Evaluation on the Stability of the Intramolecular N—H…OMe Hydrogen Bonds of Aromatic Amide Foldamers

Abstract

To evaluate the relative stability of different intramolecular N-H…OMe hydrogen bonds of aromatic amide-based foldamers, 3-, 5-, and 7-mer aromatic amide foldamers F-3, F-5 and F-7, which possess one, two, and three different amide units, have been constructed from benzene-1,3-diamine and isophthalic acid derivatives.1 H NMR experiments in CDCl 2 CDCl 2 and DMSO-d 6 showed that the hydrogen bonds formed in the central area of the foldamer backbones are least stable, whereas the hydrogen bonds formed at the two ends are most stable.1 H NMR hydrogen-deuterium exchange experiments for F-3, F-5 and F-7 in CDCl 2 CDCl 2 -CD 3 OD (19∶1, V/V) and DMSO-d 6 -CD 3 OD (19∶1, V/V) were performed. In the former less polar solvent mixture, the half-life values of the process, corresponding to amides from the central area to the end areas, were determined to be 140 h for F-3, 71.8 and 405 h for F-5, and 36.3, 216 and 314 h for F-7, respectively. In the latter more polar solvent mixture, the related values were evaluated to be 97.1 h for F-3, 69.0 and 300 h for F-5, and 13.5, 38.3 and 57.5 h for F-7, respectively. These quantitative results are consistent with the above 1 H NMR observation. To further assess the strength of the intramolecular hydrogen bonds, the three folded aromatic amide segments have also been incorporated into the main chains of dodecane-1,12-diamine-derived amide polymers to afford macromolecules P-3, P-5 and P-7. The degree of polymerization of the macromolecules was determined by GPC to be 22, 14 and 13, respectively. Force-extension curves obtained from single molecular force spectroscopy (SMFS) revealed that, in tetrachloroethane, all the three macromolecules exhibited saw-tooth force peaks, which had been attributed to the step-by-step breaking of the intramolecular hydrogen bonds of the foldamer segments. P-3 exhibited 4 peaks at ca. 83, 121, 181 and 236 pN, P-5 displayed 7 peaks at ca. 20,44, 73, 101, 130, 171 and 278 pN, and P-7 generated 8 peaks at ca. 31, 43, 50, 60, 90, 152, 173 and 221 pN. The increasing number of the force peaks observed from P-3 to P-5 and P-7 was ascribed to the increasing number of the intramolecular hydrogen bonds. It was proposed that the peaks at lower forces corresponded to the less stable hydrogen bonds, whereas those observed at higher forces were produced by the breaking of the more stable ones. The fact that the first peaks of P-3 was higher than that of P-5 and P-7 indicated that the intramolecular hydrogen bonds of P-3 were pronouncedly more stable than some of the intramolecular hydrogen bonds of P-5 and P-7, which is consistent with the above 1 H NMR and hydrogen-deuterium exchange observations. Similar results were also observed for P-5 and P-7 in hexadecane, whereas P-3 did not generate measurable force peaks possibly due to the strong absorption of its short, but more planar foldamer segments to the surface of the slide. Simulated stretching curves of the three macromolecules were also consistent with the SMFS results.

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