. K. Kalyanasundaram, Coord. Chem. Rev., 1982, 46, 159–244.
3. (a) B. Imperiali and S. L. Fisher, J. Org. Chem., 1992, 57, 757–759; (b) B. Imperiali, T. J. Prins and S. L. Fisher, J. Org. Chem., 1993, 58, 1613–1616; (c) B. Imperiali and S. L. Fisher, J. Am. Chem. Soc., 1991, 113, 8527–8528.
4. (a) D. L. Popescu, T. J. Parolin and C. Achim, J. Am. Chem. Soc., 2003, 125, 6354–6355; (b) J. I. Yeh, E. Pohl, D. Truan, W. He, G. M. Sheldrick, S. Du and C. Achim, Chem. – Eur. J., 2010, 16, 11867–11875; (c) R. M. Franzini, R. M. Watson, G. K. Patra, R. M. Breece, D. L. Tierney, M. P. Hendrich and C. Achim, Inorg. Chem., 2000, 45, 9798–9811.
5. J. Lee, D. G. Udugamasooriya, H.-S. Lim and T. Kodadek, Nat. Chem. Biol., 2010, 6, 258–260.
6. R. P. Cheng, S. L. Fisher and B. Imperiali, J. Am. Chem. Soc., 1996, 118, 11349–11356.
7. A. Torrado and B. Imperiali, J. Org. Chem., 1996, 61, 8940–8948.
8. (a) C. P. Myers and M. E. Williams, Coord. Chem. Rev., 2010, 254, 2416–2428; (b) C. P. Myers, B. P. Gilmartin and M. E. Williams, Inorg. Chem., 2008, 47, 6738–6747; (c) M. A. Case, M. R. Ghadiri, M. W. Mutz and G. L. Mc Lendon, Chirality, 1998, 10, 35–40; (d) M. R. Ghadiri, C. Soares and C. Choi, J. Am. Chem. Soc., 1992, 114, 825–831; (e) M. Lieberman, M. Tabet and T. Sasaki, J. Am. Chem. Soc., 1994, 116, 5035–5044.
9. M. R. Ghadiri and A. K. Fernholz, J. Am. Chem. Soc., 1990, 112, 9633–9635.
10. (a) M. Gochin, V. Khorosheva and M. A. Case, J. Am. Chem. Soc., 2002, 124, 11018–11028; (b) S. Tashiro and M. Shionoya, Chem. Lett., 2013, 42, 456–462.
11. (a) G. Maayan, Eur. J. Org. Chem., 2009, 5699–5710; (b) Metallofoldamers. Supramolecular Architectures from Helicates to biomimetics, ed. G. Maayan and M. Albrecht, John Wiley & Sons, Ltd, 2013; (c) J. P. Miller, M. S. Melicher and A. Schepartz, J. Am. Chem. Soc., 2014, 136, 14726–14729.
12. A. S. Knight, E. Y. Zhou, M. B. Francis and R. N. Zuckermann, Adv. Mater., 2015, 38, 5665–5691.
13. (a) S. H. Gellman, Acc. Chem. Res., 1998, 31, 173–180; (b) Foldamers: structure, properties, and applications, ed. S. Hecht, and I. Huc, Wiley-VCH, Weinheim, 2007; (c) D. Seebach and J. Cardiner, Acc. Chem. Res., 2008, 41, 1366–1375; (d) D. J. Hill, M. J. Mio, R. B. Prince, T. S. Hughes and J. S. Moore, Chem. Rev., 2001, 101, 3893–4012; (e) S. A. Fowler and H. E. Blackwell, Org. Biomol. Chem., 2009, 7, 1508–1524; (f) C. M. Goodman, S. Choi, S. Shandler and W. F. DeGrado, Nat. Chem. Biol., 2007, 3, 252–262.
14. (a) C. W. Wu, et al. , J. Am. Chem. Soc., 2003, 125, 13525–13530; (b) P. Armand, K. Kirshenbaum, A. Falicov, R. L. Jr. Dunbrack, K. A. Dill, R. N. Zuckermann and F. E. Cohen, Folding Des., 1997, 2, 369–375; (c) K. Kirshenbaum, et al. , Proc. Natl. Acad. Sci. U. S. A., 1998, 95, 4303–4308; (d) C. W. Wu, T. J. Sanborn, K. Huang, R. N. Zuckermann and A. E. Barron, J. Am. Chem. Soc., 2001, 123, 6778–6784; (e) P. Armand, et al. , Proc. Natl. Acad. Sci. U. S. A., 1998, 14, 4309–4314.
15. R. N. Zuckermann, J. M. Kerr, S. B. H. Kent and W. H. Moos, J. Am. Chem. Soc., 1992, 114, 10646.
16. (a) B. C. Lee, T. K. Chu, K. A. Dill and R. N. Zuckermann, J. Am. Chem. Soc., 2008, 130, 8847–8855; (b) G. Maayan, M. D. Ward and K. Kirshenbaum, Chem. Commun., 2009, 56–58; (c) N. Maulucci, I. Izzo, G. Bifulco, A. Aliberti, C. De Cola, D. Comegna, C. Gaeta, A. Napolitano, C. Pizza, C. Tedesco, D. Flot and F. De Riccardis, Chem. Commun., 2008, 3927–3929; (d) C. De Cola, S. Licen, D. Comegna, E. Cafaro, G. Bifulco, I. Izzo, P. Tecilla and F. De Riccardis, Org. Biomol. Chem., 2009, 7, 2851; (e) G. Della Sala, B. Nardone, F. De Riccardis and I. Izzo, Org. Biomol. Chem., 2013, 11, 726–731; (f) I. Izzo, G. Ianniello, C. De Cola, B. Nardone, L. Erra, G. Vaughan, C. Tedesco and F. De Riccardis, Org. Lett., 2013, 15, 598–601; (g) C. De Cola, G. Fiorillo, A. Meli, S. Aime, E. Gianolio, I. Izzo and F. De Riccardis, Org. Biomol. Chem., 2014, 12, 424; (h) C. Tedesco, L. Erra, I. Izzo and F. De Riccardis, CrystEngComm, 2014, 16, 3667–3687; (i) T. Zabrodski, M. Baskin, J. K. Prathap and G. Maayan, Synlett, 2014, A–F; (j) M. Baskin and G. Maayan, Biopolymers, 2015, 104, 577–584; (k) A. S. Knight, E. Y. Zhou, J. G. Pelton and M. B. Francis, J. Am. Chem. Soc., 2013, 135, 17488–17493; (l) A. S. Knight, E. Y. Zhou and M. B. Francis, Chem. Sci., 2015, 6, 4042–4048.
17. G. Maayan, Y. Dayagi, R. Arad-Yellin, L. J. W. Shimon and A. Shanzer, Polyhedron, 2013, 64, 365–370.
18. T. S. Burkoth, A. T. Fafarman, D. H. Charych, M. D. Connolly and R. N. Zuckermann, J. Am. Chem. Soc., 2003, 125, 8841–8845.
19. G. Maayan, B. Yoo and K. Kirshenbaum, Tetrahedron Lett., 2008, 49, 335.
20. M. Baskin and G. Maayan, Chem. Sci., 2016, 7, 2809–2820.
21. S. B. Y. Shin, B. Yoo, L. J. Todaro and K. Kirshenbaum, J. Am. Chem. Soc., 2007, 129, 3218–3225.
22. P. J. Kaniraj and G. Maayan, Org. Lett., 2015, 17, 2110–2113.
23. J. V. Caspar and T. J. Meyer, J. Am. Chem. Soc., 1983, 105, 5583–5590.
24. J. M. Holub, H. Jang and K. Kirshenbaum, Org. Lett., 2007, 17, 3275–3278.
25. M. H. Filby, J. Muldoon, S. Dabb, N. C. Fletcher, A. E. Ashcroft and A. J. Wilson, Chem. Commun., 2011, 47, 559–561.
26. X. Huanget, et al. , J. Am. Chem. Soc., 2002, 124, 10320–10335.
27. V. Balzani, A. Juris and M. Venturi, Chem. Rev., 1996, 96, 759–833 and ref. 21 and 37 in that review.

---

Footnote

† Electronic supplementary information (ESI) available: Detailed synthetic procedures, HPLC and MS spectra, and complementary UV spectra. See DOI: 10.1039/c6cc04346a

---