Abstract
Peptoids are oligomers of N-substituted glycine that can be readily assembled using haloacetic acids and primary amines as synthons. Here, we report the synthesis and characterization of three new heterocyclic amines, 2-(2,2′:6′,2″-terpyridine-4′-yloxy) ethylamine, 2-(1,10-phenanthroline-5-yloxy) ethylamine and 8-hydroxy-2-quinolinemethylamine, and their incorporation into a series of different peptoid oligomer sequences. Since the heterocycles are all known to coordinate metal ions, the peptidomimetic products are designed to bind metal species with the potential for applications in catalysis and materials science.
N-substituted glycine peptoid oligomers, or “peptoids”, are abiotic polypeptide mimics that are capable of adopting stable secondary structures.1 By employing a solid-phase synthesis protocol,2 a wide variety of side chains can be incorporated into peptoid sequences, as shown in Scheme 1. This “submonomer” protocol enables the generation of peptoid oligomers possessing a wide range of chemical and structural diversity.3 For example, the incorporation of nitrogen-containing heterocyclic side chains (imidazole, pyridine, etc.) in peptoid oligomers has been reported previously.4 Multidentate ligands for metal coordination such as terpyridine, phenanthroline and hydroxyquinoline,5 however, have not yet been explored in the context of peptoid synthesis. Such ligands, especially bipyridine and terpyridine, have been incorporated into other oligomeric scaffolds (e.g. PNA) and utilized for metal binding.6 Hence, the incorporation of such ligands as side chains in peptoids will enable their use for metal coordination and may further expand the functional diversity of peptoids for applications in catalysis and materials science. We report here, for the first time, the synthesis of three primary amines: 2-(2,2′:6′,2″-terpyridine-4′-yloxy) ethylamine, 2-(1,10-phenanthroline-5-yloxy) ethylamine and 8-hydroxy-2-quinolinemethylamine, and their utilization as reagents in the solid-phase synthesis of peptoid oligomers.
Scheme 1.
Solid-phase “sub-monomer” peptoid synthesis.
Primary amines 1-3 were synthesized in simple one step reactions from commercially available starting materials. Compound 1, 2-(2,2′:6′,2″-terpyridine-4′-yloxy) ethylamine, was synthesized similarly to a known procedure,7 by adding 4′-chloro-2,2′:6′,2″-terpyridine and ethanolamine to a suspension of powdered KOH in DMSO, and stirring at 40°C for 2 hours.8 Compound 2, 2-(1,10-phenanthroline-5-yloxy) ethylamine, was synthesized accordingly, but because 5-chloro-1,10-phenanthroline is much less reactive than 4′-chloro-2,2′:6′, 2″-terpyridine, a higher temperature (80°C) and a longer reaction time (6 hr) were required.9 Compound 3, 8-hydroxy-2-quinolinemethylamine, was synthesized by reduction of 8-hydroxy-2-quinolinecarbonitrile with molecular hydrogen using palladium on carbon as a catalyst.10 Conversion of the nitrile to the amine was confirmed by the 1H NMR spectrum, showing a characteristic singlet signal for the methylene amine hydrogen molecules (CH2-NH2) at δ 4.03 ppm.
N-substituted glycine oligomers 4-11 (Table 1) were synthesized as C-terminal amides in good yields and high purities by using haloacetic acids and the following primary amines: 1 (Netp); 2 (Neph); 3 (Nhq); (S)-(-)-1-phenylethylamine (Nspe); 2-methoxyethylamine (Nme); and benzylamine (Npm).11 The test sequence, Nme-Npm-Netp-Npm-Nme was evaluated at various stages of the synthesis to ensure compatibility of the heterocycles with oligomer synthesis. Figure 1 shows High Performance Liquid Chromatographic (HPLC) characterization following incorporation of Netp (trimer 4), Npm (tetramer 5), and Nme (pentamer 6).
Table 1.
Peptoid oligomer sequences.
| Peptoid | Oligomer Sequence | Oligomer length | Molecular weight | Purity* % |
|---|---|---|---|---|
| Calc: Found | ||||
| 4 | Netp-Npm-Nme | 3mer | 611.7: 612.4 | >95 |
| 5 | Npm-Netp-Npm-Nme | 4mer | 758.9: 759.5 | >95 |
| 6 | Nme-Npm-Netp-Npm-Nme | 5mer | 874.0: 874.5 | >95 |
| 7 | Nme-Npm-Neph-Npm-Nme | 5mer | 820.9: 821.5 | >90 |
| 8 | Nme-Npm-Nhq-Npm-Nme | 5mer | 755.9: 756.4 | 87 |
| 9 | Nspe-Netp-Nspe | 3mer | 671.8: 672.3 | 95 |
| 10 | Nspe-Neph-Nspe | 3mer | 618.7: 619.3 | 87 |
| 11 | Nspe-Nhq-Nspe | 3mer | 553.6: 554.3 | 56 |
As determined by analytical HPLC of crude product. Compounds 9-11 were subsequently purified by preparative HPLC to >95%. Nme = 2-methoxyethylamine; Npm = benzylamine; Netp = 2-(2,2′: 6′, 2″-terpyridine-4′-yloxy) ethylamine; Neph = 2-(1, 10-phenanthroline-5-yloxy) ethylamine and Nhq = 8-hydroxy-2-quinolinemethylamine.
Figure 1.
HPLC traces of crude peptoid oligomers 4-6 prior to purification (Offset in y-dimension only). Top: Netp-Npm-Nme trimer (4); Middle: Npm-Netp-Npm-Nme tetramer (5); Bottom: Nme-Npm-Netp-Npm-Nme pentamer (6).
As shown in Figure 1, 2-(2,2′: 6′, 2″-terpyridine-4′-yloxy) ethylamine was incorporated successfully into the peptoid sequence, resulting in peptoid 6, with high purity.
Accordingly, amines 2 and 3 were incorporated in similar sequences to form the peptoids Nme-Npm-Neph-Npm-Nme (7) and Nme-Npm-Nhq-Npm-Nme (8).
Due to the fact that only peptoid oligomers containing bulky α-chiral side chains are known to form stable secondary structures [1a,d], we were interested in evaluating amines 1-3 in the synthesis of peptoids incorporating structure-inducing (S)-(-)-1-phenylethyl glycine (Nspe) monomers. Therefore, we further synthesized peptoids 9-11, with the sequences Nspe-Netp-Nspe, Nspe-Neph-Nspe, and Nspe-Nhq-Nspe, respectively.
Crude oligomer purities ranging from 56% to 95% were obtained, as determined by HPLC. Low peptoid yields were observed with the incorporation of 8-hydroxy-2-quinolinemethylamine. Diminished yields may arise due to the variations in the length of the spacer between the heterocycle and the reactive amine functionality. In the case of 8-hydroxy-2-quinolinemethylamine, the heterocycle is positioned closer to the peptoid backbone, which may enhance side reactions such as acylation of the heterocyclic nitrogen center. Molecular weights were confirmed by electrospray mass spectrometry and were in agreement with the expected values (Table 1).
The study presented here establishes the compatibility of terpyridine, phenanthroline and hydroxyquinoline groups with the solid-phase synthesis of peptoid oligomers. Furthermore, we demonstrate the feasibility of incorporating these heterocyclic ligands in peptoids of various lengths and sequences. The results establish the opportunity for realizing peptoid metal complexes, using late transition metal ions (e.g. Co and Cu ions) as a starting point. The ability to place one or two monomers incorporating metal coordinating centers at specific positions in the context of a peptidomimetic scaffold will be exploited to direct the formation of intermolecular or intramolecular metal complexes. This may enable the control of peptoid structure and will point the way to the formation of peptoid podands, as well as foldamers with unique secondary, tertiary or quaternary structures. We have recently obtained metal complexes of peptoids bearing such ligands, which are currently under investigation in our laboratory.
Acknowledgments
This work was supported by a National Science Foundation CAREER Award (#0645361). We thank the NCRR/NIH for a Research Facilities Improvement Grant (C06RR-165720) at NYU. We gratefully acknowledge Prof. Michael Ward for his helpful comments and for the support of this study through the Molecular Design Institute.
Footnotes
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References and Notes
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- 8.4′-chloro-2, 2′: 6′,2″-terpyridine (286mg, 1mmol) and ethanolamine (100μl, 1.1mmol) were added to a stirred suspension of powdered KOH (280mg, 5mmol) in DMSO (5ml) and stirred at 40°C for 2 hours. The reaction mixture was then added to 40ml of methylene chloride and washed with water (3×). The methylene chloride solution was dried over Na2SO4 and the solvent was removed. 2-(2,2′:6′,2″-terpyridine-4′-yloxy) ethylamine (286mg, 0.97mmol) was obtained as a light yellow solid in 97% yield and used subsequently without further purification. 1H NMR (400Mhz, CDCl3): δ = 3.9 (t, 2H, Hα), 4.8 (t, 2H, Hβ), 7.25 (t, 2H, H5, 5″), 7.75 (t, 2H, H4, 4″), 8.05 (s, 2H, H3′, 5′), 8.55 (d, 2H, H3, 3″), 8.65 (d, 2H, H6, 6″) ppm. 13C NMR (400Mhz, CDCl3): δ = 40.9 (Cα), 77.1 (Cβ), 107.4 (C5, 5″), 121.3 (C4, 4″), 123.8 (C3, 3″), 136.7 (C3′, 5′), 149.0 (C6, 6″), 155.87 (C2, 2″), 156.5 (C2′, 6′) 166.8 (C4′) ppm. ESI-MS: m/z = 293.1(M+), 315.2 (M + Na+).
- 9.5-chloro-1,10-phenanthroline (560mg, 2.6mmol) and ethanolamine (175μl, 2.9mmol) were added to powder KOH (730mg, 13mmol) in DMSO (10ml) and stirred at 80°C for 6 hours. The reaction mixture was then added to 100ml of methylene chloride and washed with water (42×). The methylene chloride solution was dried over Na2SO4 and the solvent was removed. The brown solid was purified from warm methylene chloride (2×) and 2-(1,10-phenanthroline-5-yloxy) ethylamine (442mg, 1.85mmole) was obtained as a light brown solid in 71% yield and used subsequently without further purification. 1H NMR δ (400Mhz, DMSO): δ = 9.03 (dd, 2H, H2,9), 8.42 (dd, 2H, H4, 7), 7.9 (s, 1H, H6), 7.7 (dd, 2H, H3, 8) ppm. 13C NMR (400Mhz, CDCl3): δ = 40.0 (Cα), 79.5 (Cβ), 123.3 (C3, 8), 126.6 (C6), 129.1 (C6α), 130.8 (C4α), 136.2 (C4, 7), 145.8 and 147.3 (C10α, b), 150.1 (C2, 9) 151.4 (C5) ppm. ESI-MS: m/z = 240.0 (M+), 262.1 (M + Na+).
- 10.8-hydroxy-2-quinolinecarbonitrile (1gr, 5.9mmol) was dissolved in acetic acid (43ml). 10% Pd/C (220mg) was added and the solution was treated with H2 (1 atmosphere) for 14 hours. The catalyst was filtered and the solvent was removed. The crude product was re-crystallized from a mixture of CHCl3 and Et2O. The light brown solid was filtered, dissolved in CHCl3 (160ml) and treated with a 1M potassium bicarbonate solution (4ml). The product was extracted with CHCl3, dried over Na2SO4 and the solvent was removed. 8-hydroxy-2-quinolinemethylamine (66mg, 3.8mmol) was obtained as light brown solid in 65% yield and used subsequently without further purification. 1H NMR (400 MHz, DMSO): δ = 8.2 (d, 1H, H7), 7.5 (d, 1H, H2), 7.35 (dt, 2H, H5, 6), 7.08 (dd, 1H, H3), 5.2 (bs, 2H, NH2), 4.03 (s, 2H, Hα) ppm. 13C NMR (400Mhz, DMSO): δ = 46.35 (Cα), 110.0 (C7), 116.4 (C5), 119.5 (C3), 125.7 (C6), 126.5 (C4α), 135.1 (C4), 136.4 (C8α), 151.9 (C8) 159.6 (C2) ppm. ESI-MS: m/z = 174.9 (M+).
- 11.Peptoid oligomers were synthesized manually on Rink amide resin using the submonomer approach [2a]. Typically, 100mg of resin was deprotected with 2ml of 20% piperidine in N, N-dimethylformamide (DMF) for 20 minutes. This was followed by a two-step monomer addition cycle for each residue - acylation and nucleophilic amine displacement. For the haloacylation step, 0.85m1 of a 0.4 M solution of bromoacetic acid and 0.2ml of neat N, N'-diisopropylcarbodiimide (DIC) were added to the resin and mixed at room temperature for 20 minutes. For the displacement steps, a 1.0M solution of the desired amine was prepared in DMF. From this solution, 1ml was added to the resin and mixed for 20 minutes at room temperature. This two-step addition cycle was modified as follows: after incorporation of heterocyclic amines 1, 2, and 3, chloroacetic acid was used in place of bromoacetic acid, and for the next displacement steps, 2.0 M solutions of the desired amine were used and the displacement was done in 35°C for 1 hour.4